Question

What will be the heat of formation of methane if the heat of combustion of carbon is -x kj heat of formation of water is -y kj and heat of combustion of methane is z kj?

Answer 1

Answer : The heat of formation of methane is ( -x-2y+z)

Explanation : Let us write down the chemical equations for the given processes.

1) The combustion reaction of carbon is

$C (s) + O_{2} (g)\rightarrow CO_{2}(g)$   Delta H = -x

2) The formation reaction of water is

$H_{2}(g) + \frac{1}{2}O_{2} (g)\rightarrow H_{2}O(g)$     Delta H = -y

3) The combustion reaction of methane is

$CH_{4}(g) + 2O_{2} (g)\rightarrow CO_{2} (g) + 2H_{2}O(g)$ Delta H = -z

The desired equation is

Heat of formation reaction of methane which is given below.

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

To obtained the desired equation, we will reverse equation (3) and then we will add all the equations.

When we flip the equation, the sign of delta H is reversed.

The new equation is

4) $CO_{2} (g) + 2H_{2}O(g)\rightarrow CH_{4}(g) + 2O_{2} (g)$  Delta H = z

Let's multiply equation 2 by 2 .

The new equation is

5)$2H_{2}(g) + O_{2} (g)\rightarrow 2H_{2}O(g)$     Delta H = -2y

Let's add equation 1, 4 & 5

$C (s) + O_{2} (g)\rightarrow CO_{2}(g)$   Delta H = -x

$2H_{2}(g) + O_{2} (g)\rightarrow 2H_{2}O(g)$     Delta H = -2y

$CO_{2} (g) + 2H_{2}O(g)\rightarrow CH_{4}(g) + 2O_{2} (g)$  Delta H = z

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C(s) + 2H_{2} (g)\leftarrow \rightarrow CH_{4}(g)    Delta H = -x -2y+z

The heat of formation of methane is ( -x-2y+z)