# A.P. Chemistry W.S. 1 Chap. 6 20

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```							A.P. Chemistry
W.S. 1 Chap. 6 & 20

Enthalpy (H)
1. For the reaction;
S + O2    SO2               ∆Ho = -296 kJ/mol
a) How much heat is evolved when 275 g of sulfur is burned?
b) How much heat is evolved when 150 g of sulfur dioxide is produced?

2. Given the following data:
S + 3/2 O2     SO3                     ∆Ho = -395.2 kJ
2 SO2 + O2      2 SO3                  ∆Ho = -198.2 kJ
calculate ∆Ho for the reaction:
S + O2        SO2

3. Given the following data:
H2(g) + ½ O2(g)  H2O(l)               ∆Ho = -285.8 kJ
N2O5(g) + H2O(l)    2HNO3(l)          ∆Ho = -76.6 kJ
½ N2(g) + 3/2 O2(g) + ½ H2(g) HNO3(l) ∆Ho = -174.1 kJ
calculate ∆Ho for the reaction:
2N2(g) + 5O2(g)   2N2O5(g)

4. Given the following data:
2 O3(g)   3O2(g)                 ∆Ho = -427 kJ
O2(g) 2O(g)                      ∆Ho = +495 kJ
NO(g) + O3(g)   NO2(g) + O2(g)   ∆Ho = -199 kJ
calculate ∆Ho for the reaction:
NO(g) + O(g)  NO2(g)

Use the values of ∆Hof in Appendix B (p.A5-A7) to calculate ∆Ho for the following reactions:
5.
a) 2NH3(g) + 3O2(g) + 2CH4(g)   2HCN(g) + 6H2O(g)
b) Ca3(PO4)2(s) + 3H2SO4(l)   3CaSO4(s) + 2H3PO4(l)
c) NH3(g) + HCl(g)   NH4Cl(s)

6. Water gas is produced from the reaction of steam with coal:
C(s) + H2O(g) H2(g) + CO(g)
o
calculate ∆H for this reaction.

7. For the reaction:
2ClF(g) + 2NH3(g)     N2(g) + 6HF(g) + Cl2(g)       ∆Ho = -1196 kJ
calculate ∆Hof for ClF(g)

Entropy (S)
8. For each of the following pairs, choose the substance with the higher standard molar entropy (So) at the
same temperature.
a) Ar(l) or Ar(g)
b) Ar(g) or HCl(g)
c) Ar(g) or He(g)

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9. Predict the sign of the change in standard entropy, ∆So, will be for the following reactions:
a) CaCO3(s)      CaO(s) + CO2(g)
b) N2(g) + 3H2(g)    2NH3(g)
c) 2N2O5(g)        4NO2(g) + O2(g)
d) O2(g) + 2F2(g)    2OF2(g)
e) FeCl2(s) + H2(g)   Fe(s) + 2HCl(g)
f) 2Li(s) + Cl2(g)   2LiCl(s)
10. Calculate change in standard entropy (∆So)for the following reactions using the standard molar entropy (So)
values appendix B
a) CH4(g) + 2O2(g)     CO2(g) + 2H2O(l)
b) N2O4(g)      2NO2(g)
c) 2CH3OH(l) + 3O2(g)       2CO2(g) + 4H2O(g)

Gibbs Free Energy (G)
11. Consider the freezing of liquid water at -10oC. For this process what are the signs for ∆H, ∆S, and ∆G?

12. The reaction
CO2(g) + H2(g) -----> CO(g) + H2O(g)
is non-spontaneous at room temperature, but becomes spontaneous at a much higher temperature. What
can you conclude from this about the signs of ∆Ho and ∆So assuming that the enthalpy and entropy changes
are not greatly affected by the temperature change?

13. A particular reaction is spontaneous at 450K. The enthalpy change (∆H) for the reaction is +34.5 kJ. What
can you conclude about the sign and magnitude of ∆S for the reaction?

14. For the reaction:
2PbS(s) + 3O2(g)  2PbO(s) + 2SO2(g)  ∆Ho = +844 kJ and ∆So = +165 JK-1
a) Calculate the standard free energy change (∆Go) for this reaction.
b) At what temperature would this reaction become spontaneous?

15. Using the standard free energy of formation values (∆Gof) in Appendix B, Calculate the standard free
energy change (∆Go) for the following reaction and state whether it will be spontaneous at 298 K:
CH4(g) + 2O2(g) ----> CO2(g) + 2H2O(l)
a) Combine your answer for (∆Go) for this reaction and your answer in 10a) for ∆So to determine ∆Ho
for this reaction at 298K.

16. Using the standard free energy of formation values (∆Gof) in Appendix B, Calculate the standard free
energy change for the following reaction and state whether it will be spontaneous at 298 K:
a) SO2(g) + 2H2S(g) ----> 3S(rhombic) + 2H2O(g)
b) 2H2O2(aq) ---> O2(g) + 2H2O(l) (∆Gof H2O2=-134 kJ/mol)
c) HCOOH(l) ----- CO2(g) + H2(g)
d) 2H2O(l) ---> 2H2(g) + O2(g)

17. Using the values in Appendix B calculate ∆Ho, ∆So, & ∆Go for the following reaction:
H2(g) + S(rhombic) ------> H2S(g)
o
Interpret the signs of ∆H , ∆So, & ∆Go.

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Free Energy & Equilibrium(use values in Appendix B A5-A7)
18. What is the standard free energy change (∆Go) for the following reaction?
H2(g) + Cl2(g) <----> 2HCl(g)
What is the value of the thermodynamic equilibrium constant, K?

19. Calculate the standard free-energy change and the equilibrium constant, K, for the following reaction at
25oC:           CO(g) + H2O(g) <=====> CO2(g) + H2(g)

20.
a) Calculate the approximate value of the equilibrium constant (Keq) at for the reaction
C(graphite) + CO2(g) <======> 2CO(g)

b) Carbon monoxide is known to form during the combustion of carbon at high temperatures (keep
those bar-b-que grills outside you crazy suburbanites). Does the data agree with this? Explain.

Combination thermodynamics problems (H, S, G, & K)

21. CO(g) + 2 H2(g) <=====> CH3OH(l)     ∆Ho = -128.1 kJ

∆Ηfo (kJ/mol)        ∆Gfo (kJ/mol)       So(J/molK)
CO(g)              -110.5                     -137.3          +197.9
CH3OH(l)           -238.6                     -166.2          +126.8

Using the data in the table above:
a) Calculate ∆Go for the reaction above at 25 C.
b) Calculate K for the reaction above.
c) Calculate ∆So for the reaction above at 25 C.
d) In the table above, there are no data for H2. What are the values of ∆Hfo, ∆Gfo, and of the standard
entropy, So, for H2 at 25 C?

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