Gibbs Free Energy & Reaction Profiles

Question 1

Define Gibbs free energy.

Answer 1

𝐺 = 𝐻 − 𝑇𝑆.

Question 2

What is the spontaneity criterion using Δ𝐺 at constant 𝑇 and 𝑃?

Answer 2

Δ𝐺 < 0 indicates a spontaneous (exergonic) process; Δ𝐺 > 0 denotes non-spontaneity (endergonic).

Question 3

Explain why Δ𝐺 = 0 characterizes equilibrium.

Answer 3

No net driving force exists; forward and reverse rates are identical, so the system’s free energy is at a minimum.

Question 4

Describe how Δ𝐺 connects enthalpy and entropy contributions.

Answer 4

A reaction may be driven by exothermicity (Δ𝐻 < 0), entropy increase (Δ𝑆 > 0), or a favorable combination thereof; temperature modulates their relative influence.

Question 5

What vertical axis is typically used in a reaction energy diagram?

Answer 5

Potential energy (often enthalpy) of the system.

Question 6

Define activation energy (𝐸ₐ) in the diagram.

Answer 6

The energy difference between reactants and the transition state; it represents the energetic barrier to reaction.

Question 7

Explain the difference between kinetic and thermodynamic control as revealed by competing energy diagrams.

Answer 7

Kinetic control favors the pathway with the lower 𝐸ₐ (faster), whereas thermodynamic control favors the pathway with the lower final free energy (more stable product).

Question 8

Why does a catalyst not alter Δ𝐺 despite changing the diagram shape?

Answer 8

It lowers activation energy equally for forward and reverse reactions but does not affect reactant or product energies, leaving Δ𝐺 unchanged.