Determination of Enthalpy Changes (OCR A Level Chemistry A): Revision Note
Exam code: H432
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
Using a measuring cylinder, place 25.0 cm3 of 1.0 mol dm-3 hydrochloric acid into the polystyrene cup
Using a measuring cylinder, place 25.0 cm of 1.0 mol dm-3 sodium hydroxide solution into a separate polystyrene cup
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
Put a thermometer or temperature probe in each cup, stir, and record the temperature every half minute for 2½ minutes
At precisely 3 minutes, add the contents of the two cups together
DO NOT RECORD THE TEMPERATURE AT 3 MINUTES
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:
Take a temperature reading before adding the reactants for a few minutes to get a steady value
Add the second reactant and continue recording the temperature and time
Plot the graph
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:
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
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
Examiner Tips and Tricks
The exam board recommends:
The reaction of potassium carbonate with hydrochloric acid
The reaction of potassium hydrogen carbonate with hydrochloric acid
This allows the enthalpy change of reaction for the decomposition of potassium hydrogen carbonate to potassium carbonate to be determined

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
Using a measuring cylinder, place 25.0 cm3 of 1.0 mol dm-3 hydrochloric acid into the polystyrene cup
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
Using a top-pan balance, measure 3.25 g of potassium hydrogen carbonate , KHCO3
Put a thermometer or temperature probe in the cup of HCl (aq), stir, and record the temperature every half minute for 2½ minutes
At precisely 3 minutes, add the potassium carbonate to the cup of HCl (aq)
DO NOT RECORD THE TEMPERATURE AT 3 MINUTES
Continue stirring and record the temperature for 6 minutes
Plot a temperature correction graph to determine the enthalpy change for the reaction
Repeat this experiment using 2.50 g of potassium carbonate, K2CO3
Both experiments can be repeated to find an average temperature changes
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)
Apply Hess' law to calculate ΔHr (enthalpy change for the decomposition of KHCO3 to K2CO3)
Using ΔHr = 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
Worked Example
1.023 g of propan-1-ol (M = 60.11 g mol-1) was burned in a spirit burner and used to heat 200 g of water in a copper calorimeter.
The temperature of the water rose by 30 oC.
Calculate the enthalpy of combustion of propan-1-ol using this data.
Answer:
Calculate q
q = m c ΔT
q = 200 g x 4.18 J g–1 K–1 x 30 K = – 25 080 J
Calculate the amount of propan-1-ol burned
moles =
moles = = 0.01702 mol
Calculate ΔH
ΔH =
ΔH = = – 1 473 560 J
ΔH = -1 474 kJ = -1.5 x 103 kJ mol-1
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 = mcΔTApplying Hess' law to derive enthalpy changes indirectly from two reactions
Identifying and explaining sources of experimental error, such as heat loss or incomplete combustion
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