Chemical Thermodynamics Question 347
Question: The standard enthalpy of formation $ ({\Delta_{f}}H^{o}_{298}) $ for methane, $ CH_4 $ is $ -74.9kJmo{{l}^{-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:
A) The dissociation energy of the hydrogen molecule, $ H_2 $ .
B) The first four ionisation energies of carbon.
C) The dissociation energy of $ H_2 $ and enthalpy of sublimation of carbon (graphite).
D) The first four ionisation energies of carbon and electron affinity of hydrogen.
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Answer:
Correct Answer: C
Solution:
- To calculate average enthalpy of $ C-H $ bond in methane following in formations are needed (i) (i) dissociation energy of $ H_2 $ i.e. $ \frac{1}{2}H_2(g)\xrightarrow{{}}H(g);\Delta H=x(suppose) $ (ii) Sublimation energy of C(graphite) to C(g) $ C( graphite )\xrightarrow{{}}C( g );\Delta H=y( Suppose ) $ Given $ C(graphite)+2H_2(g)\xrightarrow{{}}CH_4(g); $
$ \Delta H=75kJmo{{l}^{-1}} $
