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JEE Main2014ChemistryThermodynamics (C)Actual

The standard enthalpy of formation ( _f H ^ ₂₉₈ ) for methane, CH ₄ is -74.9 ~kJ ~mol ⁻¹ . In order to calculate the average energy given out in the formation of a C - H bond from this it is necessary to know which one of the following?

Options

  1. AThe dissociation energy of the hydrogen molecule, H ₂ .
  2. BThe first four ionisation energies of carbon.
  3. CThe dissociation energy of H ₂ and enthalpy and sublimation of carbon (graphite).
  4. DThe first four ionisation energies of carbon and electron affinity of hydrogen.

Correct answer

A. The dissociation energy of the hydrogen molecule, H ₂ .

Step-by-step solution

To calculate average enthalpy of C - H bond in methane following informations are needed (i) dissociation energy of H ₂ i.e. 1 2 H ₂(g) H ( g ) ; H =x( suppose ) (ii) Sublimation energy of C (graphite) to C (g) C ( graphite ) C (g) ; H =y (Suppose) Given C ( graphite )+2 H ₂( ~g ) CH ₄( ~g ) ; H =75 ~kJ ~mol ⁻¹

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