AP Chemistry Chapter 19 Thermodynamics—Spontaneity and Entropy Key

1. Define what is meant by a spontaneous process.A spontaneous process is one that occurs without any outside intervention.

2. (a) Define entropy Entropy is a measure of disorder.

(b) Is entropy a form of energy? The quantity TS has units of energy.

3. Describe how the following changes affect the positional entropy of a substance.

(a) Increase in volume of a gas at constant T. Entropy increases; there is a greater volume accessible to the randomly moving gas molecules. Positional disorder will increase.(b) Increase in temperature of a gas at constant V. The positional entropy does not change. There is no change in volume and so, no change in the numbers of position of the molecules. The total entropy increases because the increase in temperature increases the energy disorder.(c) Increase in pressure of a gas at constant T. Entropy decreases since volume decreases.4. Which of the following processes require energy as they occur?(a) Salt dissolves in H2O. Yes; this requires external energy because it is an endothermic process. It is spontaneous since S is favorable and provides the driving force.(b) A clear solution becomes a uniform color. No

(c) A cell produces proteins from amino acids. Yes

(d) Iron rusts. No

(e) A house is built. Yes

(f) A satellite is launched into orbit. Yes

(g) A satellite falls back to earth. No

(h) Natural gas is burned in the air. No

5. Consider the following energy levels, each capable of holding two objects:

E = 2 kJ __________

E = 1 kJ __________

E = 0 kJ __XX_____

Draw all the possible arrangements of the two identical particles (represented by X) in the three energy levels. What total energy is most likely, that is, occurs the greatest number of times? Assume that the particles are indistinguishable from each other.2 kJ X X XX1 kJ X XX X0 kJ XX X XTotal E 0 kJ 1 kJ 2 kJ 2 kJ 3 kJ 4 kJThe most likely total energy is 2 kJ.

6. Do problem #5 with two particles, A and B, which can be distinguished from each other.2 kJ AB B A B A1 kJ AB B A A B0 kJ AB A B A BET 0 kJ 2 kJ 4 kJ 1 kJ 1 kJ 2 kJ 2 kJ 3 kJ 3 kJThe most likely total energy is 2 kJ (three possibilities).

7. Do problem #5 assuming each level can hold three indistinguishable particles, X.2 kJ X X1 kJ X XX X XXX0 kJ XXX XX XX X XET 0 1 2 2 3 3

2 kJ XX X XX XXX1 kJ XX X0 kJ XET 4 4 5 6A total energy of 2, 3, or 4 kJ is equally possible.

8. Which of the following involve an increase in entropy of the system? Of the three phases (solid, liquid, and gas), solids are most ordered and gases are most disordered.(a) Melting of a solid Increase

(b) Evaporation of a liquid Increase

(c) Sublimation Increase

(d) Freezing Decrease

9. Which of the following involve an increase in the entropy of the system?(a) Mixing Increase

(b) Separation Decrease

(c) Diffusion Increase

(d) Precipitation of a solid from a solution. Decrease

10. Choose the compound with the greatest positional entropy in each case.(a) 1 mol H2 (at STP) or 1 mol N2O (at STP) N2O—It is a more complex molecule, larger positional disorder.(b) 1 mol H2 (at STP) or 1 mol H2 (at 100°C, 0.5 atm) H2 (at 100°C, 0.5 atm)—Higher temperature and lower pressure means greater volume and greater positional energy.(c) 1 mol N2 (at STP) or 1 mol N2 (at 100 K, 2.0 atm) N2 (at STP)—has the greater volume.(d) 1 mol H2O(s) (at 0°C) or 1 mol H2O(l) (at 20°C) H2O(l) (at 20°C)(e) 1 mol He (at 10 K, 1 atm) or 1 mol He (at 10°C, 1 atm) He (at 10°C, 1 atm)—Higher T and greater V.(f) 1 mol He (at 500°C, 1 atm) or 1 mol He (at 500°C, 2 atm) He (at 500°C, 1 atm)—Greater volume of gas.(g) 1 mol He (at STP) or 1 mol He (at 200 K, 2 atm) He (at STP)—Higher T and greater V.(h) 1 mol He(s) (at 0 K) or 1 mol He(g) (at 5 K) He(g) (at 5 K)