Question

How does the rate of the forward reaction compare to the rate of the reverse reaction for an endergonic reaction? For an exergonic reaction? Explain.

Short answer

Forward reactions are slower in endergonic and faster in exergonic reactions.

Step-by-step solution

Understanding Reaction Types

An endergonic reaction is a chemical reaction where the energy required to initiate the process is greater than the energy released during the reaction. Conversely, an exergonic reaction releases more energy than is required to initiate it.

Rates of Forward and Reverse Reactions in Endergonic Reactions

In an endergonic reaction, the forward reaction is not spontaneous and requires a consistent input of energy. This means that the rate of the forward reaction is generally slower than the reverse reaction, as the reverse reaction tends to occur spontaneously.

Rates of Forward and Reverse Reactions in Exergonic Reactions

For an exergonic reaction, the forward reaction occurs spontaneously, releasing energy, and thus, the rate of the forward reaction is typically faster than that of the reverse reaction, which requires energy to proceed.

Explaining the Relationship

The main reason for the differences in reaction rates is due to the energy profile of the reactions. Endergonic reactions have products with higher free energy than reactants, leading to a non-spontaneous forward reaction, while exergonic reactions have products with lower free energy, leading to a spontaneous forward reaction.

Key concepts

Endergonic Reactions

An endergonic reaction is a type of chemical reaction that absorbs energy from its surroundings. This means it requires external energy input to proceed. It is characterized by a positive change in free energy (\( \Delta G > 0 \)), indicating that the products of the reaction have more energy than the reactants.

• Endergonic reactions are not spontaneous.
• They require a continuous supply of energy to drive the reaction forward.
• Examples include photosynthesis and the synthesis of large molecules like proteins.

In these reactions, because more energy is invested in the reaction than is released, the rate of the forward reaction is typically slower than the reverse reaction. The reverse reaction can often proceed spontaneously due to the release of stored energy, creating a challenge in driving the process forward without consistent energy input.

Exergonic Reactions

Exergonic reactions are the energetic opposite of endergonic reactions. They release energy during the reaction process, resulting in a net loss of free energy (\( \Delta G < 0 \)). The products have less energy than the reactants, leading to the spontaneous occurrence of the reaction.

• Exergonic reactions are considered spontaneous.
• They often occur quickly because they do not require an additional energy input.
• Common examples are cellular respiration and combustion.

In these reactions, the forward reaction rate is generally faster than the reverse reaction. This is because once it starts, it can continue releasing energy which further propels the reaction forward. Due to the spontaneous nature, the reverse reaction needs energy to go backward, making it slower compared to the forward process.

Reaction Spontaneity

Spontaneity in chemical reactions refers to the capacity of a reaction to proceed without any external energy input. The \( \Delta G \) value of a reaction is a key indicator of its spontaneity.

• A negative \( \Delta G \) indicates spontaneous reactions, as seen in exergonic reactions.
• A positive \( \Delta G \) indicates non-spontaneous reactions, common in endergonic reactions.
• Spontaneity does not imply speed; a reaction can be spontaneous but still occur slowly if the activation energy is high.

Understanding reaction spontaneity is crucial for predicting the behavior of chemical processes. While a spontaneous reaction tends to proceed naturally, a non-spontaneous reaction will require continuous energy input to maintain the progress from reactants to products. Thus, by knowing the free energy changes, scientists can determine the likelihood and conditions necessary for different reactions to occur.