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When H2 gas gets adsorbed on transition metals, hydride formation (chemisorption) takes place. Which of the following statements is NOT correct based on this observation?

1. Greater the surface area of transition metals greater will be the extent of adsorption.
2. The enthalpy change involved will be in the range of 20–40 kJ/mole.
3. The extent of adsorption will increase with increase in temperature.
4. The adsorption will be irreversible in nature.

Correct answer: The extent of adsorption will increase with increase in temperature.

Solution

Chemisorption (hydride formation) is exothermic and becomes less favorable at higher temperatures — the extent of adsorption generally decreases at elevated temperatures (desorption increases). Saying 'the extent increases with increase in temperature' is incorrect. The 20–40 kJ/mol range also corresponds to physisorption, not chemisorption, but the temperature statement is the most classically tested incorrect one.

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