Question

1) The heat of combustion for the gases hydrogen, methane and ethane are −285.8, −890.4 and −1559.9 kJ/mol respectively at 298K.

Answer 1

Answer:

The enthalpy of the reaction is 64.9 kJ/mol.

Explanation:

$H_2 + \frac{1}{2}O_2\rightarrow H_2O,\Delta H_1 =-285.8 kJ$..[1]

$CH_4 + 2O_2\rightarrow CO_2 + 2H_2O,\Delta H_2 =-890.4 kJ$..[2]

$C_2H_6 + \frac{7}{2}O_2\rightarrow 2CO_2 + 3H_2O,\Delta H_3= -1559.9 kJ$..[3]

$2CH_4(g)\rightarrow C_2H_6(g) + H_2(g),\Delta H_4=?$..[4]

2 × [2] - [1]- [3] = [4]  (Using Hess's law)

$\Delta H_4=2\times \Delta H_2 -\Delta H_1 -\Delta H_3$

$\Delta H_4=2\times (-890.4 kJ)-(-285.8 kJ) -(-1559.9 kJ)$

$\Delta H_4=64.9 kJ/mol$

The enthalpy of the reaction is 64.9 kJ/mol.