Question

The heats of formation of $C{O}_2\left(g\right)$ and $H_{2}O\left(l\right)$ are $-394kJ/mole$ and $-285.8kJ/mole$ respectively. Using the data for the following combustion reaction, calculate the heat of formation of $C_3{H}_8\left(g\right)$.
$C_3{H}_8\left(g\right)+5O_2\left(g\right)\to 4H_{2}O\left(l\right)$
$\Delta H^{\circ }=-2221.6kJ$.

• A. $212.2$
• B. $-143.3$
• C. $185.4$
• D. $-103.6$

Answer 1

Answer: (D)

$-103.6$

Correct equation is: $C_3{H}_8\left(g\right)+5O_2\left(g\right)⟶3C{O}_2\left(g\right)+4H_{2}O$

Using this we can solve the question:

$\Delta H_{f}C{O}_2=-394kJ/mol$

$\Delta H_f{H}_{2}O=-285.8kJ/mol$

$\Delta H_f{O}_2=O$

$\Delta H_f{C}_3{H}_8=?$

Given $\Delta H^o$ reaction= $-2221.6kJ$

We know:

$\Delta H^{o}reaction=\Delta H_{f}product-\Delta H_{f}reactant$

$\Rightarrow -2221.6=\left[\right(3\times -394)+(4\times -285.8\left)\right]-[\Delta H_f{C}_3{H}_8+O]$

$\Rightarrow -2221.6=-3\times 394-4\times 285.8-\Delta H_f{C}_3{H}_8$

$=-1182-1143.2-\Delta H_f{C}_3{H}_8$

$=-2325.2-\Delta H_f{C}_3{H}_8$

$\therefore \Delta H_f{C}_3{H}_8=-2325.2-2221.6$

$=-103.6kJ/mol$

Hence, option $D$ is the answer.