Chemistry Section 2 (Altius MCAT Prep)

Henderson-Hasselbach equation Use the Henderson-Hasselbach equation to demonstrate that pH = pKa at the half equivalence point.

- At the half-equivalence point the concentrations of HA and A- are equal. Therefore, the ratio of [A-]/[HA] must be one. When we plug this into the H-H equation we get: pH = pKa + log(1). The log of one is zero, so this term falls out, demonstrating that pH = pKa at the half-equivalence point.

What happens when salts of weak acids/bases are placed in water? NH3 = ? NH4+ = ? NH4NO3 = ? When the salts of weak acids or weak bases dissolve in water one of the ions will undergo?

- Dissolve, one of the ions will undergo hydrolysis to reform the weak acid or base - "weak base" - "conjugate acid" - "salt of a weak base" - hydrolysis to re-form the weak acid or the weak base

Titration definition 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 base is "titrated with" ? The analyte is? The titrant is?

- Drop by drop mixing of an acid and a base with an indicator - The terminology used in the question infers that the strong base is in the beaker, which makes it the analyte. - the strong acid, meaning the acid is being added dropwise and is therefore the "titrant." - whats in the beaker. whats being added dropwise

List the 8 MCAT strong acids: 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." Is HF a strong acid? Explain why HF is NOT a strong acid and yet the closely related HCl is a strong acid?

- HI, HBr, HCl, HNO3, HCLO4, HCLO3, H2SO4, H3O+ - The reason HF is not a strong acid and HCl is a strong acid is a matter of structure. Looking at the conjugate bases, F- is far less stable than is Cl- due to its smaller size. When a smaller molecule has to bear a full formal charge it experiences a greater charge density and therefore more instability. A larger atom such as chlorine can spread out this charge over a greater area.

Weak Acids and Weak Bases, do they disassociate completely? A general rule is that acids with a pKa > 0 or Ka < 1 and bases with pKb >0 or Kb less than one are considered? A general rule is that a base with a pKb greater than zero, or a Kb less than one, can be considered ? Anything not on the strong list is considered? Examples of Weak Acids: Examples of Weak Bases:

- No - weak acid -weak base - WEAK - H2O, H2S, NH4+, HF, HCN, H2CO3, H3PO4, acetic acid, benzoic acid. - H2O, NH3, R3N, pyridine, Mg(OH)2

Titrant and anylate How many equivalents of base can be neutralized by one equivalent of H2SO4?

- Titrant is the known concentration of acid/base that is added to the anylate which is the substance being investigated. - Two equivalents of base can be neutralized by one equivalent of H2SO4 because each sulfuric acid produces two equivalents of hydrogen ions in solution.

All equilibrium constants (Keq, Ka, Kb. Kw or Ksp) are written via? You should think of Ka or Kb just as you do Keq. A large Ka (or a small pKa) indicates? A large Kb (or a small pKb) indicates?

- Via the law of mass action with pure liquids and solids omitted. - that at equilibrium there are far more products than reactants. For an acid dissociation, this would mean a lot of dissociation ( a lot of H+ formed) and thus a very strong acid. - very strong base (i.e., a lot of OHformed—either from dissociation of a hydroxide base (i.e., NaOH) or from deprotonating water).

Multiply the base disassociation equation times the Acid Disassociation equation and what do you get? Acid A has a Kb of 1.0 x 10-9 and 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?

- [H+] [OH-] =Kw - 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.

A buffer solution contains?

- a weak acid and 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.

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.

Impact of salts on the dissolution of Weak Acids and Bases. The percent disassociation of benzoic acid (weak acid) decreases in what solution? The percent disassociation of ammonium hydroxide (weak base) decreases in what solution? Provide a potential explanation for the two observations stated above

- in a sodium benzoate solution. - 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.

Summary page The ionization of water is often called the "autoionization of water" precisely because? When we add an acid or base to water, the equilibrium of that acid or base will? Notice that the addition of either an acid or a base shifts the equilibrium for the ionization of water to the?

