In a chemical reaction, total energy that is available for use in other reactions is referred to as :

Options

A. enthalpy
B. energy of activation
C. entropy
D. free energy

Explanation

To answer the question regarding the total energy available for use in other reactions in a chemical reaction, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept of Energy in Chemical Reactions**:
- In chemical reactions, energy plays a crucial role. Different forms of energy are involved, and it's important to identify which type of energy is being referred to in the question.

2. **Evaluating the Options**:
- The question provides four options: Enthalpy, Activation Energy, Entropy, and Free Energy. We need to analyze each option to determine which one fits the definition given in the question.

3. **Analyzing Each Option**:
- **Enthalpy**: This is the total heat content of a system, which includes internal energy and the product of pressure and volume. While it is important in thermodynamics, it does not specifically refer to the energy available for work in reactions.
- **Activation Energy**: This is the minimum energy required to initiate a chemical reaction. It is not the energy available for use in subsequent reactions but rather the energy needed to start a reaction.
- **Entropy**: This is a measure of disorder or randomness in a system. While it is a significant concept in thermodynamics, it does not refer to energy available for work.
- **Free Energy**: Also known as Gibbs Free Energy, this is the energy available to perform work in a system at constant temperature and pressure. It quantifies the usable energy that can be harnessed from a chemical reaction.

4. **Conclusion**:
- After evaluating all the options, the correct answer to the question is **Free Energy**. This is the total energy that is available for use in other reactions.

### Final Answer:
The total energy that is available for use in other reactions in a chemical reaction is referred to as **Free Energy**.