Unit 9 - Applications of Thermodynamics

Question 1

Define entropy

Answer 1

Entropy (∆S) = a measure of how dispersed the energy of a system is at a specific temperature [J/mol]

Question 2

Define Gibbs Free Energy

Answer 2

Gibbs free energy = amount of energy that is free or available to do useful work

Gibbs free energy change (∆G°) = a measure of the thermodynamic favorability of a chemical or physical process [kJ/mol]

This applies to processes in which all the reactants and
products are present in a standard state (as pure elements, as solutions of 1.0 M concentration, or as gases at 1.0 atm and 298 K)

Question 3

How does Gibbs free energy change (∆G) indicate thermodynamic favorability?

Answer 3

If ΔG° < 0, the forward reaction is thermodynamically favored to occur at standard conditions. The products are favored at equilibrium and K > 1

If ΔG° > 0, the forward reaction is not thermodynamically favored to occur, so energy must be supplied from the surroundings. The reactants are favored at equilibrium and K < 1.

*ΔG° has no effect on the rate of reaction, only the tendency for it to occur (can be fast or slow)

If the forward reaction is favored, the reverse reaction is not favored, and vice versa.

ΔG° = 0 at equilibrium and for phase changes (use this to find boiling/melting point). Neither the forward or reverse process is favored. The equilibrium constant K will be close to 1.

Question 4

How do you calculate the minimum temperature that would be required for a reaction to be thermodynamically favored?

Answer 4

Use the equation ΔG° = ΔH° - TΔS°. Set ΔG° = 0 and then use the given values of ΔH° and ΔS° to solve for T.

Question 5

What does it mean for a process to be under kinetic control?

Answer 5

Processes that are thermodynamically favored, but do not proceed at a measurable rate (concentrations of reactants and products are changing very slowly).

This could be because the reaction has a high activation energy

Rate is based on kinetics instead of thermodynamics

Question 6

How do you determine if entropy or enthalpy drives a particular reaction?

Answer 6

A reaction is driven by whichever factor helps make ΔG° negative because this will cause it to be thermodynamically favorable.

1. Driven by enthalpy (ΔH°) only – negative ΔH° and negative ΔS°
2. Driven by entropy (ΔS°) only – positive ΔH° and positive ΔS°
3. Driven by both – negative ΔH° and positive ΔS°

At low temperature, enthalpy (ΔH°) is dominant while at high temperature, entropy (ΔS°) is dominant

Question 7

When will ΔS° for a reaction be close to zero?

Answer 7

EX: HCNO (g) ⇌ HNCO (g)

This reaction illustrates only a rearrangement of the atoms, with no change in phase or the number of molecules. As a result, ΔS° will be close to 0

Question 8

2 Al2O3 (s) + 3 C(s) –> 4 Al(l) + 3 CO2 (g)

Is the entropy change for this process positive or negative? Justify your answer.

Answer 8

The entropy change is positive.

There are zero moles of gas in the reactants and 3 moles of gas in the products. Gases are more dispersed/spread out than liquids or solids, and thus have much greater entropy. Therefore, the products have more entropy, so the change in entropy from reactants to products is positive