- it happens automatically in all water - directly impact the equilibrium for the ionization of water according to Le Chatelier's Principle - left

pH of various titrations WB w/ SA : pH ? WA w/ SB : pH ? SA w/ SB : pH ?

- pH < 7 - pH > 7 - pH =7

The nearly horizontal area surrounding the half-equivalence point is called? Adding a relatively large amount of titrant at this point in the titration will have little or a lot effect on pH?

- the "buffer region." - little. 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).

Hydrolysis of a salt terminology. This terminology can be a bit tricky: The "salt of a weak acid" refers to? HCO3- = ? CO32- = ? Na2 CO3 = ? Similarly, the "salt of a weak base" refers to?

- the conjugate base of that weak acid combined with a cation to form a salt. - weak acid - conjugate base - salt of a weak acid - the conjugate acid of that weak base combined with an anion to form a salt (example: NH3 = "weak base"; NH4+ = "conjugate acid"; NH4NO3 = "salt of a weak base")

Indicators are ? To set up a titration, you must know beforehand? The dissociation of the indicator and the acid/base reaction we are analyzing run simultaneously in the same beaker but are ? The amount of indicator is so small compared to titrant and analyte that we can assume ?

- weak acids that change color as they dissociate from HA into H+ and A- - the approximate pH of your equivalence point; you then select an indicator that will change color at that approximate pH - otherwise unrelated. - it has no impact on pH

To review, the equivalence point is? The End point is? Three common indicators used in the lab are methyl orange, litmus, and phenylphthalein. These indicators change color at a pH of 3.7, 6.5, and 9.3, respectively. Which indicator should be chosen to identify the equivalence point for the titration of a weak acid with a strong base?

- where [titrant] = [analyte] - where the indicator causes the color to change. There is no causal relationship between the equivalence point and the end point. - An indicator should be chosen that will change color at a pH as close to the equivalence point as possible. A titration of a weak acid with a strong base will have an equivalence point greater than seven on the pH scale. The first two indicators would change color before the equivalence point was reached, so phenylphthalein would be the appropriate choice.

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 NaSO2 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. In summary, all of the options will increase pH except for option b)

How do you increase Entropy (ΔS)?

1) Increased number of items/particles/etc. (caveat: the number of moles of gas trumps the number of moles, or particles. Thus even if 2 moles of reactants turns into 1 mole of product, if that one mole of product is a gas and the reactants are not, entropy has increased and ΔS will therefore be positive) 2) Increased volume 3) Increased temperature 4) Increased disorder (a crystal is changed to an amorphous material) or complexity (S is greater for C2H6) than for CH4)

7 Buffer problem clues

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 ofeach 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. 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 halfequivalence 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.

Calculate pH for Weak Acid, 6 steps

1) Write out the equilibrium reaction (HA-> H+ + A-) 2) Use x to represent concentrations 3) use [HA] - x for original acid 4) If a quadratic formula results, assume x is much smaller and omit 5)Solve for X using Ka=(x)(x) / [HA-x] 6) Use -log[H+] to find pH

1.) Half equivalence point= 2.) HA will continue to be deprotonated until at the equivalence point the solution contains? 3.)This is not true of a titration involving a strong acid and strong base because they both dissociate 100%. Therefore, by definition, almost immediately after adding HA 100% has been changed to A-. Therefore, SA/SB titrations?

1.) midpoint of the nearly horizontal section of the graph. 2.) pH=pKa or [HA] = [A-] BUT THEN at the equivalence point there is 100% A- and 0% HA 3.) SA/SB titrations do NOT have half equivalence points - contains 100% A and 0% HA

Titration of a WA w/ SB or WB w/ SA 1. Equivalence Point = ? 2. Because the acid and base are not both strong (i.e., dissociate 100% in water) the [H+] ? 3. This also tells us that the pH?

