Question

The heats of formation 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 $C_2{H}_2\left(g\right)$.
$2C_2{H}_2\left(g\right)+5O_2\left(g\right)\to 4C{O}_2\left(g\right)+2H_{2}O\left(I\right)$
$\Delta H^{\circ }=-2601kJ$

• A. $-238.6$
• B. 253.2
• C. 238.7
• D. 226.7

Answer 1

The correct option is D 226.7

The enthalpy change for the reaction is the difference between the total enthalpy of products and total enthalpy of reactants.
$\Delta H_r=\left[4\right(\Delta H_f{)}_{C{O}_2}+2(\Delta H_f{)}_{H_{2}O}-2(\Delta H_f{)}_{C_2{H}_2}]$
$-2601=-4\left(394\right)-2\left(285.8\right)-2(\Delta H_f{)}_{C_2{H}_2}$
$(\Delta H_f{)}_{C_2{H}_2}=226.7$
Hence, the heat of formation of $C_{2}H\left)2\right(g)$ is 226.7 kJ.
Note: The enthalpies of formation of elements in their standard states are considered to be zero. Hence, the enthalpy of $O_2\left(g\right)$ do not appear in the above expression.