A-Level Chemistry AQA 7405

Enthalpy change

Reactions can be endothermic or exothermic.

Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.

Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature (eg ∆H₂₉₈^Ɵ).

Students should be able to:

• define standard enthalpy of combustion (∆_cH^Ɵ)
• define standard enthalpy of formation (∆_fH^Ɵ).

Calorimetry

The heat change, q, in a reaction is given by the equation q = mc∆T

where m is the mass of the substance that has a temperature change ∆T and a specific heat capacity c.

Students should be able to:

• use this equation to calculate the molar enthalpy change for a reaction
• use this equation in related calculations.

Students will not be expected to recall the value of the specific heat capacity, c, of a substance.

Applications of Hess’s law

Hess’s law.

Students should be able to use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation.

Bond enthalpies

Mean bond enthalpy.

Students should be able to:

• define the term mean bond enthalpy
• use mean bond enthalpies to calculate an approximate value of ∆H for reactions in the gaseous phase
• explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law.

Required practical 2

Measurement of an enthalpy change.