Chemistry 2

Equivalence point

concentration of titrant and analyte are the same

Buffers

A buffer solution contains a weak acid and weak base, often the conjugates of each other. -In a buffer there is an equilibrium between a weak acid and its conjugate base, or between a weak base and its conjugate acid.

Units of Specific Heat/ Heat Capacity

(J/K or cal/ ̊C)

Calculating pH for Strong Acids/Bases

(e.g.,pH of a 1x10-3M HCl solution) -=The pH or pOH=-log[strong acid or base] -if we take the -log of the concentration of the acid or base directly we are assuming that the molar concentration of the acid or base is equal to the molar concentration of hydrogen ions or hydroxide ions, respectively. This cannot be exactly true because some of both of these ions (10-7M) are already present in water before addition of the acid or base. The assumption is usually safe because the molar concentration of the strong acid or base is usually many magnitudes larger than 10-7. If the difference were smaller this would weaken the validity of our assumption. -Finally, we are also assuming that the acid and base dissociate 100%. If not, even if the first assumption were true, we could not take the -log of the concentration of the acid or base directly. Say, for example, that only 75% of the acid dissociated in solution—this would be a significant difference in concentration compared to 100% dissociation. Assuming that very strong acids dissociate 100% is usually a safe assumption, although ion-pairing and other factors due reduce the effective concentration of ions

Enthalpy

*Enthalpy = the energy contained within chemical bonds* It is illustrated by the fact that we can calculate ∆H for a reaction (i.e., the change in enthalpy) by finding the difference in total bond energy between the products and the reactants.

Entropy at Equilibrium

*REACTIONS AT EQUILIBRIUM ARE AT MAXIMUM ENTROPY*

ΔHsolution

*The enthalpy value associated with the dissolution of a species into solution.*

ΔHvaporization

*The enthalpy value associated with the phase change from liquid to gas*. -The reverse process (condensation) simply interchanges products and reactants and thus the sign is just changed.

translational kinetic energy

- particles move throughout the space of a container, colliding with each other and with the walls of their container -the main form of kinetic energy for gases and liquids

Strong Acid (SA) titrated with a Strong Base (SB) Titration Curve

-A strong acid in the flask will result in the pH being low, around 1 to 2 depending on the [SA]. As strong base is added, the pH will stay low and slowly rise until the equivalence point is reached and then it will go straight vertical for around 6 pH units. The middle of the equivalence region will be pH 7. After the equivalence point, the plot will slowly go to higher pH as more strong base is added. -Equivalence Point/Stoichiometric Point = midpoint of the nearly vertical section of the graph. At this point [titrant] = [analyte]. -For example, for a solution of NaOH being titrated with HCl, at the equivalence point [HCl] = [NaOH] in the flask. -Put another way, the moles of HCl in the beaker = the moles of NaOH in the beaker. -Because HCl and NaOH are both considered "strong" (i.e., dissociate 100% in water), they will both produce the same amount of ions per mole. Thus, for titrations involving a SA and a SB, [H+] = [OH-] at the equivalence point -pH = 7 at the equivalence point. -

Weak Acid (WA) titrated with a Strong Base (SB)

-As with the strong acid titration, this one will start at a pH below 7, but since the weak acid only partially dissociates, the pH will be higher than it is for a strong acid, or usually around 3 to 5. As strong base is added, the pH will increase slowly through the buffer region and then go vertical at the equivalence point. The equivalence point won't cover as many pH units as it did for the strong acid titration and leading up to it and after it, the pH will more quickly rise than it did for the strong acid. The half way pH of the equivalence point will not be 7 as it is for a strong acid, it will be higher than 7. This is because the conjugate base of the weak acid is now present, creating a basic solution. After the equivalence point, the pH will slowly rise as more base is added -/Stoichiometric Point= midpoint of the nearly vertical section of the graph. -Again, at this point [titrant] = [analyte]. However, because the acid and base are not both "strong" (i.e., dissociate 100% in water) the [H+] does NOT equal [OH-]. -This also tells us that the pH does NOT equal 7. Notice that this is different than for SA/SB titrations.

Hydrolysis of Salts

-The "salt of a weak acid"refers to the conjugate base of that weak acid combined with a cation to form a salt (see the example below). HCO3- = "weak acid"; CO32- = "conjugate base"; Na2CO3 = "salt of a weak acid" -The "salt of a weak base" refers to the conjugate acid of that weak base combined with an anion to form a salt (see the example below). NH3 = "weak base"; NH4+ = "conjugate acid"; NH4NO3 = "salt of a weak base"

Convection (Heat exchange)

-The process of heat transfer from one location to the next by the movement of fluids. The moving fluid carries energy with it. The fluid flows from a high temperature location to a low temperature location. -Fluid movement caused by the hotter portions of a fluid rising and the cooler portions of a fluid sinking. Air currents and convection currents in water are examples -molecules move more quickly when heated and become less dense (less molecules in an area at any given time) Ex: To understand convection in fluids, let's consider the heat transfer through the water that is being heated in a pot on a stove. Of course the source of the heat is the stove burner. The metal pot that holds the water is heated by the stove burner. As the metal becomes hot, it begins to conduct heat to the water. The water at the boundary with the metal pan becomes hot. Fluids expand when heated and become less dense. So as the water at the bottom of the pot becomes hot, its density decreases. Differences in water density between the bottom of the pot and the top of the pot results in the gradual formation of circulation currents. Hot water begins to rise to the top of the pot displacing the colder water that was originally there. And the colder water that was present at the top of the pot moves towards the bottom of the pot where it is heated and begins to rise. Ex: Convection also explains how an electric heater placed on the floor of a cold room warms up the air in the room. Air present near the coils of the heater warm up. As the air warms up, it expands, becomes less dense and begins to rise. As the hot air rises, it pushes some of the cold air near the top of the room out of the way. The cold air moves towards the bottom of the room to replace the hot air that has risen Air travels along these pathways, carrying energy with it from the heater throughout the room. *These circulation currents slowly develop over time* -Heat typically does not flow through liquids and gases by means of conduction. Liquids and gases are fluids; their particles are not fixed in place; they move about the bulk of the sample of matter. THE CONVECTION METHOD OF HEAT TRANSFER ALWAYS INVOLVES THE TRANSFER OF HEAT BY THE MOVEMENT OF MATTER

