Chemistry 2

If the value of K(eq) is known, what can we infer about ∆G?

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Three kinds of Heat Exchange:

1) Convection: fluid movement caused by the hotter portions of a fluid rising and the cooler portions of a fluid sinking. 2) Radiation: electromagnetic waves emitted from a hot body into the surrounding environment. Light colors radiate and absorb less Dark colors radiate and absorb more Black Body Radiator = perfect theoretical radiator 3) Conduction: molecular collisions along a conduit Heat conduction is approximately equivalent to current flow thru a wire or water flow thru a pipe.

Entropy Increases with:

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 is positive) 2) increased volume 3) increased temperature 4) increased disorder 5) decreased pressure (increased pressure increases order by packing the same molecules into a smaller space, thus decreasing entropy)

The lab instructor asks the student to estimate the expected rate for Trial 4. Compared to the rate of Trial 3, Trial 4 should be approximately: Rate=k [NO]²[H2] Trial [NO] [H2] (mol/L*s) 3 0.001 0.001 1.95 x 10-4 4 0.008 0.016 ? A) 64 times faster B) 128 times faster C) 256 times faster D) 1024 times faster

1024 times faster

0°C = what in Keliv

273.16 K

A 0.001kg mass increases in temperature from 250K to 275K when 1000J of heat is applied. Which of the following gives the specific heat capacity? A. 4 x 104 J/kg*K B. 40 J/kg*K C. 250 J/kg*K D. 1000 J/kg*K

A; In case the units aren't given, it is good to know that specific heat is measured in either J/kg*K or in cal/g*˚C. However, since you know the units here from the answer choices, this should be easy. Just solve q = mc∆T for c and fill in the info given. You should get c = q/m∆T. Plugging and chugging we get: 1x103J/(25K*1x10-3kg), which equals 4 x 104, or answer A.

what gas has the greatest velocity and kinetic energy if the temp is 314 K? CO CO₂ SO₃

All molecules have the same KE and the smallest molecule has the greatest velocity

Which of the following gives the enthalpy of reaction (∆Hrxn), for the reaction shown below: H2(g) + F2(g) 2HF(ℓ) BOND ENERGIES Species Bond Energy (kJ/mol) H2(g) 400 F2(g) 150 HF(g) 600 A. +650kJ/mol B. +1750kJ/mol C. -50kJ/mol D. -650kJ/mol

Always pay attention to moles and multiply whatever number you use by the number of moles of the species involved in the overall reaction. In this case, you are breaking apart hydrogen and fluorine gases, so use the positive value for their bond energies; and you are forming 2 moles of HF, so use the value given in the heats of formation table for HF, but double it. Add these all together and you should get -650kJ/mol.

The hydration of ammonium nitrate is a highly exothermic dissolution reaction. Which of the following statements is NOT true of this process? A) The reaction must be spontaneous because it is both exothermic and has a favorable entropy change. B) The reaction could be spontaneous or non-spontaneous depending on the temperature at which the reaction is run. C) The products of the reaction have greater entropy than do the reactants. D) The total bond energy of all the products exceeds the total bond energy of all the reactants.

Answer A states in words what is demonstrated by the fundamental thermodynamic relationship: ∆G = ∆H - T∆S. Namely, if both the enthalpy term and the T∆S term are negative, the reaction must be spontaneous. Answer C states the conditions for having a positive entropy change, which we know this reaction does have because it is a dissolution reaction. Answer D states the conditions for being exothermic, which this reaction is, as given in the stem. Answer B WOULD be true if the signs of enthalpy change and entropy change were both negative, or both positive, but they are not in this case. We are looking for a statement that is NOT true, so B is the correct answer.

A student is using a coffee cup calorimeter to determine the enthalpy of a reaction. When assembling the apparatus, one of the coffee cups is accidentally torn along one side. The student checks to make sure the reactants aren't leaking out and decides to continue with the experiment. The value he calculates for the heat of reaction: A. will be higher than the true value B. will be lower than the true value C. will be unaffected because he is using two coffee cups D. minus the heat that escapes thru the crack, will equal the true value

B; The crack would reduce the insulating efficiency of the calorimeter and we would assume that some heat would be lost. The student will measure the change in temperature and use that value to calculate the energy released by the reaction. However, the reaction will have actually produced MORE heat that the heat required to raise the temperature by the amount shown because some of the heat escaped. Thus, his value will be lower than the actual value, Answer B. Answer D is false because the heat escaped would have to be added to this value to get the true value, not subtracted.

