Question

Would you expect \(\Delta G^{\circ}\) for the hydrolysis of a thioester to be (a) large and negative, (b) large and positive,(c) small and negative, or (d) small and positive? Give the reason for your answer.

Short answer

(a) large and negative

Step-by-step solution

- Understand the Reaction

Hydrolysis of a thioester involves breaking a thioester bond in the presence of water, resulting in the formation of a carboxylic acid and a thiol.

- Determine the Nature of the Reaction

Consider whether the hydrolysis of a thioester is spontaneous or not. Hydrolysis reactions are generally exergonic (release energy).

- Evaluate \(\text{Δ}G^{\text{0}}\)

For the hydrolysis of a thioester, the standard Gibbs free energy change (\(\text{Δ}G^{\text{0}}\)) is likely to be large because breaking the thioester bond releases a significant amount of energy.

- Determine the Sign of \(\text{Δ}G^{\text{0}}\)

Since the reaction releases a large amount of energy, the sign of \(\text{Δ}G^{\text{0}}\) is negative, indicating a spontaneous process.

- Conclusion

Combine the insights from previous steps: the hydrolysis of a thioester is highly exergonic, suggesting that \(\text{Δ}G^{\text{0}}\) is large and negative.

Key concepts

Gibbs Free Energy

Gibbs free energy, denoted as \(\text{Δ}G\), is a thermodynamic potential that helps predict whether a reaction will proceed spontaneously. It incorporates both the enthalpy (heat content) and entropy (level of disorder) of a system. The formula for Gibbs free energy is:
\[\text{Δ}G = \text{Δ}H - T\text{Δ}S\]
Where:

• \(\text{Δ}H\): Change in enthalpy
• \(\text{Δ}S\): Change in entropy
• T: Temperature in Kelvin

If \(\text{Δ}G\) is negative, the process is spontaneous. If it is positive, the process isn't spontaneous. In the case of thioester hydrolysis, we are looking at the Gibbs free energy change under standard conditions, denoted as \(\text{Δ}G^{\text{0}}\).

Exergonic Reactions

In an exergonic reaction, energy is released to the surroundings. These reactions have a negative \(\text{Δ}G\), meaning they occur spontaneously. The term 'exergonic' comes from 'exo-' meaning 'out' and 'ergon' meaning 'work'. This literally means 'work out,' signifying that work or energy is being released.
The hydrolysis of a thioester bond is a classic example of an exergonic reaction. Here, breaking the thioester bond releases a significant amount of energy. This release of energy confirms that the process is exergonic. Understanding the nature of exergonic reactions helps us predict that the standard Gibbs free energy change, \(\text{Δ}G^{\text{0}}\), for thioester hydrolysis is negative and large.

Spontaneous Reactions

Spontaneous reactions occur without the input of additional energy from outside the system. They can happen naturally, given the right conditions. Spontaneity in chemical reactions is governed by the Gibbs free energy principle. As mentioned, if \(\text{Δ}G\) is negative, the reaction is spontaneous.
For the hydrolysis of a thioester, which is naturally exergonic, this means the \(\text{Δ}G^{\text{0}}\) is not only negative but also large. This large negative value indicates that the reaction proceeds readily under standard conditions. The spontaneous nature of thioester hydrolysis underscores that the reaction does not need additional energy to occur.

Standard Free Energy Change

Standard free energy change, denoted as \(\text{Δ}G^{\text{0}}\), represents the Gibbs free energy change for a reaction under standard conditions (1 mol/L concentration, 1 atm pressure, and 298 K temperature). This value is crucial for understanding the spontaneity and energy dynamics of a reaction.
In the case of thioester hydrolysis, the \(\text{Δ}G^{\text{0}}\) is large and negative. This indicates that under standard conditions, a considerable amount of energy is released when the thioester bond is broken. Consequently, the reaction is spontaneous and exergonic. Knowing the \(\text{Δ}G^{\text{0}}\) provides insight into the feasibility and behavior of the reaction in a biological or chemical context.