CHEM 1215 Unit 5 – Thermo Practice Problems

 

Note:  You will need to use Appendix C from your text for some of these problems.

 

1.       Use your knowledge of chemistry and your every-day life experience to identify each of the following reactions or processes as spontaneous or nonspontaneous at 25oC and 1 atm pressure.

 

a.   CH4 (g) à CH4 (l)

b.   sublimation of dry ice

c.   graphite à diamond

d.   the dissolution of sodium chloride in water

e.   Br2 (g) à 2 Br (g)

 

2.      Without doing any calculations, predict whether the change in entropy will be positive, negative, or about zero.

 

a.   spilling a box of cereal

b.   NO2 (g)  +  N2O (g) à  3 NO (g)

c.   Ag+ (aq) +  Cl- (aq) à AgCl (s)

d.   melting a piece of ice

e.   CO (g) + H2O (g) à CO2 (g) + H2 (g)

 

3.      Calculate DSo for the following reaction:

 

2 Br- (aq) + 2 H2O (l) à Br2 (l) + H2 (g) + 2 OH- (aq)

 

4.      Use the standard free energies of formation to calculate DGo for the following reaction at 25oC.

 

P4O10 (s) + 6 H2O (l) à 4 H3PO4 (aq)

 

5.      The reduction of iron (III) oxide by hydrogen gas forms iron metal and water vapor.

 

a.   Calculate DGo at 230.oC for this reaction.  Assume that any temperature variation for DHo and DSo is negligible.

 

b.   Is the reaction spontaneous at this temperature?

 

c.   Calculate the temperature at which the reaction becomes spontaneous.

 

6.      Calculate the maximum amount of work that can be performed by the complete combustion of 74.3 g of liquid benzene (C6H6) if the products are cooled to room temperature.

 

7.      Using the data given in Appendix C, calculate the boiling point for methanol, CH3OH.

 

8.      The standard Gibbs free energy of formation for the magnesium ion is -454.8 kJ/mole.  (Other important values for this problem are found in Appendix C of your text.)

 

a.   Calculate DGo for the dissolution of magnesium hydroxide in water.

 

b.   Calculate Ksp for magnesium hydroxide at 25oC.

 

c.   Calculate the concentration of hydroxide ion present in a saturated aqueous solution of magnesium hydroxide.

 

9.      A saturated aqueous solution of silver phosphate contains 4.14 mg of silver ion per 300.0 mL of solution at 25oC.  Calculate DGo for the dissolution of silver phosphate in water.

 

10.   For each of the following reactions, determine the conditions under which each will be spontaneous (always, never, only high T, or only low T).

 

a.   CH3OH (l) à  CH3OH (g)

b.   CH4 (g) +  2 O2 (g) à CO2 (g) + 2 H2O (l)