Question

The heats of formaton of $C{O}_2\left(g\right)$ and $H_{2}O\left(I\right)$ are -394 kJ/mole and -285.8 kJ/mole respectively. Using the data for the following combustion reaction, calculate the heat of formation of $C_3{H}_8\left(g\right)$.

$C_3{H}_8\left(g\right)+5O_2\left(g\right)\to 3C{O}_2\left(g\right)+4H_{2}O\left(l\right)$ ; $\Delta H_{rxn}^{\circ }=-2221.6kJ$

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

Answer 1

The correct option is D $-103.6$

The enthalpy change for the reaction is the difference between the total enthalpy of products and the total enthalpy of reactants.

$\Delta H_r=\left[3\right(\Delta H_f{)}_{C{O}_2}+4(\Delta H_f{)}_{H_{2}O}-(\Delta H_f{)}_{C_3{H}_8}]$

$-2221.6=3\times (-394)-4\left(285.8\right)-(\Delta H_f{)}_{C_3{H}_8}$

$(\Delta H_f{)}_{C_3{H}_8}=-103.6kJ/mol$

Hence, the heat of formation of $C_3{H}_8\left(g\right)$ is $-103.6kJ/mol$.