Weak Base (WB) titrated with a Strong Acid (SA)

-This will look like the strong base/strong acid titration except the pH will not start as high, the equivalence region won't last for as many pH units, and the equivalence point will be at a pH below 7. The reasons for all of these are for like reasons given in the weak acid/strong base titration, except that they need to be applied to the weak base. EquivalencePoint/Stoichiometric Point= midpoint of the nearly vertical section of the graph. -Again, at this point [titrant] = [analyte]. However, because the acid and base are not both "strong" (i.e., dissociate 100% in water) the [H+] does NOT equal [OH-]. -This also tells us that the pH does NOT equal 7. Notice that this is different than for SA/SB titrations.

Strong Base (SB) titrated with a Strong Acid (SA)

-With strong base present, the pH will start high. As strong acid is added, it will slowly go to lower pH and then sharply drop when the equivalence point is reached. Again, the middle of the pH region will be pH 7. It will then slowly go to lower pH as more strong acid is added. -Equivalence Point/Stoichiometric Point = midpoint of the nearly vertical section of the graph. At this point [titrant] = [analyte]. -For example, for a solution of NaOH being titrated with HCl, at the equivalence point [HCl] = [NaOH] in the flask. -Put another way, the moles of HCl in the beaker = the moles of NaOH in the beaker. -Because HCl and NaOH are both considered "strong" (i.e., dissociate 100% in water), they will both produce the same amount of ions per mole. Thus, for titrations involving a SA and a SB, [H+] = [OH-] at the equivalence point -pH = 7 at the equivalence point.

Recognizing Buffer Problems

1) *Watch for equimolar amounts*. The maximum buffering strength occurs when the [HA] is equal to the [A-]. This is the ratio one would start with if making a buffer in the lab. Therefore, be on the lookout for equimolar amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid. 2) *Watch for conjugates*. To be a buffer, the two equimolar substances are often conjugates of each other, such as: NH3 and NH4, CH3COO- and CH3COOH, or HCO3- and CO32-. 3) *Watch for WEAK acids or bases*. The equimolar pair must be a weak base or a weak acid and its conjugate. Strong acid or strong base conjugate pairs do NOT form buffers. Q31. What would be the result if you tried to make a buffer out of a strong acid and a strong base? 4) *Watch for resistance to pH change*. Any time an acid or base is added to a solution and the pH does not change "very much," or "changes slightly," this should be a dead giveaway that the solution is a buffer. 5) *Watch for the half-equivalence point*. Remember that only solutions of weak conjugate acid/base pairs have a buffer region, and therefore they are the only solutions that have a half- equivalence point. 6) *Watch for pH = pKa.* Many students memorize this principle, but fail to recognize that what it is really saying is that pH = pKa at the midpoint of the buffer region. This is another unmistakable clue that you are dealing with a buffer. 7) *Watch for the ratio of [HA]/[A-] or [A-]/[HA]*. One particular problem on the CBT practice exams seems to stump the majority of students. The question stem asks about the ratio of an acid to its conjugate base. Those who recognize it as a buffer problem usually answer it correctly. Those who do not guess.

Calculating pH for Weak Acids

1) Write out the equilibrium equation (HA H+ + A-). 2) Use x to represent the concentration of each of the two products (or 2x, 3x etc. depending on the coefficients in the balanced equation). 3) Use "[HA] - x" for the concentration of the original acid. 4) Solve for x from the resulting equation: Ka = (x)(x)/[HA - x]. 5) If this results in a quadratic equation, assume that x is much smaller than [HA] (in step #3 above) and omit it. 6) Use -log[H+] to solve for the pH. ***see emp links

Entropy Increases with Increasing:

1) number of items/particles/etc. *Caveat: The number of moles of gas trumps the number of moles of species in any other phase. Thus, even if a reaction turns two moles of reactants into one mole of product, if that one mole of product is a gas, and the reactants are not, entropy has increased; and S will therefore be positive* . BE AWARE OF WHEN GAS IS FORMED S IS POSITIVE 2) volume- think that finding a gas molecule in a larger volume container would be more difficult than in a small container (less order) 3) temperature 4) disorder (e.g., ∆S is greater for an amorphous material than for a crystal) 5) complexity (e.g., S is greater for C2H6 than for CH4)

Q12.If∆H is positive and entropy change is negative,what will the sign of ∆G be? Q13.If the change in entropy is positive, and enthalpy is negative, the reaction is:a)spontaneous, b) non-spontaneous, c) can be either spontaneous or non-spontaneous depending on temperature. Q14. If a reactant is dissolved in solution, causing the temperature of the reaction vessel to increase, the G for this reaction must be a) positive, b) negative, or c) cannot be determined.

12. We believe it is always more effective to use a conceptual understanding of something to answer a question rather than a mathematical or memorization-based approach. In this case, if enthalpy is positive, that indicates an unfavorable change. The reaction is endothermic and energy will need to be added to move from reactants to products. At the same time we are told that entropy is negative. A negative entropy means more order—which is also an unfavorable change (things spontaneously move toward more disorder, NOT more order). So, is it likely that a process will be spontaneous if both the enthalpy change and the entropy change are unfavorable? Hardly. Therefore the sign of ∆G will be positive and the reaction will not proceed spontaneously under standard state conditions. An alternative mathematical solution would plug the signs of ∆H and ∆S into the fundamental thermodynamic relation: ∆G = ∆H - T∆S. 13. This is the opposite of the case examined in the previous question. This time both the enthalpy change and the entropy change are favorable, so we would definitely expect a negative sign for Gibbs Free Energy , or Answer a). Once again, this could be proven by plugging the signs into the fundamental thermodynamic relation. 14. *When a solute is dissolved into solution, entropy will always be positive*. A solid is far more ordered than are ions in a solution. The increase in temperature is assumed to be the result of heat evolved from the reaction, so enthalpy change must be negative (i.e., exothermic). Again, these are two favorable state changes, so according to the fundamental thermodynamic relationship, Gibbs Free Energy must be negative and the reaction must be spontaneous.

