Glossary

bond enthalpy - the average bond dissociation enthalpy for a particular bond in a range of different compounds. Precisely, it is the average enthalpy change for breaking one mole of a particular bond in a range of different compounds in the gas phase.

calorimetry – a technique used for measuring the amount of heat energy absorbed or given out by a reaction.

‘Delta H’ - ΔH – the symbol used to represent enthalpy change.

endothermic reaction – a chemical reaction in which heat energy is taken in from the surroundings. ΔH for the reaction is positive

enthalpy – a measure of the energy stored in (or heat content of) a system. Given the symbol H.

enthalpy change (for a reaction) – the heat energy exchange between the reacting system and its surroundings at constant pressure.

enthalpy level diagram – a diagram showing the amount of energy contained in both the reactants and the products for a reaction, and the difference between them. Also called an enthalpy profile diagram.

exothermic reaction – a chemical reaction in which heat energy is given out to the surroundings. ΔH for the reaction is negative.

Hess cycle - a diagram showing a reaction taking place by two or more alternative routes, all of which involve the same enthalpy change, according to Hess’s law. Also called an enthalpy cycle.

Hess’s Law - a rule stating that the overall enthalpy change for a reaction is independent of the route the reaction takes.

specific heat capacity – the amount of energy required to raise the temperature of one gram of a substance by one degree Kelvin (or Celsius).

standard enthalpy of combustion – the enthalpy change when 1 mole of a substance undergoes complete combustion under standard conditions (298 K and 100 kPa), with all substances being in their standard states.

standard enthalpy of formation – the enthalpy change when 1 mole of a substance is formed from its elements under standard conditions (298 K and 100 kPa), with all substances being in their standard states.

standard enthalpy of neutralization – the enthalpy change when 1 mole of water is produced by the reaction of an acid and a base under standard conditions (298 K and 100 kPa), with all substances being in their standard states.

standard enthalpy of reaction – the enthalpy change for a reaction under standard conditions (298 K and 100 kPa), with all substances being in their standard states.

standard state – the state a substance is in under standard conditions (298 K and 100 kPa).

Practice Exercises