Module 4.3 Entropy and Gibbs Free Energy

If an endothermic reaction is spontaneous at temperature T₁, but is not spontaneous at temperature T₂, which statement is true?

• ΔH is positive and T₁ is higher than T₂.

• ΔH is positive and T₁ is lower than T₂.

• ΔH is negative and T₁ is higher than T₂.

• ΔH is negative and T₁ is lower than T₂.

Examine the four statements below.

I. A solid changes to a liquid

II. A liquid changes to a gas

III. A molecule is broken into two or more smaller molecules

IV. A reaction occurs that results in an increase in the number of moles of gas

In which of these cases will entropy usually increase?

• I and II

• III only

• III and IV

• I, II, III and IV

The diagram represents the particles in a substance in three different states.

Which of the following applies as the solid is heated and changes to a liquid and then a gas?

• Enthalpy decreases and entropy increases

• Enthalpy decreases and entropy decreases

• Enthalpy increases and entropy increases

• Enthalpy increases and entropy decreases

When potassium metal is placed in a glass tube containing chlorine gas a violent reaction occurs at room temperature without any additional energy supplied.

2K(s) + Cl₂(g) → 2KCl(s)

Which of the following correctly identifies the values for ΔG, ΔS and ΔH for this reaction?

• ΔG > 0, ΔS > 0, ΔH < 0

• ΔG < 0, ΔS < 0, ΔH < 0

• ΔG < 0, ΔS < 0, ΔH > 0

• ΔG > 0, ΔS < 0, ΔH < 0

Examine the following reaction for the formation of ammonia gas (NH₃).

N₂(g) + 3H₂(g) → 2NH₃(g) ΔH^o = -91.8 kJ mol^-1

The current industrial conditions for this reaction. make it non-spontaneous.

Which of the following changes to the conditions will most likely make the reaction spontaneous?

• Decrease the temperature

• Increase the temperature

• Decrease the entropy

• Add a catalyst

Which statement is always true for a system undergoing an exothermic reaction?

• The temperature increases

• The activation energy decreases

• The reaction is spontaneous

• Enthalpy change increases