Standard State

A set of specific conditions chosen as the reference point for measuring and reporting enthalpy, entropy, and Gibbs free energy. -You may remember seeing these conditions specified at the top of a table of ∆H values. When values are given for standard state conditions, a superscript, called "naught," is added. The naught symbol resembles a degree symbol (∆H°). -laboratory conditions

The conjugate base of Acid A has a Kb of 1.0 x 10-9 and the conjugate base of Acid B has a Kb of 1.0 x 10-10. Which acid will create the largest decrease in pH when added in equimolar amounts to pure water?

Acid B will give the largest drop in pH. The largest decrease in pH will be caused by addition of the most acidic of the two species. You could simply recognize from the equation Kw = Ka*Kb that Kb and Ka are inversely related, and therefore recognize that the smaller Kb represents the stronger acid (because Ka and acid strength are directly related). You could also use the above equation, plugging in 1 x 10-14 for Kw, and solve for Ka in both cases. Either method leads to the conclusion that the acid with a Kb of 1 x 10-10 is the stronger acid and will therefore lower pH to the greater extent.

Lewis:

Acids accept a pair of electrons; bases donate a pair of electrons. -*AlCl3 and BF3* are two common examples of Lewis acids. -The electrophiles in all organic chemistry reactions are acting as Lewis Acids. NH3, OH- and anything else with an electron pair to donate will act as a Lewis base. -When the MCAT tests Lewis acids or Lewis bases, they always specifically label them as such. Otherwise, when you see "acid" on the MCAT, think of the Bronsted-Lowry definition. In other words, think of species that donate an acidic hydrogen.

Bronsted-Lowry

Acids donate protons (H+); bases accept protons (H+).

Arrhenius

Acids produce H+ ions in solution; bases produce OH- ions in solution. -oversimplified and rarely seen on the MCAT

Acid-Base Equilibria

All equilibrium constants (Keq, Ka, Kb. Kw or Ksp) are written via the law of mass action, with pure liquids (l) and solids (s) omitted -You should think of Ka or Kb just as you do Keq. -A large Ka (or a small pKa) indicates that at equilibrium there are far more products than reactants. -For an acid dissociation, this would mean a lot of dissociation (i.e., a lot of H+ formed) and thus a very strong acid. -Similarly, a large Kb (or a small pKb) indicates a very strong base (i.e., a lot of OH- formed—either from dissociation of a hydroxide base such as NaOH, or from deprotonating water).

Strong Acids/Strong Bases

All strong acids and strong bases dissociate 100% in water (making them good electrolytes). There is obviously a continuum of strength, not a hard, fast line, but for the MCAT we will consider a species "strong" only if it is included in the following lists:

T/F An aqueous solution with a pH of 8 is basic and there fore by definition it does not contain any unreacted H+ ions.

An aqueous solution with a pH of 8 is basic, but that does NOT mean that it does not contain any hydrogen ions. In fact, the presence of hydrogen ions is easily verified by solving the formula pH = -log[H+] for [H+]. There are 1.0 x 10-8 moles of hydrogen ions per liter of this solution. It is classified as basic because it has fewer hydrogen ions than are found in neutral water and more hydroxide ions than are found in neutral water.

Examples of Weak Acids:

Anything NOT on the strong acids list -H2O,H2S,NH4+,HF,HCN,H2CO3,H3PO4,aceticacid,benzoicacid,etc.

Examples of Weak Bases

Anything NOT on the strong base list. H2O,NH3,R3N,pyridine,Mg(OH)2,etc.

Acid HX dissociates 80% in water. Would you expect its Ka to be greater than, less than, or equal to one?

Because the acid almost fully dissociates, we know that the ratio of products over reactants would have to be greater than one. -In general terms, an acid with Ka greater than one or a pKa less than zero is considered "strong," so this acid would clearly qualify as a strong acid.

Calorimeters

Device used to calculate enthalpy change (∆H). -We are assuming that q (which is what the calorimeter actually measures) is equal to ∆H. This is true at constant pressure.

Do NOT confuse standard state with STP

Do NOT memorize any values for standard state conditions because they can be different for different tables. Later on, you will need to memorize STP conditions. If you confuse these two you can easily get into trouble. -Elements in their standard state have ∆Hformation° = zero. This is because elements in their standard state are used to define the enthalpy scale and thus there is no change in energy to create them from themselves. The enthalpy change for graphite (solid carbon in its standard state) for example, is zero. However, the enthalpy change for diamond is 2.0 kJ/mol because energy is required to form diamond from graphite

Titration

Drop by drop mixing of an acid and a base with an indicator. -procedure for determining the concentration of a solution

Radiation (Heat Exchange)

Electromagnetic waves emitted from a hot body into the surrounding environment. -Light colors radiate and absorb less -Dark colors radiate and absorb more -To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin. -The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter. -Thermal radiation can occur through matter or through a region of space that is void of matter (i.e., a vacuum). In fact, the heat received on Earth from the sun is the result of electromagnetic waves traveling through the void of space between the Earth and the sun. -The hotter the object, the more it radiates. The sun obviously radiates off more energy than a hot mug of coffee. -The temperature also affects the wavelength and frequency of the radiated waves. Objects at typical room temperatures radiate energy as infrared waves. -Being invisible to the human eye, we do not see this form of radiation. An infrared camera is capable of detecting such radiation. -s the temperature of an object increases, the wavelengths within the spectra of the emitted radiation also decrease. Hotter objects tend to emit shorter wavelength, higher frequency radiation. -Thermal radiation is a form of heat transfer because the electromagnetic radiation emitted from the source carries energy away from the source to surrounding (or distant) objects. This energy is absorbed by those objects, causing the average kinetic energy of their particles to increase and causing the temperatures to rise. In this sense, energy is transferred from one location to another by means of electromagnetic radiation.

