Quick questions on Thermochemistry - WJEC A-Level Chemistry

WJEC expects precise definitions, each "per mole" and under standard conditions. The standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states. The standard enthalpy of combustion is the enthalpy change when one mole of a substance burns completely in oxygen. The standard enthalpy of neutralisation is the enthalpy change when an acid and base react to form one mole of water.

Which Hess cycle you draw depends on the data given. With formation enthalpies, arrows point up from the elements to both reactants and products, so $\Delta H = \Sigma \Delta H_f(\text{products}) - \Sigma \Delta H_f(\text{reactants})$. With combustion enthalpies, arrows point down to the combustion products, so the formula reverses: $\Delta H = \Sigma \Delta H_c(\text{reactants}) - \Sigma \Delta H_c(\text{products})$. Matching the cycle to the type of data given is the single most important decision in these calculations.

State whether a reaction with $\Delta H = -286$ kJ mol$^{-1}$ is exothermic or endothermic. [1 mark]

Write the expression for the heat transferred in a calorimetry experiment. [1 mark]

State why bond-enthalpy calculations give only approximate values. [1 mark]

Define the standard enthalpy of formation. [1 mark]

State the Hess's law expression for $\Delta H$ using combustion data. [1 mark]