Energetics Booklet

• Enthalpy (H)

  Heat energy at constant pressure
• Enthalpy change (ΔH)

  The heat energy change measured under conditions of constant pressure
• Standard conditions

  100kPa, and a stated temperature eg 298K
• Exothermic reaction

  Heat energy is released into the surroundings
• Endothermic reaction

  Heat energy is absorbed from the surroundings
• Activation energy

  The minimum energy the particles must have to enable the reaction to proceed
• Measuring enthalpy changes

  1. q = mcΔT

  2. Enthalpy change = q/n
• Calorimetry

  • Weighing boat weighed after experiment
  • Strengths and weaknesses of using a graph to find temperature rise
  • Significant procedural error and modification
  • Possible measurement errors
  • Source of enthalpy change in terms of bonding
• Hess's Law

  The enthalpy change in a reaction is independent of the route / path taken and depends only on the initial and final states
• Standard molar enthalpy of combustion (ΔcH⁰)

  The enthalpy change when 1 mole of a substance burns completely in oxygen with all reactants AND products in their standard states at the stated temperature and 100kPa pressure
• Burning twice as many moles would give out twice as much energy so the corresponding numerical value quoted would be different
• State symbols must be correct at the stated temperature when using equations for enthalpy changes
• Standard molar enthalpy of formation

  The enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at the stated temperature and 100kPa pressure
• Enthalpy change of a reaction

  The enthalpy change (heat energy change at constant pressure) under the stated conditions, for the equation shown at the mole ratios expressed in the equation and in the physical states expressed by the state symbols
• General formula for calculating enthalpy change using combustion data


  ∆rH = Σ ∆CHreactants - Σ ∆CHproducts
• Some standard enthalpy changes are difficult to measure directly but can be determined indirectly
• Enthalpy changes of combustion

  Used to calculate enthalpies of reactions
• General formula when using combustion data: ΔrH = Σ ΔCHreactants - Σ ΔCHproducts
• Some standard enthalpy changes are difficult to measure directly but can be determined from standard enthalpies of combustion
• Enthalpy changes of formation


  Used to calculate enthalpies of reactions
• General formula when using formation data: ΔrH = Σ ΔfHproducts - Σ ΔfHreactants
• Bond enthalpies

  The enthalpy change when breaking one mole of a particular bond averaged over a range of compounds in which that bond appears
• Bond enthalpies are always endothermic (positive value of H)
• Equation using bond enthalpies

  ΔrH = Σ bonds broken - Σ bonds made
• Exam tip 1: Values calculated using bond enthalpies may differ from Hess's law due to variation in bond enthalpies in different compounds
• Exam tip 2: Bond energies for diatomic molecules are true values as they have not been averaged
• Mean bond enthalpy

  The enthalpy change when breaking one mole of a particular bond averaged over a range of compounds in which that bond appears