Answer:
[tex]\boxed{\text{a) 1.78 MJ; b) 1.25 MJ}}[/tex]
Explanation:
[tex]\rm CH$_4$ + 2O$_2 \longrightarrow \,$ CO$_2$ + 2H$_2$O; \Delta H = - 890.4 \text{ kJ}$\cdot$\text{mol}$^{-1}$[/tex]
a) Energy from 1 mol
[tex]\Delta_{\text{c}}H^{\circ} = \text{2.00 mol} \times \dfrac{\text{-890.4 kJ}}{\text{1 mol}}=\text{-1780 kJ} = \textbf{-1.78 MJ}\\\\\text{The reaction gives off } \boxed{\textbf{1.78 MJ}}[/tex]
b) Energy from 22.4 g
(i) Convert grams to moles
[tex]n = \text{22.4 g } \times \dfrac{\text{1 mol }}{\text{16.04 g}} = \text{1.399 mol}[/tex]
(ii) Convert moles to energy
[tex]\Delta_{\text{c}}H^{\circ} = \text{1.399 mol} \times \dfrac{\text{-890.4 kJ}}{\text{1 mol}}=\text{-1250 kJ} = \textbf{-1.25 MJ}\\\\\text{The reaction releases } \boxed{\textbf{1.25 MJ}}[/tex]
