Quick questions on Energetics I (Topic 7) - Edexcel A-Level Chemistry

In an exothermic reaction the products sit lower in energy than the reactants, so $\Delta H$ is negative; the activation energy $E_a$ is the hump that must be climbed first. In an endothermic reaction the products sit higher. The difference between reactant and product energy levels is $\Delta H$.

A mean bond enthalpy is the average energy to break one mole of a particular bond in the gas phase, averaged over many compounds. $$\Delta H \approx \sum(\text{bonds broken}) - \sum(\text{bonds formed}).$$ Because the values are averages, the result is only an estimate, and it strictly applies only to gaseous species.

Define the standard enthalpy of formation of a compound. [2 marks]

$0.0125\ \text{mol}$ of a fuel raises the temperature of $200\ \text{g}$ of water by $14.0\ \text{K}$. Calculate $\Delta H_c$. [3 marks]

Using $\Delta H_f^{\ominus}$ values, write the Hess expression for $\Delta H_r$ and explain the sign convention. [3 marks]