Reactivity 1.2 Energy cycles in reactions (HL)
Reactivity 1.2.3 and 1.2.4
Understandings:
  • Standard enthalpy changes of combustion, ΔHc, and enthalpy of formation, ΔHf, data are used in thermodynamic calculations (1.2.3).
  • An application of Hess’s law uses enthalpy of formation data or enthalpy of combustion data to calculate the enthalpy change (1.2.4). 
Learning outcomes:
  • Deduce equations and solutions to problems involving these terms (1.2.3).
  • Calculate enthalpy changes of a reaction using ΔHf data or ΔHc data (1.2.4). 
  • ΔH = ΣΔHc reactants − ΣΔHc products
  • ΔH = ΣΔHf products − ΣΔHf reactants
Additional notes:
  • Enthalpy of combustion and formation data are given in the data booklet. 
  • The above equations are given in the data booklet. 
Linking questions:
  • Structure 2.2 Would you expect allotropes of an element, such as diamond and graphite, to have different ΔH values?
This video covers how to calculate the enthalpy change for a reaction using enthalpy of formation data. 
This video covers how to calculate the enthalpy change for a reaction using enthalpy of combustion data. 
Reactivity 1.2.5
Understandings:
  • A Born–Haber cycle is an application of Hess’s law, used to show energy changes in the formation of an ionic compound.
Learning outcomes:
  • Interpret and determine values from a Born–Haber cycle for compounds composed of univalent and divalent ions.
Additional notes:
  • The cycle includes: ionization energies, enthalpy of atomization (using sublimation and/or bond enthalpies), electron affinities, lattice enthalpy, enthalpy of formation.
  • The construction of a complete Born–Haber cycle will not be assessed.
Linking questions:
  • Structure 2.1 What are the factors that influence the strength of lattice enthalpy in an ionic compound?
This video covers Born-Haber cycles.