Determination of Enthalpy Changes (OCR AS Chemistry)

• Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
• A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

• There are two types of calorimetry experiments for you to know:
  • Enthalpy changes of reactions in solution
  • Enthalpy changes of combustion
• In both cases you should be able to give an outline of the experiment and be able to 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) and then 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 record the temperature for an additional 6 minutes

• For the purposes of the calculations, some assumptions are made about the experiment:
  • 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 themselves may be losing heat to the surroundings, so that the true maximum temperature is never actually reached
• To overcome this problem we can use graphical analysis to determine the maximum enthalpy change

A temperature correction graph for a metal displacement reaction between zinc and copper sulfate solution. The zinc is added after 4 minutes

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  and extrapolate the cooling part 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
     • In this case, they can both be plotted as the same line, the graph can be extrapolated to determine the change in temperature at 3 minutes and the subsequent  calculation completed
  2. The initial temperatures of the HCl (aq) and NaOH (aq) are the different
     • In this case, the initial temperature of the HCl (aq) is plotted as one line and the initial temperature of the NaOH (aq) are plotted as a separate line on the same graph.
     • The temperature of the reaction mixture can be plotted and then you will do two extrapolations for the HCl (aq) and the NaOH (aq)
     • After completing the subsequent calculations for the HCl (aq) and NaOH (aq), you calculate the average of those two values to get 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
  • The published guidance materials from the exam board specify using the reactions of potassium carbonate with hydrochloric acid and potassium hydrogen carbonate with hydrochloric acid to then determine the enthalpy change of reaction for the decomposition of potassium hydrogen carbonate to potassium carbonate

• • Another experiment that is commonly used 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) and then 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 an additional 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 x c x 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, and the final temperature in order to complete the calculations
• You must also record the starting mass of the spirit burner and the final mass of the spirit burner, so that you can work out the mass of the fuel burned during the reaction
  • This will then be used to calculate the moles, which will be used to convert Q to an enthalpy change in your calculations

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

Answer:

Step 1: Calculate q

• • q = m x c x ΔT
  • q = 200 g x 4.18 J g^–1 K^–1 x 30 K = – 25 080 J

Step 2: Calculate the amount of propan-1-ol burned

• • moles 0.01702 mol

Step 3: Calculate ΔH

• • ΔH – 1 473 560 J = -1 474 kJ = -1.5 x 10³ kJ mol^-1