Determination of the enthalpy change

• Procedure


  1. Place approximately 3 g of solid potassium carbonate in a test tube. Accurately weigh the test tube and its contents.

  2. Use a burette to dispense 30 cm3 of 2 mol dm–3 hydrochloric acid into a polystyrene cup, which is supported in a beaker.

  3. Measure the temperature of the acid.

  4. Gradually add potassium carbonate to the acid, stirring all the time and monitoring the temperature of the acid.

  5. Reweigh the empty test tube.

  6. Repeat steps 1–5 using approximately 3.5 g of potassium hydrogencarbonate instead of the potassium carbonate. This time, record the lowest temperature reached.
• What is the objective ?

  to calculate the molar enthalpy change for two reactions and use Hess's Law to determine the enthalpy change for the reactions
• What are the safety precautions ?

  wear eye protection

  avoid skin contact with the reactants and products
• What are the tip to take into consideration?

  assume that the heat capacity of the final solution is the same as the heat capacity of water
• Why is it not possible to measure the enthalpy change for the decomposition of potassium hydrogen carbonate directly?

  
  

  Heat energy must be supplied; hence, the temperature change measured is not solely due to the decomposition.
• Explain why the reactions are conducted in a polystyrene cup rather than a glass beaker

  Polystyrene is a better insulator than glass. Therefore, less heat energy is lost to/gained from the surroundings, and the measured temperature changes are more accurate.