3.1.4.1 Enthalpy change
Reactions can be endothermic or exothermic.
Enthalpy change (ΔH) is the heat energy change measured
under conditions of constant pressure.
Standard enthalpy changes refer to standard
conditions, i.e. 100 kPa and a stated temperature (e.g. ΔH298Ɵ).
Students should be able to:
• define standard enthalpy of combustion (ΔcHƟ)
• define standard enthalpy of formation (ΔfHƟ).
3.1.4.2 Calorimetry
The heat change, q, in a reaction is given by the
equation q = mcΔT where m is the mass of the substance that has a temperature
change ΔT 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.
Required practical 2: Measurement of an enthalpy
change.
3.1.4.3 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.
3.1.4.4 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 ΔH for reactions in the gaseous phase
• explain why values from mean bond enthalpy calculations
differ from those determined using Hess’s law.