AQA A Level Thermodynamics
Definitions Questions and Answers
First Ionization energy - Answer-The energy required to remove an electron from each
atom in one mole of gaseous atoms
Mean bond enthalpy - Answer-The enthalpy change when one mole of gaseous
molecules each breaks a covalent bond to form two free radicals, averaged over a
range of compounds
Enthalpy - Answer-The heat content that is stored in a chemical system
Standard enthalpy change of combustion - Answer-The enthalpy change that takes
place when one mole of a substance reacts completely with oxygen under standard
conditions, all reactants and products being in their standard states.
Standard enthalpy change of formation - Answer-The enthalpy change that takes place
when one mole of a compound is formed from its constituent elements in their standard
states under standard conditions.
Enthalpy Cycle - Answer-A diagram showing alternative routes between reactants and
products that allows the indirect determination of an enthalpy change from other known
enthalpy changes using Hess' law.
Enthalpy Profile Diagram - Answer-A diagram for a reaction to compare the enthalpy of
the reactants with the enthalpy of the products
Exothermic Reaction - Answer-The reaction in which the enthalpy of the products is
smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings
Hess's Law - Answer-If a reaction can take place by more than one route and the initial
and final conditions are the same, the total enthalpy change is the same for each route
Second Ionisation Energy - Answer-The energy required to remove one electron from
each ion in one mole of gaseous +1 ions to form one mole of gaseous +2 ions
First electron affinity - Answer-The enthalpy change that takes place when one electron
is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1-
ions
Second electron affinity - Answer-The enthalpy change that takes place when one
electron is added to each ion in one mole of gaseous 1- ions to form one mole of
gaseous 2- ions.
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