Question

Which of the following processes is exothermic?

• A. \(\text{Na}_{(g)}\rightarrow \text{Na}_{(g)}^{+}\)
• B. \(\text{Cl}_{(g)}\rightarrow \text{Cl}_{(g)}^{-}\)
• C. \(\text{Cl}_{2(g)}\rightarrow \text{Cl}_{(g)}+\text{Cl}_{(g)}\)
• D. \(\text{NaCl}_{(s)}+\text{aq.}\rightarrow \text{Na}_{(aq)}^{+}+\text{Cl}_{(aq)}^{-}\)

Hint:

Gaining an electron by a halogen atom releases energy, while ionisation and bond breaking absorb energy.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
A process is exothermic if it releases energy, that is, its enthalpy change is negative.

Step 2: Analyse each option
Na(g) to Na+(g) is ionisation, which needs energy. Cl2 to 2Cl is bond dissociation, which needs energy. Dissolving NaCl(s) in water absorbs a small amount of heat.

Step 3: Electron gain
Cl(g) to Cl-(g) is electron gain by a halogen. Chlorine has a high negative electron gain enthalpy, so energy is released.

Step 4: Compare the options
Option (B) is exothermic.

Final Answer:
Adding an electron to a gaseous chlorine atom is the exothermic process. \[ \boxed{\text{(B) }\text{Cl}_{(g)}\rightarrow\text{Cl}_{(g)}^{-}} \]