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  • The standard enthalpy of formation (fHo298) for methane, CH

The standard enthalpy of formation (\DeltafHo298) for methane, CH4 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?

 

Option: 1

 the dissociation energy of the hydrogen molecule, H2.


Option: 2

the first four ionisation energies of carbon.


Option: 3

the dissociation energy of H2 and enthalpy of sublimation of carbon (graphite).


Option: 4

the first four ionisation energies of carbon and electron affinity of hydrogen.


Answers (1)

best_answer

Enthalpy of Sublimation -

Amount of enthalpy change to sublimise 1 mole solid into 1-mole vapour at a temperature below its melting point

C_{(s)}\rightarrow C_{(g)}

 

Bond dissociation enthalpy -

It is the average of enthalpy required to dissociate the said bond present in the different gaseous compounds into free atoms in the gaseous state.

\mathrm{H_{2}+Bond\, Energy\rightarrow 2H}

 

The standard eventually of form

C(s)+ 2H_{2} (g)\rightarrow CH_{4} (g)

 

Now, 

Step 1 : C_{(s)}\rightarrow C_{(g)}

Step 2 : \mathrm{H_{2}+Bond\, Energy\rightarrow 2H}

 

Hence dissociation energy of hydrogen and enthalpy of sublimation of carbon is required.

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Gaurav

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