Calculating the enthalpy change of reaction

• The general method

  1. Using q=m×c×delta T to calculate energy change for quantities used
  2. Work out the moles of the reactants used
  3. Divide q BT the number of moles of the reactant not in excess to give delta T
  4. Add a sign and unit (divide by a thousand to convert Jmol-¹ to KJmol-¹
• The heat capacity of water is 4.18 Jg-¹K-¹. In any reaction where the reactants are dissolved in water we assume that the heat capacity is the same as pure water
• Also assume that the solutions have the density of water which is 1gcm-³ e.g 25cm³ will weigh 25g
• Calculate the enthalpy change of reaction for the reaction where 25cm³ of 0.2M CuSO4 as reacted with 0.01mol (excess of Zn) the temp increased 7ºC

  1. Calculate the energy change for the amount of reactants in the test tube Q=m×c×delta T then we substitute the number in Q=25×4.18×7=731.5J (note the mass is the mass of the CuSO4 solution only. Do not include mass of Zn powder)
  2. Calculate the number of moles of the reactant not in excess moles of CuSO4 =conc×volume =0.2×(25/100) = 0.005 moles (If you are told what is in excess then you need to work out the moles of both reactants and work out using a balanced equation which one is in exess
  3. Calculate the enthalpy chnage per mole which is often called delta H (the enthalpy change of reaction) delta H=Q/mol =731.5/0.005=146300Jmol-¹ =146KJmol-¹ to 3 sf
  4. Finally add in the sign to represent the energy change if temp increases the reaction is exothermic and is given a - sign e.g -146KJmol-1