Determination of Enthalpy Changes (OCR A Level Chemistry A): Revision Note

PAG 3: Determination of enthalpy changes

• Calorimetry is a technique used to measure changes in enthalpy of chemical reactions

• A calorimeter can be a polystyrene drinking cup, a vacuum flask or metal can

• Specific heat capacity is the energy needed to raise the temperature of 1 g of a substance by 1 K

• The specific heat capacity of water is 4.18 J g^-1 K^-1

• The energy transferred as heat can be calculated by:

q = m c ΔT

• Where:

  • q = heat energy transferred, J

  • m = mass of water, g

  • c = specific heat capacity, J g^-1 K^-1

  • ΔT = temperature change, K

• There are two types of calorimetry experiments for you to know:

  • Enthalpy changes of reactions in solution

  • Enthalpy changes of combustion

• For both calorimetry experiments, you should be able to:

  • Outline the experiment

  • Process experimental data using calculations or graphical methods

PAG 3.1: Determination of the enthalpy change of neutralisation

• The principle of these calorimetry experiments is to mix stoichiometric quantities of acid and alkali then measure the temperature change over the course of a few minutes

• The apparatus needed to carry out an enthalpy of reaction in solution calorimetry experiment is shown above

Sample method for a neutralisation reaction

1. Using a measuring cylinder, place 25.0 cm³ of 1.0 mol dm^-3 hydrochloric acid into the polystyrene cup

2. Using a measuring cylinder, place 25.0 cm of 1.0 mol dm^-3 sodium hydroxide solution into a separate polystyrene cup 

3. Draw a table to record:

   • The initial temperature of the HCl (aq) and NaOH (aq)

   • The temperature of the combined reaction mixture every half minute up to 9.5 minutes

4. Put a thermometer or temperature probe in each cup, stir, and record the temperature every half minute for 2½ minutes

5. At precisely 3 minutes, add the contents of the two cups together

   • DO NOT RECORD THE TEMPERATURE AT 3 MINUTES

6. Continue stirring and recording the temperature for 6 minutes

• For the purposes of the calculations, the following experimental assumptions are made:

  • That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g^-1 K^-1

  • That the density of the solution is the same as pure water, i.e. 1 g cm^-3

  • The specific heat capacity of the container is ignored

  • The reaction is complete

  • There are negligible heat losses

Temperature correction graphs

• For reactions which are not instantaneous there may be a delay before the maximum temperature is reached

• During that delay the substances may lose heat to the surroundings

  • This means that the true maximum temperature is never reached

• To overcome this problem we can use graphical analysis to determine the maximum enthalpy change

The steps to make a temperature correction graph are:

1. Take a temperature reading before adding the reactants for a few minutes to get a steady value

2. Add the second reactant and continue recording the temperature and time

3. Plot the graph 

4. Extrapolate the cooling section of the graph until you intersect the time at which the second reactant was added

• For the neutralisation experiment, there can be two main possibilities:

  1. The initial temperatures of the HCl (aq) and NaOH (aq) are the same

     • They can both be plotted as the same line

     • The graph can be extrapolated to determine the change in temperature at 3 minutes

     • The subsequent q = m c ΔT calculation can be completed

  2. The initial temperatures of the HCl (aq) and NaOH (aq) are the different

     • The initial temperature of the HCl (aq) is plotted as one line

     • The initial temperature of the NaOH (aq) is plotted as a separate line on the same graph

     • The temperature of the reaction mixture can be plotted

     • Two extrapolations are performed

       • One for the HCl (aq) and another for the NaOH (aq)

     • The subsequent q = m c ΔT calculations for the HCl (aq) and NaOH (aq) are completed

     • Calculating the average of those two values gives the overall enthalpy change of neutralisation

PAG 3.2 Determination of an enthalpy change of reaction by Hess' law

• This practical is an extended variation of PAG 3.1

• Another common experiment for this Hess' law approach is hydrated copper sulfate and anhydrous copper sulfate to form copper sulfate solution

Sample method for a neutralisation reaction

1. Using a measuring cylinder, place 25.0 cm³ of 1.0 mol dm^-3 hydrochloric acid into the polystyrene cup

2. Draw a table to record:

   • The initial temperature of the HCl (aq)

   • The temperature of the combined reaction mixture every half minute up to 9.5 minutes

3. Using a top-pan balance, measure 3.25 g of potassium hydrogen carbonate , KHCO₃ 

4. Put a thermometer or temperature probe in the cup of HCl (aq), stir, and record the temperature every half minute for 2½ minutes

5. At precisely 3 minutes, add the potassium carbonate to the cup of HCl (aq)

   • DO NOT RECORD THE TEMPERATURE AT 3 MINUTES

6. Continue stirring and record the temperature for 6 minutes

7. Plot a temperature correction graph to determine the enthalpy change for the reaction

8. Repeat this experiment using 2.50 g of potassium carbonate, K₂CO₃ 

   • Both experiments can be repeated to find an average temperature changes

9. Use the equation q = m c ΔT to calculate:

   • ΔH₁ (enthalpy change of reaction for KHCO₃ with HCl)

   • ΔH₂ (enthalpy change of reaction for K₂CO₃ with HCl)

10. Apply Hess' law to calculate ΔH_r (enthalpy change for the decomposition of KHCO₃ to K₂CO₃)

    • Using ΔH_r = ΔH₁ - ΔH₂

PAG 3.3 Determination of Enthalpy Changes of Combustion

• The principle here is to use the heat released by a combustion reaction to increase the heat content of water

• A typical simple calorimeter is used to measure the temperature changes to the water

A simple combustion calorimeter

• To complete this experiment, the following steps will need to be completed:

• It is important that you record:

  • The starting temperature

  • The final temperature

  • The starting mass of the spirit burner

  • The final mass of the spirit burner

• Using this information, the mass of the fuel burned during the reaction and the temperature change can be deduced

• This mass of fuel burned can be used to calculate the moles of fuel burned

• The moles of fuel burned can be used to convert q to an enthalpy change

Key points to consider

• Not all the heat produced by the combustion reaction is transferred to the water

  • Some heat is lost to the surroundings

  • Some heat is absorbed by the calorimeter

• To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter

• Shielding can be used to reduce draughts

• In this experiment the main sources of error are

  • Heat losses

  • Incomplete combustion

Practical skills reminder

• These calorimetry-based experiments develop a range of essential skills, including:

  • Accurately measuring temperature changes using a thermometer or probe

  • Measuring mass using a top-pan balance (e.g. for solids or spirit burners)

  • Handling volumes of liquids using measuring cylinders and polystyrene cups

  • Recording and plotting time–temperature data to apply graphical corrections

  • Calculating enthalpy changes using experimental data and the formula
    q = mcΔT

  • Applying Hess' law to derive enthalpy changes indirectly from two reactions

  • Identifying and explaining sources of experimental error, such as heat loss or incomplete combustion