The First Law of Thermodynamics:

Energy neither be created nor destroyed 1) The total energy of an isolated system is always constant. 2) The energy change in a closed system is equal to the heat absorbed by that system plus any work done on that system by its surroundings. ∆E=q+w

Entropy (∆S)

Entropy=a measure of the randomness or disorder in a system. -Reactions and other processes will be more likely to be spontaneous if they increase entropy. As a reaction proceeds, if randomness increases, energy will be released and thus be available to do work. -If randomness decreases, energy is required to "create" this increased order, and that amount of energy will thus be unavailable to do work.

A pan of water is placed upon an electric heating element on a stove. Describe all types of heat exchange expected to occur in this scenario.

First, it is important to note that this question is not as straightforward as it may seem if we do not know the temperature of the pan of water. -Most students will assume the pan of water is cold. However, this is a good reminder that more careful thinking is usually rewarded on the MCAT. -If the pot and water happened to be at the same temperature as the heating element then no heat exchange would occur. -If the pan is hotter than the element (say the element isn't even turned on yet), then heat flow will occur in the opposite direction to that most students will propose. -Assuming the pot is cold and the heating element is hot: Heat will be transferred from the element to the pan via conduction because they are in contact with one another. -Heat will be transferred from the pan to the water via conduction because they are also in contact with one another. -The water in the bottom of the pan will heat up first and that will cause this hotter part of the liquid to rise while the cooler liquid above sinks—which is convection. Finally, any part of the system that is hotter than the environment will radiate heat into the air— including the heating element itself, the pan, and the water. (You can feel the heat from a red hot burner without touching it.)

Strong Bases List

Group IA hydroxides (NaOH,KOH,etc.), NH2-, H-, Ca(OH)2, Sr(OH)2, Ba(OH)2, Na2O, CaO.

Strong Acids List

HI , HBr, HCl, HNO3, HClO4, HClO3, H2SO4, H3O+ -Note: H3O+ is a borderline strong acid. It does have a pKa < 0, but just barely (pKa = -1.7). Many texts use it as a line of demarcation: acids stronger than hydronium ion are "strong" and acids weaker than hydronium ion are "weak." -Note:*HF is NOT a strong acid*, but is often mistakenly labeled a ssuch.

The Second Law of Thermodynamics

Heat cannot be changed completely into work in a cyclical process. - Entropy in an isolated system can never decrease.

Positive vs. Negative Gibs Free Energy

If energy is available and the system can do work, Gibbs Free Energy is negative. -If energy must be added to the reaction (e.g., heat must be added to the system) to make it proceed, Gibbs Free Energy is positive. -negative∆G=Spontaneous process; free energy available to do work. positive∆G=Non-spontaneous process; no free energy available; energy is required.

The Zeroth Law of Thermodynamics

If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with object B, then objects A and C must be in thermal equilibrium with each other *For the MCAT, Just Remember This*: Everything tends to move toward thermal equilibrium with everything else. Objects with higher temperatures will always equilibrate over time with their surroundings, including other objects with which they are in contact. Finally, if two objects are in thermal equilibrium, by definition they have the same temperature.

For the same system, which heat capacity will be greater, the constant volume heat capacity or the constant pressure heat capacity?

If the volume is held constant, then 100% of the energy added will go toward an increase in temperature. -If the pressure is held constant the volume can still change and therefore some of the added heat will go toward pv work. - If we think of heat capacity as "the amount of energy we can add before the system increases by one temperature unit," it is fairly easy to see that the system capable of pv work will be able to absorb more heat before increasing by one degree Celsius or Kelvin. (greater heat capacity)

Caloric Theory

In caloric theory, heat was the fluid and the fluid that moved was the heat. DONT CONFUSE WITH CONVECTION The convection method of heat transfer always involves the transfer of heat by the movement of matter.

Indicators

Indicators are weak acids that change color as they dissociate from HA into H+ and A-. -To set up a titration, you must know beforehand the approximate pH of your equivalence point; you then select an indicator that will change color at that approximate pH. -The dissociation of the indicator and the acid/base reaction we are analyzing run simultaneously in the same beaker but are otherwise unrelated. -The amount of indicator is so small compared to titrant and analyte that we can assume it has no impact on pH.

Units of Entropy

J/K

Gibs Free Energy Units

Joules

Units of Enthalpy

Joules (J)

Equation that shows the direct relationship between kinetic energy and temperature

KE = 3/2(kB)(T) where kB is Boltzmann's constant

Acid Dissociation:

Ka = [H+][A-]/[HA]

Base Dissociation

Kb = [OH-][HA]/[A-] Ka*Kb = Kw = 10-14 (at 25°C); because ([H+][A-]/[HA])*([OH-][HA]/[A-]) = [H+][OH-] = Kw

Celsius to Kelvin

Kelvin= degrees celsius + 273 *ALWAYS USE KELVIN IN FORMULAS* (unless specified)

Ionization of Water

Kw=[H3O+][OH-]=10-14 (at25°C; This caveat should make sense because you learned earlier that temperature is the one thing that does change Keq.) pKw = pH + pOH = 14 pKa+pKb=14

Three common indicators in the Lab: Methyl Orange Litmus Phenolphthalein

Methyl Orange- 3.7 Litmus- 6.5 Phenolphthalein- 9.3

Conduction (Heat exchange)

