Quick questions on Enthalpy and thermochemistry: TCE Chemistry (Tasmania)

An energy profile plots enthalpy against reaction progress. Reactants and products sit at different enthalpy levels, and the activation energy $E_a$ is the height of the peak above the reactants. For an exothermic reaction the products lie below the reactants ($\Delta H < 0$); for an endothermic reaction the products lie above ($\Delta H > 0$). The vertical gap between reactants and products is $\Delta H$.

Chemists define standard enthalpy changes so values can be compared fairly. The standard enthalpy of combustion ($\Delta H_c^\circ$) is the heat released when one mole of a substance burns completely in oxygen under standard conditions. The standard enthalpy of formation ($\Delta H_f^\circ$) is the heat change when one mole of a compound forms from its elements in their standard states; elements in their standard state are defined as zero. The standard enthalpy of neutralisation is the heat released when an acid and base react to form one mole of water, and for strong acid and strong base it is almost constant (about $-57\ \text{kJ mol}^{-1}$) because the reaction is essentially $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$ in every case.

A school calorimetry result is almost always less exothermic than the data-book value, for predictable reasons: heat is lost to the surroundings and to the apparatus rather than all going into the water; combustion may be incomplete, leaving soot and producing less energy; and some fuel evaporates without burning. Recognising these systematic errors, and stating that they make the measured $\Delta H$ less negative, is frequently worth a mark in TASC questions.