1.4 Energetics

• Enthalpy change


  Heat change in a reaction under constant pressure
• Endothermic reaction


  • Reactions that absorb energy from the surroundings
  • Requires input of heat energy
  • Products higher in energy than reactant so ΔH is positive
  • e.g. thermal decomposition of calcium carbonate, melting/boiling
• Exothermic reaction


  • Reactions that release energy to the surroundings
  • Products low in energy than reactants ΔH is negative
  • e.g. combustion of fuels / oxidations of carbohydrates such as glucose in respiration
• Signs for ΔH for exothermic/endothermic

  • ΔH is POSITIVE for endothermic as the SYSTEM gains energy
  • ΔH is NEGATIVE for exothermic as the system LOSES energy
• Mean bond enthalpy


  The enthalpy needed to break a covalent bond, averaged over different molecules
• Bond making


  • To form bonds energy is released
  • Bonds are made when products are being produced and this is an exothermic process so ΔH is negative
• Bond Breaking


  • To break a bond energy needs to be absorbed
  • Bonds are broken in the reactants and this is an endothermic process so ΔH is positive
• Formula for enthalpy change when working with bond energies

  ΔH = Σ bond energies broken - Σ bond energies made
• Calorimetry (ΔHc of an alcohol)


  1. Used to work out the enthalpy change of combustion

  2. Measure how much of the fuel has burned to work out the mass
• Method to measure ΔHc of an alcohol

  1. Weigh spirit burned before and after combustion

  2. Measure mass (or volume) of water

  3. Water in calorimeter

  4. Burn alcohol to heat water

  5. Measure temperature change in water
• Sources of error in calorimetry

  • Heat Loss
  • Incomplete Reaction
  • Approximation in Heat capacity
  • Density taken to be the same as Water
• We add a lid and a windshield to prevent heat loss in calorimetry
• Error in calorimetry


  • Heat loss
  • Incomplete combustion
  • Heat absorption by the calorimeter
• Calorimetry experiment of a solution

  • Add acid first and then measure the temperature
  • Add alkali, stir and measure the temperature change
• Method for measuring ΔHsoln of MgCl2


  1. Measure _____ cm3 water using ______

  2. Into polystyrene cup

  3. Record initial temperature (min 2 measurement)

  4. Add ___g of MgCl2 (weigh by difference)

  5. Record T for every minute for ____ min
• Prevent heat loss in calorimetry experiment of a solution

  Use a polystyrene cup
• Method for measuring ΔHsoln of MgCl2


  1. Measure _____ cm3 water using ______

  2. Record initial temperature (min 2 measurements)

  3. Add ___g of MgCl2 (weigh by difference)

  4. Record T for every minute for ____ mins

  5. Plot t vs time

  6. Extrapolate line to when solid was added

  7. Find ΔT
• HESS LAW: The total enthalpy of a reaction is independent of the route taken
• Enthalpy of formation


  Enthalpy change when one mole of a substance is produced from its elements under standard conditions
• Enthalpy of combustion


  Enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Trends in enthalpies of combustion in a homologous series: Constant rise as the number of carbon atoms rises. This is because there is a constant amount and type of extra bonds being broken and made
• Standard enthalpy of formation

  The enthalpy change at constant pressure when 1 mole of a compound is formed from its constituent elements with the reactants and products in standard states
• Standard enthalpy of combustion

  The enthalpy change at constant pressure when 1 mole of a compound is completely burned in oxygen under standard conditions of 298K and 100kPa
• State why the value for the standard enthalpy of formation of Mn(s) is zero: Because it is an element
• State why the enthalpy change of formation may not be 0 for an element: It may not be in its standard state
• State the meaning of the term mean bond enthalpy for a bond: The enthalpy change to break 1 mol of covalent bonds arranged over a range of compounds/molecules
• When calculating enthalpy change during the reaction of two solutions, what moles do you use? The moles of the limiting reagent. Might have to use stoichiometry
• Give one reason why titanium cannot be extracted directly from titanium(IV) oxide using carbon: Ti is not produced, TiC (carbide) will be produced (brittle)