Question

The standard enthalpy of formation of $C_2H_{4(g)}, CO_{2(g)}$ and $H_2O_{(l)}$ are $52, -394$ and $-286\, kJ\, mol^{-1}$ respectively. Then the amount of heat evolved by burning $7\, g$ of $C_2H_{4(g)}$ is

• A. $1412\, kJ$
• B. $9884\, kJ$
• C. $353\, kJ$
• D. $76\, kJ$

Answer 1

Answer: (C)

$353\, kJ$

Answer 2

Enthalpy of combustion, is the heat energy that is absorbed or released when one mole of a substance undergoes complete combustion in the presence of excess oxygen under the standard conditions.

Given 2C+2H→C2H4 (g) △H1 = 52KJ mol-1

C+O2→ CO2 (g) △H2 = -394 KJ mol-1

H2 + $\frac{1}{2}$O2 → H2O (g) △H3 = -286 KJ mol-1

For Combustion of C2H4

C2H4 + 3O2 → 2CO2 +2H2O

For △ H → 2 [△H2+△H1] - [△H1]

= 2 [-394+(-286)]- 52

= -1360 - 52

= -1412 KJ mol-1