Measuring enthalpy changes

Measuring enthalpy changes with calorimetry

  • measure the energy transferred in a reaction as heat using a calorimeter.

  • Combustion calorimetry - used to find the enthalpy change when a fuel is burned. e.g enthalpy change of combustion

  • Solution calorimetry - used to find the enthalpy for reactions occurring i aqueous solution such as enthalpy change of neutralisation

Combustion calorimetry

  1. a weighed fuel sample is burnt underneath a calorimeter contaning a known mass of water

  2. As fuel combusts heat is transferred to the water. A thermometer measures temperature change

  3. Temperature change along with mass and heat capacity of water used to calculate heat energy transferred

Inaccuracy in enthalpy changes

  • Heat loss - heat escaping to the surroundings means less is absorbed by the water, leading to lower measured enthalpy change

  • Incomplete combustion - when fuel doesn’t completely burn, less heat produces leading to lower enthalpy change

  • Fuel evaporation - if some fuel evaporates before burning, it reduces amount of heat generated, leading to decreased the measured the enthalpy change

Solution Calorimetry

  1. Add a measured volume of one reactant to a polystyrene cup and record initial temperature

  2. Add a measured volume of a second reactant and quickly seal container with a lid

  3. Carefully stir the mixture with the thermometer and record temperature at regular intervals

Creating an accurate temperature-time graph

  • During a calorimetry experiment, the temperature fluctuates throughout, increasing rapidly then decreasing gradually

  1. Plot data - Temperature against time

  2. Establish baseline - Draw line of best fit for initial temperature before reactions commences

  3. Post reaction line - After peak reaction temperature is reached, draw another line of best fit at the start of the reaction gives the accurate temperature change

  4. Extrapolation - extend both lines to intersect at reactions start time

  5. True temperature change - The difference between the two extrapolated lines of best fit at start of reaction to gives accurate temperature

Calculating enthalpy change

  • q=mc∆T

  • q - heat change (J)

  • m - mass of water (g)

  • c - specific heat capacity (J/g/K)

  • ∆T - temperature change (K)

Then use

  • ∆H = q/n

  • ∆H - enthalpy change (kJ/mol)

  • q - heat energy (kJ)

  • n - moles of limiting reactant (mol)