1.) midpoint of the nearly vertical section [titrant] = [analyte] 2.) [H+] DOES NOT EQUAL [OH-] 3.) DOES NOT EQUAL 7 - This is why it is very important that you clearly master these two unique types of titrations and keep their behavior clearly separated in your mind. An MCAT question will simply ask: "What is the pH of the solution at the equivalence point?" or an answer choice will say "The concentration of hydroxide ions equals the concentration of hydrogen ions at point B." You will need to remember that the first thing you must do on such a question is decide which type of titration is being performed.

Conversion between pH and pOH pH + pOH = ?

14

What is a black body radiator?

A black body is a theoretical object that absorbs 100% of the radiation that hits it. Therefore it reflects no radiation and appears perfectly black. "Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. The radiated energy can be considered to be produced by standing wave or resonant modes of the cavity which is radiating. The amount of radiation emitted in a given frequency range should be proportional to the number of modes in that range. The best of classical physics suggested that all modes had an equal chance of being produced, and that the number of modes went up proportional to the square of the frequency. But the predicted continual increase in radiated energy with frequency (dubbed the "ultraviolet catastrophe") did not happen. Nature knew better.

Which of the calorimeters described above allows for pv work?

A coffee cup calorimeter allows for pv work because pressure is constant.

pH Scale

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.

Standard State

A set of specific conditions chosen as the reference point for measuring and reporting enthalpy, entropy, and Gibbs free energy. DON'T CONFUSE STANDARD STATE WITH STP.

How does the hydrogen ion concentration differ for two solutions, one with a pH = 2 and the other with pH = 4? The solution with a ph of 2 has a factor of how many more H+ ions?

A solution with a pH of 2 has [H+] = 10^-2, while a solution of pH 4 has [H+] = 10^-4, thus the pH 2 solution has 10^2 or 100 times the [H+] of a solution with a pH of 4

Specific Heat Capacity

Above we defined Heat Capacity as the energy absorbed/unit change in temperature for a system. This "system" could include a solution, the container holding the solution, and even a thermometer or stirring rod. Specific Heat Capacity, however, describes energy absorption for one individual susbtance only and is defined per unite mass. Little "c" is used instead of big "C". Formula : c= q/m (change in) T Specific Heat of Water = 1.0 cal/ g (degree C) or 4.18 J/g (degree C)

Lewis acid-base definition

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.

Bronsted-Lowry acid-base definition

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

Arrhenius acid-base definition

Acids produce H+ ions in solution; bases produce OH- ions in solution.

Strong acids/ strong bases

All strong acids and strong bases dissociate 100% in water (making them good electrolytes).

When are reactions at their maximum entropy?

At equilibrium

How are the signs of Work defined in chemistry?

By convention, work done on the system is positive, work done by the system is negative (chemistry definition).

Which type of calorimeter provides constant pressure and which type provides constant volume?

Coffee cup calorimeter proceeds at constant pressure Bomb calorimeter proceeds at constant volume

Entropy (ΔS)

Definition: THINK OF ENTROPY AS: Entropy = a measure of the randomness or disorder in a system. Units: Joules/K 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. + ΔS = increased randomness, and thus MORE energy to do work. - ΔS = decreased randomness, and thus LESS energy to do work. Reactions at equilibrium are at maximum entropy. Entropy increases with increasing number, temperature, volume, moles of gas, etc. AND decreases with a decrease in any of these. Pressure is the weird one in this set, since pressure can be increased by decreasing volume with a constant amount of gas phase molecules, which would decrease entropy. Pressure can also be increased by adding more molecules to a constant volume at constant temperature, which would increase entropy, as noted above. Remember that any kind of increase in order or organization will be a decrease in entropy and vice versa.

