enthalpy changes

• Enthalpy change


  Energy transferred between system (chemicals) and surroundings
• Exothermic change


  • Energy transferred from system (chemicals) to surroundings
  • Products have less energy than reactants
  • ∆H is negative
• Endothermic change


  • Energy transferred from surroundings to system (chemicals)
  • Products have more energy than reactants
  • ∆H is positive
• Activation energy


  Minimum energy particles need to collide to start a reaction
• Common oxidation exothermic processes are combustion of fuels and oxidation of carbohydrates like glucose in respiration
• Standard conditions


  • 100 kPa pressure
  • 298 K (room temperature or 25oC)
  • Solutions at 1mol dm-3
  • All substances in their normal state at 298K
• Enthalpy change of reaction (∆rH)

  Enthalpy change when moles of reactants as specified in balanced equation react
• Standard enthalpy change of formation (∆fH)

  Enthalpy change when 1 mole of compound is formed from its elements under standard conditions
• Standard enthalpy change of combustion (∆cH)

  Enthalpy change when 1 mole of substance is combusted completely in oxygen under standard conditions
• Enthalpy change of neutralisation (∆neutH)

  Enthalpy change when solutions of an acid and an alkali react under standard conditions to produce 1 mole of water
• Measuring enthalpy change experimentally (calorimetric method)

  1. Measure mass of solution

  2. Measure heat capacity of solution

  3. Measure temperature change

  4. Calculate energy change

  5. Calculate moles of reactant

  6. Calculate enthalpy change per mole
• Errors in calorimetric method include energy transfer from surroundings, approximation in specific heat capacity, neglecting heat capacity of calorimeter, incomplete reaction, and assuming density of solution is same as water
• Calculating enthalpy change from experimental data

  • Example 1: Reaction of CuSO4 and Zn
  • Example 2: Neutralisation of HCl and NaOH
  • Example 3: Combustion of propan-1-ol
• Calculating enthalpy change of combustion

  1. Calculate energy change to heat up water

  2. Calculate moles of alcohol combusted

  3. Calculate enthalpy change per mole
• Enthalpies of combustion can be calculated by using calorimetry
• Errors in calorimetry method

  • Energy losses from calorimeter
  • Incomplete combustion of fuel
  • Incomplete transfer of energy
  • Evaporation of fuel after weighing
  • Heat capacity of calorimeter not included
  • Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment
• Mean bond enthalpy


  The enthalpy change when one mole of (gaseous covalent) bonds is broken (averaged over different molecules)
• Mean bond enthalpies are used because every single bond in a compound has a slightly different bond energy
• In an exothermic reaction

  More energy is released when making bonds than is absorbed when breaking bonds
• In an endothermic reaction


  More energy is absorbed when breaking bonds than is released when making bonds
• Calculating enthalpy of combustion using mean bond enthalpies


  1. Sum bond energies broken in reactants

  2. Sum bond energies made in products

  3. Enthalpy change = Sum bond energies broken - Sum bond energies made
• Hess's law


  The total enthalpy change for a reaction is independent of the route by which the chemical change takes place
• Hess's law is a version of the first law of thermodynamics, which is that energy is always conserved
• Using Hess's law to determine enthalpy changes from enthalpy changes of formation


  Enthalpy change = Sum of formation enthalpies of products - Sum of formation enthalpies of reactants
• Using Hess's law to determine enthalpy changes from enthalpy changes of combustion


  Enthalpy change = Sum of combustion enthalpies of reactants - Sum of combustion enthalpies of products
• Elements in standard states have formation enthalpy of 0