Which Calorimeter proceeds at constant volume?

Bomb Calorimeter

A student examines the heat of formation of MgCl2(s) using both a bomb calorimeter and a coffee-cup calorimeter. The student's results from the bomb calorimeter give a higher ∆Hrxn than his results from the coffee cup calorimeter. Which of the following provides the best explanation? A. Coffee cup calorimeters are less accurate than bomb calorimeters B. Bomb calorimeters are less accurate than coffee cup calorimeters C. Both are equally accurate, but coffee cup calorimeters allow for pv work, while bomb calorimeters do not. D. Both are equally accurate, but the reaction mixture increases in volume in a bomb calorimeter, but does not in a coffee cup calorimeter

C; Answers A and B are false because neither calorimeter is known to be particularly more accurate; they merely serve different purposes. Answer D is false because volume changes in the coffee-cup calorimeter, NOT in the bomb calorimeter. Answer C is correct because the change in volume that occurs in a coffee-cup calorimeter means that some of the energy input from the reaction went to PV work instead of raising the temperature of the vessel. This would give a lower value for the heat of reaction in the coffee cup calorimeter as compared to the bomb calorimeter where no PV work can be done.

Which of the following represent an example of heat conduction and heat radiation, respectively? I. Air flow within a convection oven and heat created by a household radiator II. A chicken sitting on her eggs to keep them warm and heat escaping from the skin after exercise III. A heating blanket wrapped around a victim just rescued from a blizzard and heat escaping from a non-insulated, current carrying wire A. I only B. II only C. II & III D. I, II & III

C; Statement I does not qualify because air flow in a convection oven is a clear example of convection, as the name implies. When you think of convection, think of hot and cold air naturally flowing and creating currents in a fluid—such as air or ocean currents. Statements II and III Both describe an example of conduction first—which is natural flow from hot to cold due to two things being in contact—and then radiation—which is the flow of energy from an object that is hotter than its environment into the environment via electromagnetic radiation.

What causes reactions?

COLLISIONS CAUSE REACTIONS 1)reactants must collide with enough energy to overcome the Energy of Activation 2)The reactants must be in the correct spatial arrangement

Enthalpy of Reaction (∆H_reaction)

Calculated most often by adding half-reactions from a table. If the half-reaction proceeds in the same direction, use the number directly. If it proceeds in the opposite direction, change the sign. Remember to multiply the number given in the table by the coefficient in the balanced equation. It can also be calculated by adding the bond energies. If the bond is formed, the bond energy is negative; if it is broken, the bond energy is positive.

What is Rate measured in?

Change in Molarity of the reactants per second (M/s)

Which Calorimeter proceeds at constant pressure?

Coffee cup calorimeter

Which of the Calorimeters allows for PV work?

Coffee cup calorimeter

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

Constant pressure (because more energy will be needed to increase the temp of the substance if it can expand)

For a reaction in which A and B combine to form C, and B is known to be in excess, a non-linear plot of ln[A}versus time indicates which of the following? A. the reaction is first order B. the reaction may be first order C. the reaction is not first order with respect to A, but may be first order with respect to B D. the reaction is not first order

D; Recall that these rate order graphs ONLY work under one of two conditions ("work" meaning any of them will give you a linear line): 1) the reaction has only one reactant, or 2) there are two or more reactants but only ONE of them is in the rate law. So, a non-linear graph means that either the reaction does have only one species in the rate law but it is NOT the order with respect to that species associated with that graph, or it could mean that both A and B are in the rate law—in which case none of the graphs would give a linear line. These facts, applied to this problem, show that Answer A is clearly false. Under any circumstance, if the reaction were first order this graph would give a linear line, BECAUSE B is in excess and it therefore must be zero order. However, a graph of 1/[A] could also give a linear line if the reaction is second order with respect to A. Answer B is false for the same reason; this is NOT first order. Answer C is false because B is in excess. Answer D is all that we really know for sure.

Entropy (∆S)

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. + ∆S = increased randomness, and thus MORE energy available to do work. - ∆S = decreased randomness, and thus LESS energy available to do work. Reactions at equilibrium are at maximum entropy.

