Question

Consider the following reactions, C(s) + O2(g) → CO2(g), \(\Delta\) H = -94\(\text{ kcal}\) 2CO(g) + O2 → 2CO2(g), \(\Delta\) H = -135.2\(\text{ kcal}\) Then, the heat of formation of CO (g) is:

• A. 26.4 kcal
• B. -26.4 kcal
• C. 41.2 kcal
• D. -41.2 kcal

Hint:

Hess's Law allows the calculation of enthalpy change by summing the enthalpy changes of multiple steps to obtain the overall reaction.

Answer 1

The Correct Option is B

Using Hess's Law, we can combine the given reactions to find the heat of formation of CO (g).
The first reaction is the formation of CO\(_2\) from C(s) and O\(_2\).
The second reaction is the formation of CO\(_2\) from CO(g) and O\(_2\). We want to find the heat of formation of CO(g), which involves forming CO(g) from its elements (C and O\(_2\)). Starting with: 1) \( \text{C(s) + O}_2 \text{(g) → CO}_2 \text{(g), } \Delta H = -94 \text{ kcal} \) 2) \( 2\text{CO(g) + O}_2 \text{ → 2CO}_2 \text{(g), } \Delta H = -135.2 \text{ kcal} \) By reversing the second reaction and halving the amounts to align with the desired equation, we get: \[ \text{CO(g) + O}_2 \text{ → CO}_2 \text{(g), } \Delta H = -67.6 \text{ kcal} \] Now, we subtract the two reactions to form the desired reaction: \[ \text{C(s) + O}_2 \text{ → CO(g), } \Delta H = -26.4 \text{ kcal} \]
Thus, the correct answer is (b) -26.4 kcal.