CHM 2046C Sample Module 13 Name: ___Answers_____

Part B Entropy: Disorder and Spontaneity 10 points

1. What six types of processes or reactions lead to an increase in entropy?

1. Changes from solid to liquid to gas.
2. Breakdown of a large molecule into smaller molecules.
3. Breakdown of an ionic solid
4. Reactions in which the number of gas molecules increases
5. Making a solution of a mixture.
6. The expansion of a gas.

2. How are entropy, enthalpy, and the spontaneity of a reaction related?

The contributing, driving forces for a spontaneous reaction are an increase in entropy and a decrease in enthalpy of the system. Both result in an increase in the entropy of the universe.

The reaction is spontaneous if ∆S^o_sys is positive or ∆H^o_sys is negative.

The possibilities are:

∆H^osys	∆S^osys	Result
- (exothermic)	+	Reaction must be spontaneous
- (exothermic)	-	Depends upon T, more favorable at low temperature
+ (endothermic)	+	Depends upon T, more favorable at high temperature
+ (endothermic)	-	Reaction cannot be spontaneous

3. Which of the following processes are spontaneous and which are nonspontaneous:

(a) diffusion of perfume molecules from one side of the room to the other (Spontaneous)

(b) Heat flow from a cold piece of metal (2^oC)to hot water(70^oC) when the cold metal is dropped into the hot water. (nonspontaneous)

(c) Decomposition of rust (Fe₂O₃∙H₂O) to iron metal, oxygen gas, and water. (nonspontaneous)

(d) Decomposition of solid CaCO₃ to solid CaO and gaseous CO₂ at 25^oC and 1 atm pressure (K_p = 1.4 x 10 ^-23) (nonspontaneous)

4. Predict the sign of ∆S in the system for each of the following processes:

(a) CO₂ (s) ---> CO₂ (g) (sublimation of dry ice) (positive)

(b) CaSO₄(s) ---> CaO(s) + SO₂(g) (positive)

(c) N₂(g) + 3 H₂ (g) ---> 2 NH₃ (g) (negative)

(d) I₂(s) ---> I₂ (aq) (dissolution of iodine in water) (positive)