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

Gibb's free energy is negative, we also know that K must be greater than one from the equation: ∆G = -RTlnK

The Second Law of Thermodynamics

Heat cannot be changed completely into work in a cyclic process; And entropy in an isolated system can never decrease Basically, perpetual motion is impossible

Heterogeneous catalysts

Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture

Homogeneous catalysts

Homogeneous catalysts function in the same phase as the reactants. Typically homogeneous catalysts are dissolved in a solvent with the substrates.

Pressure-Volume (PV) work

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 the PV work

If ∆H is positive and entropy is negative, what will ∆G be?

Positive

The Third Law of Thermodynamics

Pure substances at absolute zero have an entropy of zero

For a reaction with two reactants, A and B, a graph of 1/[A] vs. time is non-linear. Which of the following is known? 1)Reactant A cannot be second order 2)Reactant B must be involved in the rate law 3)Reactant B cannot be in excess

Reactant A cannot be second order

For a reaction with two reactants, A and B, a graph of ln[A] vs. time is linear. Which of the following is known? 1)Reactant A must be first order 2)Reactant B could be first order 3)Reactant B cannot be involved in the rate law 4)Reactant B must be in excess

Reactant A must be first order Reactant B could be first order

A student submits the following partial data table for a lab project in which students analyzed the reaction of nitric oxide with hydrogen gas. It is known that the reaction is second order with respect to nitric oxide. 2 NO(g) + 2 H2(g) N2(g) + 2 H2O(g) Trial [NO] [H2] (mol/L*s) 1 0.004 0.004 1.25 x 10-2 2 0.002 0.004 7.8 x 10-4 3 0.001 0.001 1.95 x 10-4 4 0.008 0.016 ? • 1) The order of the above reaction with respect to hydrogen gas is: A) zero order B) first order C) second order D) cannot be determined without additional information

Solution: There are no trials across which [H2] changes, while [NO] remains constant. Normally, this is what we look for first to predict rate order. However, we were given the information that it is 2nd order with respect to NO. If we divide the rate for Trial 1 by the rate for Trial 3 we see that the rate decreased by a factor of 64. Knowing the order of NO, we can say that when [NO] went down by a factor of 4, the rate should have gone down by a factor of 16. It actually went down by a factor of 64, or 16*4. Thus the effect of the change in the [H2] must have accounted for this additional 4-fold decrease. Using x = yz we can see that if concentration went down by 4 and rate also went down by 4, the reaction must be first order with respect to H2. It will help to recall form the manipulating equations section that if two variables are changed by factors x and y, respectively, the overall change to the equation is equal to x times y. B is the correct answer.

Coffee Cup Calorimeter

Solve using: q=mc(deltaT)

Bomb Calorimeter

Solve using: q = C∆T. This does NOT give enthalpy, but change in internal energy, usually called ∆U or ∆E. Use heat capacity (big C) instead of specific heat capacity (little c).

What is the Specific Heat of Water

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

If 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.

Spontaneous

Enthalpy (∆H)

THINK OF ENTHALPY AS: Enthalpy = the energy contained within the bonds. Units: Joules Standard State: a complicated set of circumstances chosen as the reference point for measuring enthalpy, entropy and Gibbs Energy. DON'T confuse with STP. Elements in their standard state have delta H° = zero. This is because elements in their standard state are not formed, or created and thus there is no change in bond energy or associated flow of energy. The enthalpy change for graphite, for example, is zero. However, the enthalpy change for diamond is 2 kJ/mol because there is energy required to form diamond out of graphite.

Gibbs Free Energy (∆G)

THINK OF GIBB'S FREE ENERGY AS: ∆G = 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, Gibb's Free Energy is said to be negative. If energy must be added to the reaction (i.e., work must be done on the system) to make it proceed, Gibb's Free Energy is said to be positive Units: Joules - ∆G = SPONTANEOUS, free energy available to do work. + ∆G = NON-SPONTANEOUS, no free energy available, energy must be added.

The Zeroth Law of Thermodynamics

Temperature exists KE= 3/2kT (where k is the boltzman's constant)

Isothermal

Temperature is constant (heat can flow in or out) An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0 but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.

Enthalpy of Vaporization (∆H_vaporization)

The enthalpy value associated with a species dissolving into solution. We'll discuss this in more detail when we cover solution chemistry.

Enthalpy of solution (∆H_solution)

The enthalpy value associated with a species dissolving into solution. We'll discuss this in more detail when we cover solution chemistry.

