18.3.1 Gibbs Free Energy

Question 1

Gibbs Free Energy

Answer 1

• Spontaneity depends on change in enthalpy (ΔH) and change in entropy (ΔS).
• Gibbs free energy change (ΔG) is a thermodynamic state function that combines enthalpy and entropy, and reflects the spontaneity of a reaction. ΔG = ΔH – ΔDS
• When ΔG < 0, the forward reaction is spontaneous.
When ΔG > 0, the reverse reaction is spontaneous.
When ΔG = 0, the reaction is at equilibrium

Question 2

note

Answer 2

• In order for a process to be spontaneous, it must be
  accompanied by an overall increase in the entropy of the universe. This is stated mathematically as ΔS > q/T.
• At constant pressure, heat (q) is equal to the enthalpy change (ΔH).
• Substituting this into the expression for entropy change of a spontaneous process and rearranging yields the expression ΔH – TΔS < 0 (for a spontaneous process). The expression ΔH – TΔS is the Gibbs free energy change (ΔG).
• Free energy change (ΔG) is a thermodynamic state function that combines enthalpy and entropy, and reflects the spontaneity of a reaction.
• For example, in order for the formation of ice to be a
  spontaneous process, the free energy change must be less than zero.
• The value of ΔH is negative because freezing is an exothermic process (heat is released). The term TΔS is also negative because the system is becoming more ordered, thus the value of ΔS is negative (entropy is decreasing).
• At 273 K, ΔH is equal to TΔS, and the terms cancel each other out, causing ΔG to be zero. The system is at equilibrium. Water freezes at this temperature.
• At temperatures below 273 K, ice formation is a spontaneous process. As the temperature decreases toward zero, the value for TΔS increases toward zero. As TΔS becomes larger than ΔH, the value of ΔG becomes negative, showing that the process is spontaneous.

Question 3

Look at the diagram for the fusion of water (from liquid to solid).

Which statement about Point B is not correct?

Answer 3

At Point B, only solid water is present.

Question 4

Which statement about spontaneous processes is not correct?

Answer 4

In a spontaneous event, the change in entropy of the universe can be negative.

Question 5

Which statement about a reaction is not correct?

Answer 5

A reaction that is exothermic and causes a decrease in the entropy of the system cannot be a spontaneous reaction.

Question 6

Which statement about a reaction in which liquid water freezes is not correct?

Answer 6

ΔS is positive.

Question 7

Which statement about Gibbs free energy is not correct?

Answer 7

If ΔG > 0, then the forward reaction is spontaneous.

Question 8

Look at the diagram for the fusion of water (from liquid to solid).

Which statement about this diagram is not correct?

Answer 8

The value for ΔH changes much more than the value for TΔS as the temperature is lowered or raised.

Question 9

Look at the diagram for the fusion of water (from liquid to solid).

Which statement about this diagram is not correct?

Answer 9

At Point A, the water exists as both solid and liquid phases.

Question 10

Which statement about ΔG, Gibbs free energy change, is not correct?

Answer 10

If ΔG = 0, the reaction is spontaneous and favors the forward process.

Question 11

Which statement about nonspontaneous reactions is not correct?

Answer 11

If the entropy increases and the heat content increases, then the reaction will always be nonspontaneous.

Question 12

Look at the decomposition reaction for sodium bicarbonate
2NaHCO3(s) <==> Na2CO3(s) + H2O(g) + CO2(g)
Suppose that ΔH = 64.5 kJ/mol and ΔS = 0.167 kJ/mol*K. Which statement about this reaction is correct? Assume that ΔH and ΔS are constant for all the given temperatures

Answer 12

At about 386 K, the reaction is at equilibrium.