SCH4U    Entropy and Gibbs Free Energy Review                Answers
1.
When solid potassium iodide is dissolved in water, a cooling of the mixture occurs because the solution process is endothermic for these substances.  Explain, in terms of what happens to the molecules and ions, why this mixing occurs spontaneously?

2.
Predict the algebraic sign of the entropy change for the following reactions?
(a) PCl3(g) + Cl2(g) ----> PCl5(g)

(b) SO2(g) + CaO(s) -----> CaSO3(s)

(c) CO2(g) + H2O(l) ----> H2CO3(aq)

(d) Ni(s) + 2 HCl(aq) ---> H2(g) + NiCl(aq)

(e) I2(s) ----> I2(g)

(f) Cl2(g) + Br2(g) ----> 2 BrCl(g)

(g) NH3(g) + HCl(g) ---> NH4Cl(s)

(h) CaO(s) + H2O(l) ----> Ca(OH)2(s)

3.
Show that ΔS for the melting of ice is positive.

4
Calculate the entropy change in J/mol K for each of the following reactions.

(a) CaO(s)  + 2 HCl(g) --->  CaCl2(s)  + H2O(l)

(b) C2H4(g)  + H2(g)  --->  C2H6(g)

5.
What would be the algebraic signs of ΔS for the following reactions?

(a) Br2(l) + Cl2(g) ---> 2 BrCl(g)

(b) CaO(s) + CO2(g) ---> CaCO3(s)

6.
Calculate ΔSo (in J/mol K) for the following reaction.

(a) H2O(l) + SO3(g) ---> H2SO4(l)
   
(b) 2 KCl(s)  + H2SO4(l) ---> K2SO4(s)  + 2 HCl(g)

(c) C2H4 + H2O(g) ---> C2H5OH(l)

7.
Predict the probability of the following reactions by  approximating the sign of ΔG. Classify each reaction as exothermic or endothermic.

a)  H2O(l) ----> H2(g)  +  ½ O2(g)   
ΔH=  +285 kJ
                                                     T
ΔS =  -245 kJ   

b) C6H14(g)   ---->   6 C(s) + 7 H2(g)  
 ΔH = +167 kJ
                                                           T
ΔS = +168 kJ

c) 2 Fe(s) + ½ N2(g)  ---->   Fe2N(s)      
ΔH = -3.8 kJ
                                                             T
ΔS = -14.6 kJ

d) HCl(g) + H2O(l)  ---->   H3O+(aq) + Cl-(aq)    
ΔH = -75.3 kJ
                                                                        T
ΔS = -39.3 kJ

8.
Phosgene, COCl2, was used as a war gas during World War I.  It reacts with the moisture in the lungs to produce HCl, which causes the lungs to fill with fluid, leading to death of the victim. COCl2 has a standard entropy, ΔSo = +284 J/mol K and ΔHfo = -223 kJ/mol. Use this information and appropriate tables to calculate ΔGfo for COCl2(g) in kJ/mol.

9.
Aluminum oxidizes rather easily, but forms a thin protective coating of Al2O3 that prevents further oxidation of the aluminum beneath.  Use the data for  ΔHfo and ΔSo to calculate ΔGfo for Al2O3(s) in kJ/mol.

10.
Calcualte ΔGo in kJ/mole for the following reactions, using the appropriate data tables.
(a) SO3(g) + H2O(l) ----->  H2SO4(l)

(b) 2 NH4Cl(s)  +  CaO(s)  ----->  CaCl2(s)  +  H2O(l)  +  2 NH3(g)

(c) CaSO4(s) +  2 HCl(g)  -----> CaCl(s) + H2SO4(l)

(d) C2H4(g)  +  H2O(l)  -----> C2H5OH(l)

(e) Ca(s) + 2 H2SO4(l)  -----> CaSO4(s)  +  SO2(g)  +  2 H2O(l)

11.
Plaster of Paris, CaSO4·½H2O(s), reacts with liquid water to form gypsum, CaSO4·2H2O(s). Write a chemical equation  for the reaction and calculate ΔGo in kJ/mol, using the appropriate data tables.

12.
Given the following,

    4 NO(g)  ----->  2 N2O(g)  +  O2(g)     
ΔGo = -139.56 kJ

    2 NO(g)  +  O2(g)  ----->  2 NO2(g)   
ΔGo = -69.70 kJ

    calculate  
ΔGo for the reaction

        2 N2O(g)  +  3 O2(g)    ---->   4 NO2(g)

13.
Calculate ΔG in kJ/mol at 55oC for the following reaction.

    CH4(g)  +  Cl2(g)   -------->  CH3Cl(g)   +   HCl(g)

14.
For the reaction at 98.2 K,

        2 NO2(g)   ---->   N2O4(g)

a)   The values of
ΔHo and ΔSo are -58.03 kJ mol-1 and -176.61 J K-1 mol-1 respectively. What is the value of ΔGo at 298.2 K?  
b)   At what temperature is
ΔGo = 0?
c)   Is
ΔG negative above, or below, this temperature?