Molecular collisions carry heat along a conduit. -Recall that temperature is a reflection of the average kinetic energy of the molecules. -High energy molecules collide with their neighbors, which in turn collide with their neighbors until eventually the energy is spread equally throughout. -Heat conduction is roughly analogous to current flow through a wire. -Conductive heat flow involves the transfer of heat from one location to another in the absence of any material flow. There is nothing physical or material moving the heat-only energy is transferred -The more energetic particles will lose a little kinetic energy and the less energetic particles will gain a little kinetic energy. Temperature is a measure of the average amount of kinetic energy possessed by the particles in a sample of matter. So on average, there are more particles in the higher temperature object with greater kinetic energy than there are in the lower temperature object. -The collisions of our little bangers and wigglers will continue to transfer energy until the temperatures of the two objects are identical. -*there is no net transfer of physical stuff between the objects. Nothing material moves across the boundary. The changes in temperature are wholly explained as the result of the gains and losses of kinetic energy during collisions* HEAT TRANSFER THROUGH SOLIDS

Do SA/SB titrations have a half equivalence point?

No because strong acids and strong bases both dissociate 100%. Therefore, by definition, almost immediately after adding HA 100% has been changed to A-.

Example of Conduction: experiment in which a metal can containing hot water was placed within a Styrofoam cup containing cold water

On average, the particles with the greatest kinetic energy are the particles of the hot water. -Being a fluid, those particles move about with translational kinetic energy and bang upon the particles of the metal can. -As the hot water particles bang upon the particles of the metal can, they transfer energy to the metal can. -This warms the metal can up. Most metals are good thermal conductors so they warm up quite quickly throughout the bulk of the can. The can assumes nearly the same temperature as the hot water. -Being a solid, the metal can consists of little wigglers. The wigglers at the outer perimeter of the metal can bang upon particles in the cold water. The collisions between the particles of the metal can and the particles of the cold water result in the transfer of energy to the cold water. -This slowly warms the cold water up. The interaction between the particles of the hot water, the metal can and the cold water results in a transfer of energy outward from the hot water to the cold water. -The average kinetic energy of the hot water particles gradually decreases; the average kinetic energy of the cold-water particles gradually increases; and eventually, thermal equilibrium would be reached at the point that the particles of the hot water and the cold water have the same average kinetic energy. -At the macroscopic level, one would observe a decrease in temperature of the hot water and an increase in temperature of the cold water.

What happens when heat enters a system? Does the temperature always increase? Is any increase in temperature always exactly proportional to the heat absorbed by that system? (Hint: Think of adding energy to a sealed steel container vs. adding energy to a balloon; remember that temperature is the average kinetic energy of the molecules.)

PV work is the work necessary to produce an increase in volume. For example, when a sealed balloon is heated, the gases inside the balloon will expand and must do work on the rubber walls of the balloon and the air around it to accomplish this expansion. Because some of the heat energy added to the balloon was used for pv work, only the remaining portion of the heat will go toward increasing the average kinetic energy of the molecules (i.e., temperature). -when heat enters a system, if the system is capable of volume change, heat can go to pv work, increased temperature, or both. For this reason, the addition of a certain amount of heat will NOT necessarily be exactly proportional to the resultant increase in temperature. If the system is not capable of changing volume then no pv work can be done, so all of the added heat will go toward an increase in temperature.

Important Note

Pay special attention to where the titration curve starts. Strong acids titrated with anything (strong or weak base) should start low on the y-axis. Strong bases titrated with anything should start high on the y-axis. The end of the titration curve should correlate with the pH of the titrant. For example, if you're titrating with a strong base, near the end of the titration the curve should flatten out at a point high on the y-axis. If titrating with a weak base, however, the curve should end much lower

The Third Law of Thermodynamics

Pure crystalline substances at absolute zero have an entropy of zero.

End point

Solution indicator changes color, complete neutralization has occurred

Bomb Calorimeter

Solve using: q = C∆T. This does NOT give enthalpy, but change in internal energy, ∆U or ∆E. Use heat capacity (big C) instead of specific heat capacity (little c). -A coffee cup calorimeter is great for measuring heat flow in a solution, but it can't be used for reactions which involve gases, since they would escape from the cup. The coffee cup calorimeter can't be used for high temperature reactions, either, since these would melt the cup. - In a bomb calorimeter, the reaction takes place in a sealed metal container, which is placed in the water in an insulated container. Heat flow from the reaction crosses the walls of the sealed container to the water. The temperature difference of the water is measured, just as it was for a coffee cup calorimeter. Analysis of the heat flow is a bit more complex than it was for the coffee cup calorimeter because the heat flow into the metal parts of the calorimeter must be taken into account: -qreaction = - (qwater + qbomb) where qwater = 4.18 J/(g·°C) x mwater x Δt The bomb has a fixed mass and specific heat. The mass of the bomb multiplied by its specific heat is sometimes termed the calorimeter constant, denoted by the symbol C with units of joules per degree Celsius. The calorimeter constant is determined experimentally and will vary from one calorimeter to the next. The heat flow of the bomb is: qbomb = C x Δt *The bomb calorimeter is a sealed steel container, meaning volume cannot change. However, because volume is constant any gases produced or consumed during the reaction will change the pressure.*

