Question:
Which of the following physical transformations is an exothermic?
Which of the following physical transformations is an exothermic?
Show Hint
Check which of the four processes releases energy instead of absorbing it.
Updated On: Nov 12, 2026
- Enthalpy of ionisation of one mole of gaseous sodium atom.
- Electron gain enthalpy of one mole of gas phase atoms of chlorine.
- Enthalpy of atomization of one mole of methane gas.
- Enthalpy of solution of one mole of sodium chloride in specified amount of water.
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept
An exothermic change releases heat, so its enthalpy change is negative.
Step 2: Check each option
Ionisation of gaseous Na needs energy, so it is endothermic.
Atomisation of methane needs energy to break the C-H bonds, so it is endothermic.
Dissolving NaCl in water is slightly endothermic, because the lattice enthalpy is a little larger than the hydration enthalpy.
Step 3: Electron gain
When a gaseous chlorine atom gains an electron, it attains the stable noble gas configuration and releases energy. This change is exothermic.
Final Answer:
The exothermic change is the electron gain of gaseous chlorine atoms, which is option (B). \[ \boxed{\text{(B) }\text{Electron gain enthalpy of one mole of gas phase atoms of chlorine}} \]
An exothermic change releases heat, so its enthalpy change is negative.
Step 2: Check each option
Ionisation of gaseous Na needs energy, so it is endothermic.
Atomisation of methane needs energy to break the C-H bonds, so it is endothermic.
Dissolving NaCl in water is slightly endothermic, because the lattice enthalpy is a little larger than the hydration enthalpy.
Step 3: Electron gain
When a gaseous chlorine atom gains an electron, it attains the stable noble gas configuration and releases energy. This change is exothermic.
Final Answer:
The exothermic change is the electron gain of gaseous chlorine atoms, which is option (B). \[ \boxed{\text{(B) }\text{Electron gain enthalpy of one mole of gas phase atoms of chlorine}} \]
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