Introduction
The law of conservation of energy allows the use of mathematical operations to determine energy changes that cannot be found experimentally.
Reactivity 1.2.1 - Bond-breaking absorbs and bond-forming releases energy.
- Calculate the enthalpy change of a reaction from given average bond enthalpy data.
Guidance
- Include explanation of why bond enthalpy data are average values and may differ from those measured experimentally.
- Average bond enthalpy values are given in the data booklet.
Tools and links
- Structure 2.2 - How would you expect bond enthalpy data to relate to bond length and polarity?
- Reactivity 3.4 - How does the strength of a carbon–halogen bond affect the rate of a nucleophilic substitution reaction?
Reactivity 1.2.2 - Hess’s law states that the enthalpy change for a reaction is independent of the pathway between the initial and final states.
- Apply Hess’s law to calculate enthalpy changes in multistep reactions.
Guidance
Tools and links
Reactivity 1.2.3 - Standard enthalpy changes of combustion, ΔHc⦵, and formation, ΔHf⦵, data are used in thermodynamic calculations. (HL)
- Deduce equations and solutions to problems involving these terms.
Guidance
- Enthalpy of combustion and formation data aregiven in the data booklet.
Tools and links
- Structure 2.2 - Would you expect allotropes of an element, such as diamond and graphite, to have different ΔHf⦵ values?
Reactivity 1.2.4 - An application of Hess’s law uses enthalpy of formation data or enthalpy of combustion data to calculate the enthalpy change of a reaction. (HL)
- Calculate enthalpy changes of a reaction using ΔHf⦵ data or ΔHc⦵ data:
- ΔH⦵ = Σ (ΔHf⦵products) − Σ (ΔHf⦵reactants)
- ΔH⦵ = Σ (ΔHc⦵reactants) − Σ (ΔHc⦵products)
Guidance
- The equations to determine the enthalpy change of a reaction using ΔHf⦵ data or ΔHc⦵ data are given in the data booklet.
Tools and links
Reactivity 1.2.5 - A Born–Haber cycle is an application of Hess’s law, used to show energy changes in the formation of an ionic compound. (HL)
- Interpret and determine values from a Born–Haber cycle for compounds composed of univalent and divalent ions.
Guidance
- 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.
Tools and links
- Structure 2.1 - What are the factors that influence the strength of lattice enthalpy in an ionic compound?
In Reactivity 1.2 - Energy cycles in reactions