3.32 AS Physical Chemistry - Enthalpy changes

Enthalpy is that part of the internal chemical energy of a substance that can result in a heat change, either by release or absorption of heat energy. There are two types of enthalpy change:

Although it is not possible to measure the chemical potential energy directly, we can measure the change in heat energy and know that this is due to the same change in chemical potential energy (law of conservation of energy)

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Hess' law

Hess' law is one manifestation of the law of conservation of energy. It says that the overall energy change between two states is always the same, regardless of the route taken between them.

Hess' law allows us to calculate unknow energy changes by choosing a different route between them.


The enthalpy of hydration of copper(II) sulfate

The enthalpy change for the following reaction cannot be measured directly

CuSO4(s) + 5H2O(l) CuSO4.5H2O(s)

However, we can measure the change of energy when both the reactants and the products are dissolved in a large volume of water. Then we can apply Hess' law to calculate the enthalpy change of the reaction above:

CuSO4(s) + nH2O CuSO4(aq)

CuSO4.5H2O(s) + nH2O CuSO4(aq)

Hence, we can construct a Hess cycle to go from CuSO4(s) to CuSO4.5H2O(s) via CuSO4(aq)

Experimental procedure

Note: The masses of compounds chosen are approximately the same number of moles and give approximately the same molarity solution when dissolved

Part I

Part II

Data recording and analysis

All weighings (masses) and readings must be recorded, preferably in a table.

The energy change for each trial can be calculated from the formula:

Enthalpy change = mass of water (kg) x specific heat capacity of water x temperature change.

ΔH = m x c x ΔT

Energy per mole = energy change/ number of moles of salt used

Energy per mole = ΔH/moles

You should obtain an average of the values over all of the trials carried out. This will help to remove random errors.

This can then be used to find out the enthalpy change of the reaction:

CuSO4(s) + 5H2O(l) CuSO4.5H2O(s)

Applying Hess' law:

ΔH(anhydrous salt) - ΔH(hydrated salt) = ΔH(reaction)


The experiment has several inaccuracies that should be discussed:


AS Inorganic Chemistry Experiments

Task Possible context
Make up a volumetric solution The preparation of a standard solution of sodium carbonate
Carry out a simple acid-base titration Determine the concentration of unknown hydrochloric acid by titration
Carry out some inorganic tests Tests for anions

AS Physical Chemistry Experiments

Task Possible context
Measure an enthalpy change Use Hess's law to find an unknown enthalpy change, such as the reaction of anhydrous copper(II) sulfate with water to produce hydrated crystals
Determine the Mr of a volatile liquid or the Mr of a gas Determine the Mr of hexane or the Mr of carbon dioxide
Investigate how the rate of a reaction changes with temperature. Investigate the rate of reaction of sodium thiosulfate with acid at different temperatures

AS Organic Chemistry

Task Possible context
Distil a product from a reaction The preparation of ethanal from the oxidation of ethanol or the preparation of cyclohexene from the dehydration of cyclohexanol
Carry out some organic tests Tests for alkene, alcohol, acid, aldehyde
Investigate the combustion of alcohols Use a calorimetric method to measure the enthalpies of combustion in an homologous series of alcohols