Question

The heat of combustion of methane, carbon monoxide and hydrogen are \(P\), \(Q\) and \(R\) respectively. For the reaction
\[ CH_4 + H_2O \longrightarrow CO + 3H_2 \] the heat of reaction is given by

• A. \(P - Q - 3R\)
• B. \(Q + 3R - P\)
• C. \(P - Q - R\)
• D. \(Q + R - P\)

Hint:

For heat of reaction, use: Heat of reactant combustion − Heat of product combustion.

Answer 1

The Correct Option is A

The heat of reaction can be computed using Hess's law.
We use heats of combustion of reactants and products to derive the heat of the given reaction.
Combustion reactions:
\[ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O (\text{heat } = P) \] \[ CO + \frac{1}{2}O_2 \rightarrow CO_2 (\text{heat } = Q) \] \[ H_2 + \frac{1}{2}O_2 \rightarrow H_2O (\text{heat } = R) \] To obtain the target reaction, we reverse combustion of CO and \(H_2\), and combine with combustion of methane.
Final heat of reaction becomes:
\[ \Delta H = P - Q - 3R \] Thus the correct answer is option (A).