18.2.2 Entropy and Temperature

Question 1

Entropy and Temperature

Answer 1

• The entropy change for a reversible process at equilibrium is equal to the heat of the system divided by the temperature of the system.
• The change in entropy for a reaction is the sum of the entropy of the products minus the sum of the entropy of the reactants.
• By the third law of thermodynamics, a perfect crystal at absolute zero (0 K) has perfect order (S = 0).

Question 2

note

Answer 2

• Entropy, a measure of the disorder in a system, can be defined mathematically.
• The entropy change for a reversible process (at equilibrium) is equal to the heat of the system divided by the temperature of the system.
• For a spontaneous process, the change in entropy is greater than the heat of the system divided by the temperature of the system.
• The change in entropy for a reaction is determined by taking the sum of the entropy of the products minus the sum of the entropy of the reactants. Unlike enthalpy, it is possible to discuss the absolute entropy (S) of a substance. The process on the left resulted in products with greater order (lower entropy) than the reactants; consequently, the change in entropy is negative.
• The third law of thermodynamics provides a reference point for absolute entropy.
• By the third law of thermodynamics, a perfect crystal at
  absolute zero (0 K) has perfect order (S = 0).

Question 3

With respect to the third law of thermodynamics, which statement is not correct?

Answer 3

At 0 K for a substance, almost all of the thermal energy has been removed from the system.

Question 4

Look at the plot for entropy versus temperature.

Which of the following describes Range B?

Answer 4

the entropy of fusion

Question 5

The entropy change for a reversible process can be defined by the expression ΔS = q / T. Which statement about entropy change is not correct?

Answer 5

The term T represents the change in temperature.

Question 6

Which statement about ΔS is not correct?

Answer 6

For a spontaneous process (at constant pressure), ΔH − TΔS > 0.

Question 7

Which statement about heat capacity is not correct?

Answer 7

The molecules in a solid can easily rotate. This gives them a great capacity for storing heat energy.

Question 8

Which statement about entropy or the second law of thermodynamics is not correct?

Answer 8

Entropy remains constant for any spontaneous process.

Question 9

Which statement about investigating (mathematically) whether or not the reaction below is spontaneous is incorrect?
2H2(g) + O2(g) <==> 2H2O(g)

Answer 9

The sum of the absolute entropies of the reactants is SH2 + SO2

Question 10

Look at the plot for entropy, (S°) vs. T (K).

Which statement about this plot is not correct?

Answer 10

There are two ranges in the plot where the temperature of the substance is simply increasing. The value for S° at any point in those ranges can be calculated using the formula

Question 11

The second law of thermodynamics says that the entropy of the universe tends to increase. In other words, the universe prefers maximum disorder. With respect to entropy, which statement is not correct?

Answer 11

The temperature does not affect the absolute entropy value.