Enthalpy of fusion (∆H_fussion)

The enthalpy value associated with the phase change from solid to liquid OR from liquid to solid.

Enthalpy of combustion (∆H_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 a LOW heat of combustion with a stable molecule.

Enthalply of Formation (∆H_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.

Rates of Multi-Step Reactions:

The slow step ALWAYS determines the rate. If slow step is first, the rate law is written as if it were the only step. If the slow step is second (based on some assumptions) the rate law is the rate law of the slow step with the intermediate added.

Overall Order of a reaction is...

The some of the exponents in the rate law

Beaker A contains 2.55 g of FeCl3 dissolved in 100mL of water. Beaker B contains 10.20 g of FeCl3 dissolved in 400mL of water. If the specific heat capacity of the solution in Beaker A is known to be 1.33 cal/g˚C, what is the expected specific heat capacity of the solution in Beaker B? A) 1.33 cal/g B) 2.66 cal/g˚C C) 5.32 cal/g˚C D) Solutions do not have specific heat capacities

The two solutions have the same molar concentration of FeCl3, Beaker B simply contains four times as much of it. Specific Heat Capacity is an intensive property and thus should not vary with amount. This makes A the best answer. Note, however, that had the question asked for Heat Capacity—which refers to the heat absorption of the system as a whole instead of per gram or per mole—then Beaker B would indeed have approximately four times the Heat Capacity.

Kinetics vs Thermodynamics

Thermodynamics tells if if a reaction will go Kinetics tells us how fast it will go Thermodynamics is not about things moving and changing but instead about how stable they are in one state versus another, while kinetics is about how quickly or slowly species react.

Rate Order Graphs:

These graphs will only be linear when the reactant graphed is from a single reactant reaction OR when it is part of a multiple reactant reaction where ALL other reactants are NOT in the rate law (i.e., zero order). Zero Order: [A] vs. time is linear (i.e., yields a straight line) with slope = -k First Order: ln[A] vs. time is linear with slope =-k Second Order: 1/[A] vs. time is linear with slope = k Third Order: ½[A]2 vs. time is linear with slope = k

How to you find the "order" of each reactant

To calculate the "order" of each reactant using experimental data: 1) Find two trials where the [reactant] in question changed, but ALL OTHER PARAMETERS DID NOT (i.e., concentration of other reactants, temperature, etc.) 2) Note the factor by which the reactant concentration changed 3) Note the factor by which the rate changed across those same two trials 4) Use the following equation: XY = Z ; where X = the factor by which the [reactant] changed, Z = the factor by which the rate changed, and Y = the order of the reactant.

What is the purpose of a calorimeter?

Used to calculate enthaly (∆H). We are assuming that q (which is what the calorimeter actually measures) is equal to ∆H. This is true at constant pressure

Catalysts

any substance that increases reaction rate without itself being consumed in the process

The First Law of Thermodynamics

delta E= q +w In chemistry, by convention, the work done ON a system is positive, work done BY the system is negative. (like Endothermic is Positive Exothermic is Negative)

What does a higher heat capacity mean?

it can absorb more energy per unit temperature (its harder to heat up)

How will increasing [products] affect the rate of reaction?

it will go down

Isobaric

pressure is constant An isobaric process is a thermodynamic process in which the pressure stays constant. The term derives from the Greek isos, (equal), and barus, (heavy). The heat transferred to the system does work but also changes the internal energy of the system: The yellow area represents the work done

Heat Capacity

the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount. In the International System of Units (SI), heat capacity is expressed in units of joule(s) (J) per kelvin (K). C = q/∆T C=the heat capacity q=heat flow in T=change in temp of object

Specific Heat Capacity (Specific Heat)

we defined Heat Capacity as energy/change in temperature of a system. This could be a single substance or a complex system such as a liquid, the container and a thermometer. Specific Heat Capacity, however, is for a given substance only and is thought of as the heat capacity per unit mass. Little "c" is used instead of big "C". Formula: c = q/m∆T ; often re-written as: q = mc∆T Specific Heat of water = 1.0 cal/g˚C or 4.18 J/g˚C

What is an equation that relates ∆G and K

∆G = -RTlnK(eq) or K(eq)=e^(-∆G/RT)

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. As described above, if randomness increases (+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 simply 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.