Thermodynamics Practice Problems

3 C 2 H 2 (g) C 6 H 6 (g) What is the standard enthalpy change, ΔH o , for the reaction represented above? (ΔH o f of C 2 H 2 (g) is 230 kJ mol -1 ; ΔH o f of C 6 H 6 (g) is 83 kJ mol -1 .) (A) -607 kJ (B) -147 kJ (C) -19 kJ (D) +19 kJ (E) +773 kJ

A

A 10. g cube of copper at a temperature T 1 is placed in an insulated cup containing 10. g of water at a temperature T 2 . If T 1 > T 2 , which of the following is true of the system when it has attained thermal equilibrium? (The specific heat of copper is 0.385 J / (g o C) and the specific heat of water is 4.18 J / (g o C).) a. The temperature of the copper changed more than the temperature of the water. b. The temperature of the water changed more than the temperature of the ;copper. c. The temperature of the water and the copper changed by the same amount. d. The relative temperature changes of the copper and the water cannot be determined without knowing T 1 and T 2

A

4 NH 3 (g) + 3 O 2 (g) 2 N 2 (g) + 6 H 2 O(g) If the standard molar heats of formation of ammonia, NH 3 (g), and gaseous water, H 2 O(g), are -46 kJ / mol and -242 kJ / mol, respectively, what is the value of ΔH 298 o for the reaction represented above? (A) -190 kJ / mol rxn (B) -290 kJ / mol rxn (C) -580 kJ / mol rxn (D) -1270 kJ / mol rxn (E) -1640 kJ / mol rxn

D

Which of the following processes involves the greatest increase in entropy? a. (A) SO 3 (g) + H 2 (g) SO 2 (g) + H 2 O(g) b. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) c. Ag + (aq) + Cl - (aq) AgCl(s) d. C 2 H 2 (g) + 2 H 2 (g) C 2 H 6 (g) e.MgSO 3 (s) MgO(s) + SO 2 (g)

E

When a magnesium wire is dipped into a solution of lead (II) nitrate, a black deposit forms on the wire. Which of the following can be concluded from this observation? (A) The standard reduction potential, E o , for Pb 2+ (aq) is greater than that for Mg 2+ (aq). (B) Mg(s) is less easily oxidized than Pb(s) (C) An external source of potential must have been supplied. (D) The magnesium wire will be the cathode of a Mg / Pb cell. (E) Pb(s) can spontaneously displace Mg 2+ (aq) from solution.

A

Under which of the following conditions can an endothermic reaction be thermodynamically favorable? (A) ΔG is positive (B) ΔS is negative (C) T ΔS > ΔH (D) TΔS = 0 (E) There are no conditions under which an endothermic reaction can be thermodynamically favorable.

C

In real calorimeters, most of the heat released by the bomb is absorbed by water, but a certain amount is also absorbed by the metal and insulation surrounding the water tank. A certain calorimeter absorbs 24 J/oC. If 50.0 g of 52.7oC water is mixed with the calorimeter's original 50.0 g of 22.3oC water, what will be the final temperature of the mixture?

Tf= 36.7C -(50g)(4.18)(Tf-52.7)=50g(4.18)(tf-22.3C)+24(Tf-22.3)

. If 26 g of water at 18oC are mixed with 49 g of water at 70oC, find the final temperature of the system.

Tf=52.1C 26g(4.18)(Tf-18)=-(49g)(4.18)(Tf-70C)

Which of the following reactions is not thermodynamically favored at low temperatures but becomes favored as the temperature increases? Reaction ΔH o (kJ / mol rxn ) ΔS o (J / (mol rxn K)) a. 2 CO(g) + O 2 (g) 2 CO 2 (g) -566 -173 b. 2 H 2 O(g) 2 H 2 (g) + O 2 (g) 484 90.0 c. 2 N 2 O(g) 2 N 2 (g) + O 2 (g) -164 149 d. PbCl 2 (s) Pb 2+ (aq) + 2Cl - (aq) 23.4 -12.5

B

Which of the following equations represents a reaction for which the standard entropy change is positive (ΔS o > 0) ?

C