Calorimeters

Device used to calculate enthalpy change (change enthalpy). We are assuming that q (which is what the calorimeter actually measures) is equal to enthalpy change. This is true at constant pressure. Know how each type of calorimeter works: Coffee Cup Calorimeter: Solve using: q=C ΔT. Bomb Calorimeter: Solve using: q=C (change in T). This does NOT give enthalpy, but change in internal energy, (Δ U) or (Δ E). Use heat capacity (big C) instead of specific heat capacity (little c).

Radiation

Electromagnetic waves emitted from a hot body into the surrounding environment. Which is the flow of energy from an object that is hotter than its environment into the environment via electromagnetic radiation. Light colors radiate and absorb less. Dark colors radiate and absorb more.

The first law of Thermodynamics

Energy cannot be created or destroyed. ΔE= q + W 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.

Enthalpy

Enthalpy = the energy contained within chemical bonds. This is a simplified definition, but one that works perfectly for the purpose of doing the MCAT. It is illustrated by the fact that we can calculate ΔH for a reaction (change in enthalpy) by finding the difference in total bond energy between the products and the reactants. Units: Joules

A pan of water is placed upon an electric heating element on a stove. Describe all types of heat exchange 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 was hotter than the element (say the element wasn't even turned on yet), then heat flow would 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.)

Convection

Fluid movement caused by the hotter portions of a fluid rising and the cooler portions of a fluid sinking. Think of hot and cold air naturally flowing and creating currents in a fluid—such as air or ocean currents. Air currents and convection currents in water are examples.

List the 8 strong bases for the MCAT

Group IA hydroxides (LiOH, NaOH, KOH, RbOH) NH2-, H-, Ca(OH)2, Sr(OH)2, Ba(OH)2, Na2O, CaO *Disassociate 100%

Ionization of water: H2O + H2O --> H3O+ + OH- H3O+ is the same as? Kw=[H3O+] [OH-] = ? pKw=pH + pOH = ? pKa + pKb = ? Show how the equation above for pKw is derived from the equation for Kw.

H3O+ is the same as? = as H+ Kw=[H3O+] [OH-] = 10^-14 pKw=pH + pOH = 14 pKa + pKb = 14 Starting with Kw = [H3O+][OH-] = 10^-14, we take the negative log of all terms, yielding: -logKw = -log[H3O+] + -log][OH-] = -log(10-14). The middle two terms come from the log rule that states: logAB = logA + logB. The first term can be replaced with pKa by definition, the second term with pH by definition, the third term with pOH by definition, and the fourth term by 14 because 14 is the -log of 10^-14. This leaves: pKw = pH + pOH = 14

The Second law of Thermodynamics

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

If the value of Keq is known, what can we infer about ΔG?

If Keq > 1 ΔG is negative the reaction is spontaneous; If Keq < 1 ΔG is positive and the reaction is nonspontaneous

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. 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. It is much like asking how many gallons of water can be added to Tank A vs. Tank B? Tank A and Tank B are both 5-gallon tanks, but Tank B is connected via a hose to a reserve tank that holds 2 gallons. So, you can add 5 gallons to Tank A before it is "full" (analogous to a one unit increase in temperature). However, you can add 7 gallons to Tank B before it is full (the reserve tank being analogous to pv work). We would therefore say that Tank B has the higher "water capacity" in terms of our analogy. This indicates that the constant pressure heat capacity (allows for pv work; i.e., includes the 2-gallon reserve tank) will always be more than the constant volume heat capacity (does not allow for pv work; i.e., no reserve tank) for the same system.

Provide a conceptual definition for the terms isobaric and isothermal

Isobaric: constant pressure (an isobaric process is a change in a system in which the pressure remains constant) Isothermal: constant temperature (an isothermal process is a change in a system in which the temperature remains constant

Kinetic Energy

KE = 3/2 Kb T Where Kb is Boltzmann's constant; this shows the direct relationship between temperature and kinetic energy.

Acid Disassociation Ka= ? Because the acid almost fully dissociates, what do we know about the ratio of products over reactants? In general terms, an acid with Ka greater than one or a pKa less than zero is considered?

