Consider the reactions given below. On the basis of these reactions find out which of the

the algebraic relationships is correct?

(i) C(g) +4H (g) CH4(g); 
Hr =x kJ mol-1
(ii) C(graphite) + 2H2 (g)   CH4; 
Hr  = y kJ mol-1

1. x=y 2. x=2y
3. x>y 4. x<y

Hint: Enthalpy of formation of the compound can be positive or negative
depending on the type of reaction.

Explanation:

In a chemical reaction, the reactant first converts into its atomic state,
then combined, and formed a product.

In the first reaction, C and H are present in their atomic state hence
they will combine and released X amount of energy.

In the second reaction, H2 molecule first converts into atomic hydrogen,
then combined with a carbon atom and formed a product.

The overall Hr for both reactions are as follows:

\({i. \Delta H_r=x=- energy \ of \ the \ bonds \ being \ formed \ (four \ \mathrm{C}-\mathrm{H} \ bond \ formed)}\)
\(ii. \Delta \mathrm{H}_{\mathrm{r}}=\mathrm{y}= \ energy \ of \ the \ bond \ being \ broken \ ( \mathrm{H}-\mathrm{H} \ bond \ break) - \)
\( \ energy \ of \ the \ bonds \ being \ formed \ (four \ \mathrm{C}-\mathrm{H} \ bond \ formed)\)

In both the reaction's energy of the bonds being formed is equal
but in the second reaction, extra energy is used to break the hydrogen molecule bond. Thus, x>y.