Boardworks AS Chemistry Energetics Teacher notes bomb calorimeter – A type of calorimeter that virtually eliminates heat loss and so provides very accurate values for enthalpies of combustion. bond dissociation enthalpy – The energy required to break a particular bond. calorimetry – A technique used for measuring the amount of heat energy absorbed or given out by a reaction. ΔH – ‘Delta H’, the symbol used to represent enthalpy change. endothermic reaction – A chemical reaction during 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. Given the symbol DH. enthalpy 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 a Hess cycle. enthalpy level diagram – A diagram showing the amount of energy contained in both the reactants and the products for a reaction, and therefore the difference between them. Also called an enthalpy profile diagram. exothermic reaction – A chemical reaction during which heat energy is given out to the surroundings. DH for the reaction is negative. first law of thermodynamics – A law stating that the increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount of energy lost as a result of the work done by the system. It is sometimes stated in the form ‘energy cannot be created or destroyed; it can only change form’. heat – Energy transferred from one body to another due to a temperature difference. 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. mean bond enthalpy – The average (mean) 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. 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 atomization – T he enthalpy change when 1 mole of gaseous atoms of a substance are formed from the substance in its standard state under standard conditions (298 K and 100 kPa). 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). thermodynamics – A branch of study concerned with the transfer of heat and work and the conversion between heat and other forms of energy. In its modern form it is based on four laws, the zeroth, first, second and third laws of thermodynamics.
