UNC-Charlotte AP Chemistry Exam Review Session

UNC-Charlotte AP Chemistry Exam Review Session

Thermochemistry & Thermodynamics – Most Likely Questions

Contrasting “heat” – a non-state function to “enthalpy” – a state function.

2. Given the standard molar enthalpy of formation for reactants and products, calculate the heat of a reaction.

3. Given heat of reaction and some of the heat of formation values for reactants and products, calculate

the heat of formation of the missing species.

4. Given molar heat of reaction, calculate the heat (absorbed or released), given the mass of a single reactant

or product.

5. Explain why the standard molar enthalpy of formation for any element in its most stable form is zero.

6. Given the standard molar entropy of formation for reactants and products, calculate

the entropy of a reaction.

7. A) Using the Gibbs-Helmholtz equation, calculate the free energy of a reaction.

B) Importance of the units on S vs. H or G!!!

8. A) Using the Gibbs-Helmholtz equation and the H and S values (and signs), predict whether or not

a reaction is “thermodynamically favored (spontaneous).

B) Based on changing conditions (for example, moving from a “spontaneous” to a “nonspontaneous”

reaction as temperature changes, predicting the signs for enthalpy and entropy.

9. Relating Go to Eo and to K

10. Relating Go to G

11. Using the Gibbs-Helmholtz equation, calculate the F. Pt / B. Pt / M. Pt / condensation point

by using the appropriate values for entropy and enthalpy.

12. Calorimetry problems – open / closed / isolated systems

13. Change-of-state problems

14. Three Laws of Thermodynamics and what they mean

15. Combustion Reactions – at standard conditions vs. higher temperatures (like over 100oC)

16. Using / Calculating bond energies in calculating enthalpies of reactions

17. Comparing the entropy of different (related) substances

18. Using Hess’s Law of Summation

19. Performing calculations for Heat of Neutralization lab

20. Energy diagram – exothermic vs. endothermic / why the need for activation energy

Sample Thermochemistry / Thermodynamics Free Response Essays

1970

Consider the first ionization of sulfurous acid: H2SO3(aq) H+(aq) + HSO3-(aq)

Certain related thermodynamic data are provided below:

H2SO3(aq) H+(aq) HSO3-(aq)

------

HfkJ/mole -608.8 0 -635.5

Sjoules/mole K 234.3 0 108.8

(a) Calculate the value of G at 25C for the ionization reaction.

(b) Calculate the value of K at 25C for the ionization reaction.

and ions present.

1972

For the given reaction, the following data are available: Br2 + 2 Fe2+(aq) 2 Br-(aq) + 2 Fe3+(aq)

2 Br-(aq) Br2(l)+ 2e- (SRP) E = -1.07 volts

Fe2+(aq) Fe3+(aq)+ e- (SRP) E = -0.77 volts

S, joule/mole C

Br2(l) 245Fe2+(aq) -27.1

Br-(aq) 82.0Fe3+(aq) -70.1

(a) Determine S

(b) Determine G

1974

WO3(s) + 3 H2(g) W(s) + 3 H2O(g)

Tungsten is obtained commercially by the reduction of WO3 with hydrogen according to the equation above. The following data related to this reaction are available:

WO3(s)H2O(g)

Hf(kilocalories/mole)-200.84-57.8

Gf(kilocalories/mole)-182.47-54.6

(a)What is the value of the equilibrium constant for the system represented above?

(b)Calculate S at 25C for the reaction indicated by the equation above.

(c)Find the temperature at which the reaction mixture is in equilibrium at 1 atmosphere.

1975 D

2 Cu + S  Cu2S

For the reaction above, H, G, and S are all negative. Which of the substances would predominate in an equilibrium mixture of copper, sulfur, and copper(I) sulfide at 298K? Explain how you drew your conclusion about the predominant substance present at equilibrium. Why must a mixture of copper and sulfur be heated in order to produce copper(I) sulfide?

1978 B

Standard Entropy

Substancecal/deg mole

N2(g)45.8

H2(g)31.2

NH3(g)46.0

Ammonia can be produced by the following reaction: N2(g) + 3 H2(g) 2 NH3(g)

TheGibbsfreeenergyofformationGfofNH3(g) is -3.94 kilocalories per mole.

(a)Calculate the value for H for the reaction above 298K.

(b)Can the yield of ammonia be increased by raising the temperature? Explain.

(c)What is the equilibrium constant for the reaction above at 298K?

(d)If 235 milliliters of H2 gas measured at 25C and 570 millimeters Hg were completely converted to ammonia and the ammonia were dissolved in sufficient water to make 0.5000 liter of solution, what would be the molarity of the resulting solution?

1979 B

HfS

Compound(kilocalories/mole)(calories/mole K)

H2O(l) -68.316.7

CO2(g) -94.151.1

O2(g) 0.0 49.0

C3H8 ? 64.5

When 1.000 gram of propane gas, C3H8, is burned at 25C and 1.00 atmosphere, H2O(l) and CO2(g) are formed with the evolution of 12.03 kilocalories.

(a)Write a balanced equation for the combustion reaction.

(b)Calculate the molar enthalpy of combustion, Hcomb, of propane.

(c)Calculate the standard molar enthalpy of formation, Hf, of propane gas.

(d)Calculate the entropy change, Scomb, for the reaction and account for the sign Scomb.

1981 D

PCl5(g)  PCl3(g) + Cl2(g)

For the reaction above, H = +22.1 kilocalories per mole at 25C

(a)Does the tendency of reactions to proceed to a state of minimum energy favor the formation of the products of this reaction? Explain

(b)Does the tendency of reactions to proceed to a state of maximum entropy favor the formation of the products of this reaction? Explain.

(c)State whether an increase in temperature drives this reaction to the right, to the left, or has no effect. Explain.

