Energetics
Endothermic Reaction: Reaction in which the enthalpy of the
products is greater than the enthalpy of the reactants ∆H +ve
e.g. melting, boiling, evaporation, sublimation, evaporation
Exothermic Reaction: Reaction in which the enthalpy of the
products is less than the enthalpy of reactants ∆H -ve .e.g
condensation, freezing
∆H units = kJ mol-1
Standard Conditions : 298K (25oc) and 100kPa
Standard Enthalpy Change: The change in heat energy at constant pressure, measured at standard conditions
Standard Enthalpy of Combustion: Enthalpy change when one mol of a substance is completely burnt in oxygen
under standard conditions, kJ mol-1
Standard Enthalpy of Neutralisation: Enthalpy change when 1 mol of water is produced in a neutralisation
reaction under standard conditions, kJ mol-1
Standard Enthalpy of Formation: Enthalpy change when 1 mol of a compound is formed from its elements
under standard conditions, kJ mol-1
Enthalpy of Reaction: Enthalpy change when the number of moles of substances are as written in the equation,
under standard conditions. kJ
q=mc∆t q=change in energy, joules m=mass in grams of substance that undergoes temperature change
c=specific heat capacity (energy required to raise the temp. of a 1g substance by 1oc (4.2)
∆T= temperature change in 0c or K
Determining enthalpy of neutralisation
Add an exact vol. of a known conc. of alkali into a polystyrene cup and measure initial temp.
Add an exact vol. of known conc. of acid to the polystyrene cup with stirring
Measure the highest temp. reached and calculate ∆T
Calculate temp. change using q=mc∆t
Calculate energy change for 1 mol of water by dividing by the no. of moles of water produced and
then divide by 1000 to get into kJ mol-1
Determining enthalpy of combustion
Measure mass of liquid fuel used (could be change in mass of spirit burner)
Measure initial temp. of a known vol. of water in a small beaker
Allow fuel in spirit burner to burn and to heat the water
Measure the highest temp. reached and calculate temp. change q=mc∆t
Calculate the energy change for 1 mol of fuel by dividing by the number of moles burnt and then
divide by 1000 to get a value in kJ mol-1
Hess’s Law: Enthalpy change of a reaction is independent of the route taken, provided the initial and final
conditions are the same
Conservation of Energy: Energy cannot be created or destroyed, but it can be changed from one form to
another
Average bond enthalpy: Energy required to break 1 mol of a given bond averaged over many compounds
Endothermic Reaction: Reaction in which the enthalpy of the
products is greater than the enthalpy of the reactants ∆H +ve
e.g. melting, boiling, evaporation, sublimation, evaporation
Exothermic Reaction: Reaction in which the enthalpy of the
products is less than the enthalpy of reactants ∆H -ve .e.g
condensation, freezing
∆H units = kJ mol-1
Standard Conditions : 298K (25oc) and 100kPa
Standard Enthalpy Change: The change in heat energy at constant pressure, measured at standard conditions
Standard Enthalpy of Combustion: Enthalpy change when one mol of a substance is completely burnt in oxygen
under standard conditions, kJ mol-1
Standard Enthalpy of Neutralisation: Enthalpy change when 1 mol of water is produced in a neutralisation
reaction under standard conditions, kJ mol-1
Standard Enthalpy of Formation: Enthalpy change when 1 mol of a compound is formed from its elements
under standard conditions, kJ mol-1
Enthalpy of Reaction: Enthalpy change when the number of moles of substances are as written in the equation,
under standard conditions. kJ
q=mc∆t q=change in energy, joules m=mass in grams of substance that undergoes temperature change
c=specific heat capacity (energy required to raise the temp. of a 1g substance by 1oc (4.2)
∆T= temperature change in 0c or K
Determining enthalpy of neutralisation
Add an exact vol. of a known conc. of alkali into a polystyrene cup and measure initial temp.
Add an exact vol. of known conc. of acid to the polystyrene cup with stirring
Measure the highest temp. reached and calculate ∆T
Calculate temp. change using q=mc∆t
Calculate energy change for 1 mol of water by dividing by the no. of moles of water produced and
then divide by 1000 to get into kJ mol-1
Determining enthalpy of combustion
Measure mass of liquid fuel used (could be change in mass of spirit burner)
Measure initial temp. of a known vol. of water in a small beaker
Allow fuel in spirit burner to burn and to heat the water
Measure the highest temp. reached and calculate temp. change q=mc∆t
Calculate the energy change for 1 mol of fuel by dividing by the number of moles burnt and then
divide by 1000 to get a value in kJ mol-1
Hess’s Law: Enthalpy change of a reaction is independent of the route taken, provided the initial and final
conditions are the same
Conservation of Energy: Energy cannot be created or destroyed, but it can be changed from one form to
another
Average bond enthalpy: Energy required to break 1 mol of a given bond averaged over many compounds