Coffee Cup Calorimeter

Solve using: q=mc∆T A coffee cup calorimeter is essentially a polystyrene (Styrofoam) cup with a lid. The cup is partially filled with a known volume of water and a thermometer is inserted through the lid of the cup so that its bulb is below the water surface. When a chemical reaction occurs in the coffee cup calorimeter, the heat of the reaction if absorbed by the water. The change in the water temperature is used to calculate the amount of heat that has been absorbed (used to make products, so water temperature decreases) or evolved (lost to the water, so its temperature increases) in the reaction. Heat flow is calculated using the relation: q = (specific heat) x m x Δt where q is heat flow, m is mass in grams, and Δt is the change in temperature. The specific heat is the amount of heat required to raise the temperature of 1 gram of a substance 1 degree Celsius. The specific heat of water is 4.18 J/(g·°C). For example, consider a chemical reaction which occurs in 200 grams of water with an initial temperature of 25.0°C. The reaction is allowed to proceed in the coffee cup calorimeter. As a result of the reaction, the temperature of the water changes to 31.0°C. The heat flow is calculated: qwater = 4.18 J/(g·°C) x 200 g x (31.0°C - 25.0°C) qwater = +5.0 x 103 J In other words, the products of the reaction evolved 5000 J of heat, which was lost to the water. The enthalpy change, ΔH, for the reaction is equal in magnitude but opposite in sign to the heat flow for the water: ΔHreaction = -(qwater) Recall that for an exothermic reaction, ΔH < 0; qwater is positive. The water absorbs heat from the reaction and an increase in temperature is seen. For an endothermic reaction, ΔH > 0; qwater is negative. The water supplies heat for the reaction and a decrease in temperature is seen. -A coffee cup calorimeter is great for measuring heat flow in a solution, but it can't be used for reactions which involve gases, since they would escape from the cup. The coffee cup calorimeter can't be used for high temperature reactions, either, since these would melt the cup. -The coffee cup calorimeter allows for an increase in volume of the solution inside the coffee cup, but remains at atmospheric pressure throughout.

Specific Heat of Water

Specific Heat of Water = 1.0 cal/g ̊C or 4.18 J/g ̊C

Constant Volume vs. Constant Pressure Heat Capacities

Systems can be confined by rigid walls (constant volume) or can be open to the atmosphere like water in a beaker (constant pressure).

Acid/Conjugate Acid and Base/Conjugate Base:

The "conjugate base" of an acid is the acid minus its hydrogen (e.g., HCl = acid; Cl- = conjugate base). -The "conjugate acid" of a base is the base plus a hydrogen (e.g., NH3 = base; NH4+ = conjugate acid). Which species you call the acid/base or the conjugate acid/base is arbitrary (e.g., you could call HCl the acid and Cl- the conjugate base or call Cl- the base and HCl the conjugate acid).

Heat Capacity

The amount of energy (Joules or Calories) a system must absorb to give a unit change in temperature (J/K or cal/ ̊C). Formula, where C is the heat capacity, q is heat (or other energy) and T is temperature: C=q/∆T

ΔHfusion:

The enthalpy value associated with the phase change from liquid to solid. -The sign changes for the reverse process (melting).

ΔHcombustion

The enthalpy value for the combustion of a compound with O2 to form CO2 and water. *A high heat of combustion is associated with an unstable molecule and a low heat of combustion with a stable molecule.* (remember energy coordinate diagram, higher energy molecule will have larger difference in energy)

Beakers 1 and 2 contain 0.25 L of water and 0.5 L of water, respectively. How does the heat capacity of the water in Beaker 1 compare to that of the water in Beaker 2? How do the specific heat capacities compare?

The heat capacity of Beaker 2 will be greater than that of Beaker 1 because there is more water available to absorb heat in Beaker 2. However, the specific heat capacity of water in both beakers will be identical - specific heat capacity is an intensive property.

Conduction Through The Bulk of an Object

The mug is at room temperature - maybe at 26°C. Then suppose we fill the ceramic coffee mug with hot coffee at a temperature of 80°C. The mug quickly warms up. Energy first flows into the particles at the boundary between the hot coffee and the ceramic mug. But then it flows through the bulk of the ceramic to all parts of the ceramic mug. -the ceramic particles at the boundary between the hot coffee and the mug warm up, they attain a kinetic energy that is much higher than their neighbors. As they wiggle more vigorously, they bang into their neighbors and increase their vibrational kinetic energy. -Soon the entire coffee mug is warm and your hand feels it. -particle-to-particle interaction is very common in ceramic materials such as a coffee mug

Buffer Region

The nearly horizontal area surrounding the half-equivalence point is called the "buffer region." -Adding a relatively large amount of titrant at this point in the titration will have little effect on pH. -In fact, that is the very reason the graph is flat in this region—the volume of titrant added increases (x-axis) with little or no increase in pH (y-axis).

pH Scale:

The pH scale is a mathematical ranking system for the acidity or basicity of aqueous solutions. The solution being ranked could be water only, water + acid, water + base, or water + base + acid. Notice, however, that water is always there. In fact, as you'll see in the next section, the pH scale ranks solutions based not so much on the acids or bases themselves, but on how those acids or bases influence the equilibrium for the ionization of water. pH of pure H2O at 25°C=7.0, and the [H+]=[OH]; -pH>7= basic, and the [H+]<[OH-]; pH<7=acidic, and the [H+] > [OH-]; pH = 7 is defined as "neutral" and the [H+] = [OH-

What would be the result if you tried to make a buffer out of a strong acid and a strong base?

The pH would change drastically when acid or base was added to it. There would be no weak acid or base to consume the added base or acid. -you will never have acid and base in the solution at once because they completely dissociate

Impact of Salts on the Dissolution of Weak Acids and Weak Bases:

The percent dissociation of benzoic acid (weakacid) decreases in a sodium benzoate solution. The percent dissociation of ammonium hydroxide (weak base) decreases in an ammonium chloride solution. Both observations are easily explained by Le Chatelier's principle. In case one, sodium benzoate dissociates to release benzoate ions, which shift the acid dissociation equilibrium for benzoic acid to the left. Similarly, ammonium chloride dissociates to release ammonium ions, which shift the base dissociation equilibrium for ammonium hydroxide to the left.

Standard enthalpy change of reaction, ΔH°

The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state.

If a question states: "A strong base is titrated with a strong acid," which one is being added drop- wise and which one is in the beaker? Which solution is referred to as the titrant? Which solution is referred to as the anylate?

