Question

$2C\left(s\right)+2O_2\left(g\right)\to 2C{O}_2\left(g\right),\Delta H=-787kJ$

$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(l\right),\Delta H=-286kJ$

$C_2{H}_2\left(g\right)+\frac{5}{2}{O}_2\left(g\right)\to 2C{O}_2\left(g\right)+H_{2}O\left(l\right),\Delta H=-1310kJ$

From the above data, heat of formation of acetylene is:

• A. $+1802kJ$
• B. $-1802kJ$
• C. $-800kJ$
• D. $+237kJ$

Answer 1

The correct option is D $+237kJ$

The thermochemical reactions are as given below.

$2C\left(s\right)+2O_2\left(g\right)\to 2C{O}_2\left(g\right)\Delta H=-787kJ$

$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(l\right)\Delta H=-286kJ$

$C_2{H}_2\left(g\right)+\frac{5}{2}{O}_2\left(g\right)\to 2C{O}_2\left(g\right)+H_{2}O\left(l\right)\Delta H=-1310kJ$

The reaction (3) is reversed and added to reactions (1) and (2) to obtain the reaction for the formation of acetylene which is $2C\left(s\right)+H_2\left(g\right)\to C_2{H}_2\left(g\right)$.
Hence, the heat of formation of acetylene is $-787-286+1310=237$ kJ