Select equation having endothermic step:

Options

A. `S^(-)(g) to S^(2-)(g)`
B. `Na^(+)(g) +Cl^(-)(g) to NaCl(s)`
C. `N(g) to N^(-)(g)`
D. `Al^(2+)(g) to Al^(3+)(g)`

Answer

To determine which equation has an endothermic step, we need to analyze the given options based on the concepts of endothermic and exothermic processes.

### Step-by-Step Solution:

1. **Understand Endothermic Processes**:
- An endothermic process is one that absorbs energy from its surroundings. This means that energy is required for the reaction to proceed.

2. **Analyze Each Option**:
- **Option 1: S- to S-2**:
- This process involves the addition of an electron (gaining an electron). The energy released during this process is known as Electron Gain Enthalpy. Since energy is released, this is an exothermic process.

- **Option 2: Na+ + Cl- → NaCl**:
- This reaction involves the formation of an ionic compound from its ions. The energy released when cations and anions come together is called lattice energy. Since energy is released, this is also an exothermic process.

- **Option 3: N- to N-**:
- Similar to Option 1, this involves gaining an electron. Again, energy is released, making this an exothermic process.

- **Option 4: Al+2 to Al+3**:
- This process involves the removal of an electron from Al+2 to form Al+3. Removing an electron requires energy, which is known as ionization energy. Since energy is absorbed for this process to occur, it is an endothermic process.

3. **Conclusion**:
- The only option that involves an endothermic step is **Option 4: Al+2 to Al+3**.

### Final Answer:
The equation having an endothermic step is **Option 4: Al+2 to Al+3**.