Measuring the enthalpy change for a reaction experimentally

• For a reaction in solution we use the following equation

  Energy change = mass of solution × heat capacity × temperature change

  Q(J)=m(g)×Cp(J g-¹K-¹)×delta T (K)
• This equation (Q=m×Cp×deltaT) will only give the energy for the actual quantities used. Normally this value is converted into the energy change per mole of one of the reactants (The enthalpy change of reaction)
• The calorimetric method is one type of experiment is one in which substances are mixed in an insulated container and the temperature rise measured. This could be a solid dissolving or reacting in a solution or it could be two solutions reacting together
• General calorimetric method

  • Washes the equipment (cup and pipettes) with the solutions to be used
  • Dry the cup after washing
  • Put polystyrene cup in beaker for insulation and support
  • Clamp thermometer into place making sure the thermometer bulb is immersed in liquid
  • Measure the initial temperature of the solution or both solutions if 2 are used
  • Transfers reagents to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance
  • Stirs mixture
  • Measures final temperature
• The calorimetric method could be a solid dissolving or reacting in a solution or it could be two solution reacting together
• If the reaction is slow then the exact temperature can be difficult to obtain as cooling occurs simultaneously with the reaction. To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together. We also take the temoerature of the reactants for a few minutes before they are added together to get a better average temperature. If th two reactants are solutions then the temperature of both solutions need to be measured before addition and an average temperature is used

  
• Errors in this method

  • Heat transfer from surroundings (usually loss)
  • Approximation in specific heat capacity of the solutions have the heat capacity of water
  • Neglecting the specific heat capacity of the calorimeter - we ignore any heat absorbed by the apparatus
  • Reaction of dissolving may be incomplete or slow
  • Density of solution is taken to be the same as water