Thermodynamics is concerned with

Options

A. total energy of the system
B. energy changes in a system
C. rate of a chemical change
D. mass changes in nuclear reactions.

Explanation

To solve the question "Thermodynamics is concerned with," we will analyze the given options and determine which one aligns best with the principles of thermodynamics.

### Step-by-Step Solution:

1. **Understanding Thermodynamics**:
- Thermodynamics is the branch of physical science that deals with the relationships between heat and other forms of energy. It focuses on how energy is transferred and transformed in physical and chemical processes.

2. **Reviewing the Options**:
- **Option A: Total energy of the system**: While thermodynamics does consider total energy, it is more focused on changes in energy rather than static total energy.
- **Option B: Energy change in a system**: This option aligns with the core principle of thermodynamics, which is concerned with how energy changes occur within a system, particularly through heat and work.
- **Option C: Rate of a chemical change**: Thermodynamics does not directly deal with the rate of reactions; that is more related to kinetics.
- **Option D: Mass changes in nuclear reaction**: While mass-energy equivalence is a concept in thermodynamics, it is not the primary focus of thermodynamics as a whole.

3. **Applying the First Law of Thermodynamics**:
- The first law of thermodynamics states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (Q) plus the work done on the system (W): ΔU = Q + W. This emphasizes that thermodynamics is fundamentally about energy changes.

4. **Conclusion**:
- Based on the analysis, thermodynamics is primarily concerned with **energy change in a system**. Therefore, the correct answer is **Option B**.

### Final Answer:
**Thermodynamics is concerned with energy change in a system (Option B).**