The terminology used in the question infers that the strong base is in the beaker, which makes it the analyte. The base is "titrated with" the strong acid, meaning the acid is being added dropwise and is therefore the "titrant."

ΔHRXN the enthalpy change for a reaction:

This is usually calculated by adding reactions (and their associated enthalpy changes) from a table. You must select the reactions from the table that—when added together—will produce the net reaction for which you are calculating ∆HRXN. -To calculate ∆HRXN you will add all of the values given for each of the reactions you use, paying careful attention to signs and stoichiometry. -If the reaction proceeds in the same direction as it would in the net reaction, use the value given directly. -If it proceeds in the opposite direction, change the sign of the value given. -You MUST multiply the number given in the table by the coefficient in the balanced net reaction. -For example, suppose one of the reactions given was the formation of liquid water from the elements hydrogen and oxygen: 1/2O2 + H2 -> H2O ∆H°f = -285 kJ/mol, but in your net reaction water is a reactant rather than a product. You would need to change the sign of ∆H° to be positive and combine it with the enthalpy changes from the other reactions used. If your balanced net reaction contained two moles of water, you would need to double the value given, and so forth.

Weak Acids/Weak Bases

Weak acids and bases do not dissociate readily in solution. -As a general rule, an acid with a pKa greater than zero, or a Ka less than one, can be considered a weak acid. -Similarly, a base with a pKb greater than zero, or a Kb less than one, can be considered a weak base.

Thermal Equillibrium

When this state of thermal equilibrium has been reached, the average kinetic energy of both objects' particles is equal. - At thermal equilibrium, there are an equal number of collisions resulting in an energy gain as there are collisions resulting in an energy loss. -On average, there is no net energy transfer resulting from the collisions of particles at the perimeter.

Calculating ΔHRXN Using Bond Energies

You can also calculate the enthalpy change of a reaction using bond energies. -This is not quite as common on the MCAT as the method of adding reactions (but it is a bit more intuitive in our opinion). -To do so, simply add up the bond energies of all of the products and reactants. If a bond is broken during the reaction, energy is required, so the bond energy should be given a positive sign. -If a bond is formed, energy is released, so the bond energy should be given a negative sign. Once again, multiply all bond energy values by their coefficients in the balanced equation. *DONT MULTIPLY COEFFICIENTS WHEN CALCULATING E° OR ELECTROCHEMICAL REACTIONS*

Adding acid or base to water relates to Le Chatlier's Principle

When we add an acid or base to water, the equilibrium of that acid or base will directly impact the equilibrium for the ionization of water according to Le Chatelier's Principle. -Looking at the formula, H2O + H2O <-> H3O+ OH-, we can see that adding an acid will shift the equilibrium to the left. -This will use up hydroxide ions. Each hydroxide ion that reacts will also use up one hydronium ion, but remember that we just added extra hydronium ions in the form of the acid. -The net result will be more hydronium ions relative to hydroxide ions, and therefore a lower pH (Remember that the [OH-] equaled the [H+] before we added the acid). -Similarly, if we add a base to neutral water, it will also shift the reaction to the left, but this time we will be using up H3O+ ions. The net result will be more hydroxide ions relative to hydronium ions and therefore a higher pH. -Notice that the addition of either an acid or a base shifts the equilibrium for the ionization of water to the left! -Of course, we could add both an acid and a base to the same solution of water— creating three equilibriums in the same solution. In that case, the acid and base equilibriums would have competing influences on the equilibrium for the ionization of water. If the acid and base were of equal strength, there would be no net effect and the pH would remain neutral. -The equilibrium for the ionization of water is always present in aqueous solutions. -Kw, Ka and Kb are used to describe these three equilibriums. -At 25°C, Kw = Ka*Kb; this should make sense because 1) we demonstrated above that this is mathematically true, and 2) if we always remain at 25°C the Kw for the ionization of water should never change—per our rule that temperature is the only thing that changes Keq.

Pressure-Volume (PV) Work:

Work is energy transfer via a force (physics), or via a change in volume at constant pressure (chemistry). -PV Work = P∆V (requires constant pressure, any change in volume tells you there is pv work) -On a pressure vs. volume graph, the area under the curve is pvwork.

Temperature

a measure of the average amount of kinetic energy possessed by the particles in a sample of matter. *VERY IMPORTANT DEFINITION* -The more the particles vibrate, translate and rotate, the greater the temperature of the object.

Black Body Radiator

a theoretically perfect body that absorbs all energy incident upon it (or produced within it) and then emits 100% of this energy as electromagnetic radiation.

For which of the following titrations will the[OH-]=[H+]at the equivalence point?For which titrations will [titrant] = [analyte] at the equivalence point? a) SA with SB, b) WB with SA, c) WA with SB, d) WA with WB.

a) At the equivalence point of the titration of a strong acid with a strong base the [OH-] will equal the [H+] AND the [analyte] will equal the [titrant] in the flask; b) At the equivalence point of the titration of a strong acid with a weak base the [OH-] does NOT equal the [H+], but the [analyte] will equal the [titrant]; c) At the equivalence point of a titration between a weak acid and a strong base the [OH-] will NOT equal [H+], but the [analyte] will equal the [titrant]; d) At the equivalence point of a titration between a weak acid and a weak base the [OH-] will NOT equal [H+], but the [analyte] will equal the [titrant] (remember WA/WB titrations are rarely attempted or useful). The pattern is that the hydroxide and hydrogen ions will be equal at the equivalence point for any "strong/strong" titration, but NOT for any other titrations. The concentration of the analyte will equal the concentration of the titrant at the equivalence point for all titrations.

