Aqa A level Chemistry Energetics
Definitions and Formulas Questions
and Answers
Standard Enthalpy Of Formation - answerThe enthalpy change that takes place when
one mole of a compound is formed from its constituent elements under standard
conditions; all products and reactants in their standard states
Standard Enthalpy Of Combustion - answerThe enthalpy change that takes place when
one mole of a compound completely reacts with oxygen under standard conditions with
all reactants and products being in their standard states
Bond Enthalpy - answerThe energy required to break one mole of gaseous bonds to
form gaseous atoms
Mean Bond Enthalpy - answerThe average enthalpy change required to break or form
one mole of a covalent bond in the gaseous state, that has been averaged over many
different compounds
Standard Enthalpy Change - answerThe heat energy change at constant pressure and
under standard conditions
Standard Conditions - answer298K and 100kPa
Standard Enthalpy Of Reaction - answerThe enthalpy change for a chemical reaction
under standard conditions and all substances in their standard states
Hess' Law - answerThe total enthalpy change of a reaction is independent of the route
taken
Formula to use when provided with (mean) bond enthalpies - answer∆Hr = Σ total
energy needed to break all bonds in reactants - Σ total energy release to form new
bonds in products
If you are provided with Combustion Data (∆cH) - answer∆Hr = Σ∆cH of reactants -
Σ∆cH of products
If you are provided with Formation Data (∆fH) - answer∆Hr = Σ∆fH of products - Σ∆fH of
reactants
Definitions and Formulas Questions
and Answers
Standard Enthalpy Of Formation - answerThe enthalpy change that takes place when
one mole of a compound is formed from its constituent elements under standard
conditions; all products and reactants in their standard states
Standard Enthalpy Of Combustion - answerThe enthalpy change that takes place when
one mole of a compound completely reacts with oxygen under standard conditions with
all reactants and products being in their standard states
Bond Enthalpy - answerThe energy required to break one mole of gaseous bonds to
form gaseous atoms
Mean Bond Enthalpy - answerThe average enthalpy change required to break or form
one mole of a covalent bond in the gaseous state, that has been averaged over many
different compounds
Standard Enthalpy Change - answerThe heat energy change at constant pressure and
under standard conditions
Standard Conditions - answer298K and 100kPa
Standard Enthalpy Of Reaction - answerThe enthalpy change for a chemical reaction
under standard conditions and all substances in their standard states
Hess' Law - answerThe total enthalpy change of a reaction is independent of the route
taken
Formula to use when provided with (mean) bond enthalpies - answer∆Hr = Σ total
energy needed to break all bonds in reactants - Σ total energy release to form new
bonds in products
If you are provided with Combustion Data (∆cH) - answer∆Hr = Σ∆cH of reactants -
Σ∆cH of products
If you are provided with Formation Data (∆fH) - answer∆Hr = Σ∆fH of products - Σ∆fH of
reactants