(d)State whether a decrease in the volume of the system at constant temperature drives this reaction to the right, to the left or has no effect. Explain?

1984 B

Standard Heat ofAbsolute

Formation, Hf,Entropy, S,

Substancein kJ mol-1 in J mol-1 K-1

------

C(s) 0.00 5.69

CO2(g) -393.5 213.6

H2(g) 0.00 130.6

H2O(l) -285.85 69.91

O2(g) 0.00 205.0

C3H7COOH(l) ? 226.3

The enthalpy change for the combustion of butyric acid at 25C, Hcomb, is -2,183.5 kilojoules per mole. The combustion reaction is

C3H7COOH(l) + 5 O2(g) 4CO2(g) + 4H2O(l)

(a)From the above data, calculate the standard heat of formation, Hf, for butyric acid.

(b)Write a correctly balanced equation for the formation of butyric acid from its elements.

(c)Calculate the standard entropy change, Sf, for the formation of butyric acid at 25C. The entropy change, S, for the combustion reaction above is -117.1 J K-1 at 25C.

(d)Calculate the standard free energy of formation, Gf, for butyric acid at 25C.

1985 D

(a)When liquid water is introduced into an evacuated vessel at 25C, some of the water vaporizes. Predict how the enthalpy, entropy, free energy, and temperature change in the system during this process. Explain the basis for each of your predictions.

(b)When a large amount of ammonium chloride is added to water at 25C, some of it dissolves and the temperature of the system decreases. Predict how the enthalpy, entropy, and free energy change in the system during this process. Explain the basis for each of your predictions.

(c)If the temperature of the aqueous ammonium chloride system in part (b) were to be increased to 30C, predict how the solubility of the ammonium chloride would be affected. Explain the basis for each of your predictions.

1993 D

2 C4H10(g) + 13 O2(g) 8 CO2(g) + 10 H2O(l)

The reaction represented above is spontaneous at 25C. Assume that all reactants and products are in their standard state.

(a)Predict the sign of S for the reaction and justify your prediction.

(b)What is the sign of G for the reaction? How would the sign and magnitude of G be affected by an increase in temperature to 50C? Explain your answer.

(c)What must be the sign of H for the reaction at 25C? How does the total bond energy of the reactants compare to that of the products?

(d)When the reactants are place together in a container, no change is observed even though the reaction is known to be spontaneous. Explain this observation.

1995 B

Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking.

(a)Write a balanced equation for the complete combustion of propane gas, which yields CO2(g) and H2O(l).

(b)Calculate the volume of air at 30C and 1.00 atmosphere that is needed to burn completely 10.0 grams of propane. Assume that air is 21.0 percent O2 by volume.

(c)The heat of combustion of propane is -2,220.1 kJ/mol. Calculate the heat of formation, Hf, of propane given that Hf of H2O(l) = -285.3 kJ/mol and Hf of CO2(g) = -393.5 kJ/mol.

(d)Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00 kilograms of water (specific heat = 4.18 J/g.K), calculate the increase in temperature of water.

1996 B

C2H2(g) + 2 H2(g) C2H6(g)

Information about the substances involved in the reaction represented above is summarized in the following tables.

Substance / S(J/molK) / Hf(kJ/mol) / Bond / Bond Energy (kJ/mol)
C2H2(g) / 200.9 / 226.7 / C-C / 347
H2(g) / 130.7 / 0 / C=C / 611
C2H6(g) / - - - - / -84.7 / C-H / 414
H-H / 436

(a)If the value of the standard entropy change, S, for the reaction is -232.7 joules per moleKelvin, calculate the standard molar entropy, S, of C2H6gas.

(b)Calculate the value of the standard free-energy change, G, for the reaction. What does the sign of G indicate about the reaction above?

(c)Calculate the value of the equilibrium constant, K, for the reaction at 298 K.

(d)Calculate the value of the CC bond energy in C2H2 in kilojoules per mole.

1998 B

C6H5OH(s) + 7 O2(g) 6 CO2(g) + 3 H2O(l)

When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow.

Substance / Standard Heat of Formation, Hâ; at 25C (kJ/mol) / Absolute Entropy, S, at 25C (J/molòK)
C(graphite) / 0.00 / 5.69
CO2(g) / -393.5 / 213.6
H2(g) / 0.00 / 130.6
H2O(l) / -285.85 / 69.91
O2(g) / 0.00 / 205.0
C6H5OH(s) / ? / 144.0

(a)Calculate the molar heat of combustion of phenol in kilojoules per mole at 25C.

(b)Calculate the standard heat of formation, Hâ, of phenol in kilojoules per mole at 25C.

(c)Calculate the value of the standard free-energy change, G, for the combustion of phenol at 25C.

(d)If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110.C. (Assume no oxygen remains unreacted and that all products are gaseous.)

1999 D (Required)

Answer the following question in terms of thermodynamic principles and concepts of kinetic molecular theory.

(a)Consider the reaction represented below, which is spontaneous at 298 K.

CO2(g) + 2 NH3(g) CO(NH2)2(s) + H2O(l) Hº298 = –134 kJ

(i)For the reaction, indicate whether the standard entropy change, Sº298, is positive, negative, or zero. Justify your answer.

(ii)Which factor, the change in enthalpy, Hº298, or the change in entropy, Sº298, provides the principle driving force for the reaction at 298 K? Explain.

(iii)For the reaction, how is the value of the standard free energy change, Gº, affected by an increase in temperature? Explain.

(b)Some reactions that are predicted by their sign of Gº to be spontaneous at room temperature do not proceed at a measurable rate at room temperature.

(i)Account for this apparent contradiction.

(ii)A suitable catalyst increases the rate of such a reaction. What effect does the catalyst have on Gº for the reaction? Explain.