Summary Notes 3.1.8 - Thermodynamics (A-level only)

Taylor Notes Thermodynamics

Hess’ Law
The enthalpy change for a reaction is independent of the route taken

Standard Enthalpy of Formation
The enthalpy change when one mol of a substance is formed from its constituent elements where all
species are in their standard states and under standard conditions




Examples of Enthalpy of Formation Calculations
2KHCO3(s) K2SO3(s) H2O(l) CO2(g)
2KHCO3(s)  K2SO3(s) + H2O(l) + CO2(g) -1
ΔfH / kJ mol -959 -1146 -286 -394




Standard Enthalpy of Combustion ΔcH
The Enthalpy Change when 1 mol of a substance is completely combusted in oxygen under standard
conditions where all species are in their standard state

C8H18(l) + 25/2O2(g)  8CO2(g) + 9H2O(l)

, Taylor Notes Thermodynamics

Examples of Enthalpy of Combustion Calculation




Mean Bond Dissociation Enthalpy
The Enthalpy Change required to break a gaseous covalent bond into gaseous atoms which is averaged
over a range of molecules

Enthalpy Change = Bonds Broken – Bonds Formed

Examples of a Mean Bond Dissociation Calculation
C-H F-F C-F H–F
CH4 + 4F2  CF2 + 4HF
ΔdisH 413 158 484 562




Calorimetry Calculations
Equations to Use