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