Hydrolysis of which of the following salts in solution will increase the pH of the solution? a) NaNO2, b) NH4Cl, c) NaF, d) NaClO2, e) CH3COONa, f) NaCl.

a) Hydrolysis of NaNO2 will result in the reaction of a nitrite ion with water to form HNO2 and hydroxide ion, INCREASING pH; b) hydrolysis of NH4Cl will result in reaction of NH4+ with water to form NH3 and H3O+, DECREASING pH; c) hydrolysis of NaF will result in fluoride ion reacting with water to form HF and OH-, INCREASING pH; d) hydrolysis of NaClO2 will result in reaction of ClO2- with water to form HClO2 and OH-, INCREASING pH; e) hydrolysis of CH3COONa will result in reaction of acetate with water to form CH3COOH and OH-, INCREASING pH; f) hydrolysis of NaCl will result in Cl- reacting with water to form HCl and OH-, INCREASING pH. In summary, all of the options will increase pH except for option b).

Closed System

closed system is a system that can exchange energy with its surroundings but not mass.

Model of Matter

consisting of particles which vibrate (wiggle about a fixed position), translate (move from one location to another) and even rotate (revolve about an imaginary axis).

Specific Heat Capacity

describes energy absorption *for one individual substance only* and is defined per unit mass. Little "c" is used instead of big "C". -This "system" heat capacity may refer to a solution, the container holding the solution, and even a thermometer or stirring rod c = q/m∆T ; often re-written as: q = mc∆T

Flow of Heat

heat as a flow of energy from a higher temperature object to a lower temperature object. -It is the temperature difference between the two neighboring objects that causes this heat transfer. The heat transfer continues until the two objects have reached thermal equilibrium and are at the same temperature. -metal can containing hot water is placed within a Styrofoam cup containing cold water. Heat is transferred from the hot water to the cold water until both samples have the same temperature.

Forced Convection

involves fluid being forced from one location to another by fans, pumps and other devices. -Many home heating systems involve force air heating. Air is heated at a furnace and blown by fans through ductwork and released into rooms at vent locations. This is an example of forced convection. The movement of the fluid from the hot location (near the furnace) to the cool location (the rooms throughout the house) is driven or forced by a fan.

Isolated System

isolated system is a system for which neither mass nor energy can be exchanged with the surroundings.

Each of the above ΔH items has an analogous definition for changes in entropy (∆S) and Gibbs free energy (∆G) (i.e., ∆Gformation, ∆Gvaporization, etc. Because the basic science principles are identical, but these other quantities aren't seen as frequently, they could very likely appear on the MCAT.

know the meaning of each term!!!!!!!

buffering region

look at aka region is + or - 1 from the pea

Half-EquivalencePoint

mid point of the nearly horizontal section of the graph. *Here the pH = pKa.* -We can also say that [HA] = [A-] at the half-equivalence point.

Work done by a system is

negative

Negative ∆S

negative∆S=decreased randomness, and thus less energy available to do work

When the salts of weak acids or weak bases dissolve in water

one of the ions will undergo hydrolysis to re-form the weak acid or the weak base:

Three Important pH Equations:

pH = -log[H+] pOH = -log[OH-] pH+pOH=14

pH for SA w/ SB

pH = roughly 7

pH for WA w/ WB

pH = roughly 7

pH for WB w/ SA

pH<7

pH for WA w/ SB

pH>7

vibrational kinetic energy

particles can also vibrate about a fixed position -the main form of kinetic energy for solids

Work done on a system is

positive- added to the system

Positive ∆S

positive∆S= increased randomness, and thus more energy available to do work.

Amphoteric

substances can act as either an acid or a base (e.g., H2O)

One "equivalent"

the amount of acid or base necessary to produce or consume one mole of [H+] ions amount necessary to deprotonate .

Conduction through metal

thermal conductivity in metals occurs by the movement of free electrons. -Outer shell electrons of metal atoms are shared among atoms and are free to move throughout the bulk of the metal. -These electrons carry the energy from the skillet to the skillet handle. -heat transfer through metals occurs without any movement of atoms from the skillet to the skillet handle.

ΔHformation

ΔHformation: The enthalpy value for the formation of a compound from its elements in their standard states. -If the number is negative, formation is an exothermic process, if it is positive, the process is endothermic.

Gibbs Free Energy (∆G)

∆G = the amount of "free" or "useful" energy available to do work (excluding pv work) as a result of running an isothermal, isobaric reaction (constant temperature and pressure)

The Fundamental Thermodynamic Relation:

∆G=∆H-T∆S Recall that enthalpy is basically the change in bond energy from reactants to products. - If there were no change in randomness during the reaction, the amount of energy available to do work (ΔG) would be exactly equal to enthalpy (∆H). -As described above, if randomness increases (positive ΔS), energy will be released and that energy (in addition to ΔH) will also be available to do work (creating a larger, more negative ΔG). -By contrast, if randomness decreases, energy will be "used" to create this order, decreasing the amount of energy available to do work. -The "T" term in the equation converts entropy into joules (J/K*K = J). -You may recall that energy can also be used to increase temperature or to expand the volume (PV work), but neither occurs here because the system is both isothermal and isobaric. *GIBS FREE ENERGY DEPENDS ON ENTHALPY AND ENTROPY(x temperature)*

Equation that relates the equilibrium constant to the Gibbs free energy

∆G° = - RTln(Keq) (Note: Remember that the ln of any positive number less than 1 is negative) A useful rearrangement of the above equation: Keq = e-ΔG/RT -If the value of K is greater than one then the reaction will be spontaneous. -To be more precise, if we are starting at the standard state conditions, then we know the reaction will proceed spontaneously. - We know this because the natural log of a number greater than one is positive, which will result in a negative Gibbs Free Energy per the equation given. -If the value of K is less than one then the reaction will not proceed spontaneously from standard state conditions. -We know this because the natural log of a number less than one is negative. -This negative will counteract the existing negative in front of the equation and therefore the sign of Gibbs Free Energy will be positive. If K = 1, then ∆G = 0 because the natural log of one is zero. Again, a key principle in this section is that kinetics is "walled off" from thermodynamics: the value of K will not tell us anything at all about the rate of the reaction.