Ka= [H+] [A-] / HA - the ratio of products over reactants would have to be greater than one. - "strong," so this acid would clearly qualify as a strong acid.

Base Disassociation Kb= ? Ka*Kb = Kw = 10^-14 (at 25°C); because? T/F An aqueous solution with a pH of 8 is basic and therefore by definition it does not contain any unreacted H+ ions.

Kb= [OH-][HA] / [A-] - because ([H+][A-]/[HA])*([OH-][HA]/[A-]) = [H+][OH] = Kw - 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.

Conduction

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. Which is natural flow from hot to cold due to two things being in contact Heat conduction is roughly analogous to current flow through a wire.

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, but an increase in temperature is NOT the only "place" where added energy can go.)

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). So, when heat enters a system, if the system is capable of volume change, heat can go to either 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.

If ΔH is positive and entropy is negative, what will ΔG be?

Positive (nonspontaneous)

The Third law of Thermodynamics

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

If a reactant is dissolved in solution, causing the temperature of the reaction vessel to increase, the ΔG for this reaction must be a) positive, b) negative, or c) cannot be determined.

Reactants are dissolved in solution, meaning bonds are being broken and energy is released so the reaction is exothermic or ΔH is negative. Temperature increases which increases the disorder of the system, so ΔS is positive. With a negative enthalpy and a positive entropy, you have a negative free energy (-ΔG) meaning the reaction is spontaneous.

Bomb Calorimeter

Solve using: q=CΔT. This does NOT give enthalpy, but change in internal energy, ΔU or ΔE. Use heat capacity (big C) instead of specific heat capacity (little c). • A bomb calorimeter is used to measure heat flows for gases and high temperature reactions. • A bomb calorimeter works in the same manner as a coffee cup calorimeter, with one big difference. In a coffee cup calorimeter, the reaction takes place in the water. 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: • The pressure within a bomb calorimeter often changes during a reaction, so the heat flow may not be equal in magnitude to the enthalpy change.

Coffee Cup Calorimeter

Solve using: q=mcΔT • 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. • Recall that for an exothermic reaction, ΔH < 0; q water is positive. The water absorbs heat from the reaction and an increase in temperature is seen. For an endothermic reaction, ΔH > 0; q water is negative. The water supplies heat for the reaction and a decrease in temperature is seen.

If a reaction is spontaneous, what can we infer about the rate of that reaction?

Spontaneity does not mean greater speed of the reaction. For example the process of graphite decaying into diamonds is a spontaneous reaction, but the reaction is extremely slow. Rate is independent of spontaneity.

If entropy is positive and enthalpy is negative, the reaction is: a) spontaneous, b) nonspontaneous, c) can be either spontaneous or non-spontaneous depending on temperature.

Spontaneous because a negative enthalpy and positive entropy gives a negative free energy (ΔG) meaning the system has more energy to do work. Free energy only depends on temperature if the signs of enthalpy and entropy are the same.

What is STP?

Standard temperature and pressure: T=273 K or 0 C P= 1atm

Provide a conceptual definition for temperature.

Temperature is the change in a molecule's kinetic energy. Temperature is the property of matter which reflects the quantity of energy of motion of the component particles. Kinetic energy of a molcule is the energy a molecule possesses due to its motion

Acid/Conjugate Acid and Base/Conjugate Base

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

"One equivalent" definition

The amount of acid or base necessary to produce or consume one mole of H+ ions.

Heat Capacity

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

ΔH solution

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

ΔH vaporization

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.

ΔH fusion

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

ΔH combustion

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 LOW HEAT OF COMBUSTION with a STABLE molecule. Either bond energy or heats of formation can tell us relative stability. A high positive bond energy tells us something is very stable, while a large negative heat of formation tells us the amount of energy released when the compound is formed is high and thus the compound is very stable.

ΔH formation

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.

Strong acid titrated with strong base

The first part of this graph will be the same as the first part for a weak acid titrated with a strong base.

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. An intensive property is a bulk property, meaning that it is a physical property of a system that does not depend on the system size or the amount of material in the system.

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 reactions for which you are calculating ΔHrxn. To calculate ΔHrxn 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 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.

How do you convert Celsius to Kelvin?

To convert from Celsius to Kelvin: K= T ˚C +273; so zero degrees Celsius is the same as 273 K. Know how to convert Celsius to Kelvin; always use Kelvin in formulas, unless Celsius is specified.

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 ΔV (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 pv work.

How is the sign of work defined in physics?

Work is positive when it is done in the same direction as the velocity Work is negative when it is done in the opposite direction of the velocity

Calculating ΔHrxn using Bond Energies

You can also calculate the enthalpy change of a reaction using bond energies. To do so, simply add up the bond energies of all 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.

Titration of SA w/ SB or SB w/ SA 1. Equivalence Point/Stoichiometric Point = ? 2. For a solution of NaOH being titrated with HCl, at the equivalence point [HCl] = ? 3.For titrations involving a SA and a SB, [H+] = [OH-] at ? 4.By definition, if [H+] and [OH-] are exactly equal, pH = ?

[titrant] = [analyte] [H+] =[OH-] pH=7 1.) midpoint of the nearly vertical section of the graph. 2.) [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. 3.) the equivalence point 4.) 7 at the equivalence point.

What is the sign for the ln of any number less than one?

negative

Calculate pH for Strong Acid/Base What must we assume to make this calculation?

pH or pOH = -log [strong acid/base] We calculate pH and pOH by taking the negative log of the concentration of H+ ions and OH- ions respectively. Therefore, 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. This seems like a very likely MCAT topic because they tend to ask many questions that require a comparison between how we assume things to be for calculation purposes and what they are actually like in reality (real vs. ideal fluids, real vs. ideal gases, ignoring air resistance, etc.).

pH equation pH of pure H2O at 25°C = ? pH > 7 = ? pH < 7 = ?

pH=-log [H+] pH of pure H2O at 25°C = 7 and the [H+] = [ OH-] , pH = 7 is defined as "neutral" pH > 7 = Basic and the [H+] < [ OH-] pH < 7 = Acidic and the [H+] > [ OH-]

pOH equation

pOH= -log[OH-]

Amphoteric

substances can act as either an acid or a base (ex. H2O)

Elements in their Standard State have an ΔH ˚ of?

zero This is because elements in their standard state are not formed, or created and thus there is no change in bond energy or association flow of energy. The enthalpy change for graphite (solid carbon in its standard state) for example, is zero. However, the enthalpy change for diamond is 2kJ/mol because energy is required to form diamond out of graphite.

ΔG = -RTlnKeq

ΔG = -RTln(Keq) Relates the equilibrium constant to the Gibbs free energy. (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) Keq = 1, ΔG=0 and no ATP is required or produced. They hydrolysis of ATP generates approximately -33 kJ/mol under biological conditions. This should be considered anytime ATP is a product or reactant in a biological reaction.

The Fundamental Thermodynamic Relation

ΔG = ΔH - T ΔS 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 (ΔH). 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.

Gibbs Free Energy (ΔG)

ΔG= ΔH - TΔS Definition: THINK OF GIBBS FREE ENERGY AS: ΔG = the amount of "free" or "useful" energy available to do work (excluding pv work; as a result of running an isothermal, isobaric reaction). If energy is available, Gibbs Free Energy is said to be negative. If energy must be added to the reaction (i.e., work must be done on a system) to make it proceed, Gibbs Free Energy is said to be positive. Units: Joules - ΔG = Spontaneous; free energy available to do work + ΔG = Nonspontaneous; no free energy available; energy is required

Name the 5 types of ΔH

ΔH reaction ΔH formation ΔH combustion ΔH vaporization ΔH fusion