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:

• 1 Exothermic change - chemical potential energy changes to heat energy
• 2 Endothermic change - heat energy changes to chemical potential energy

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.

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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

• Accurately weigh out approximately 5g of anhydrous copper(II) sulfate (Mr = 159.5) using a weighing bottle
• Measure out 50 ml water into a weighed polystyrene beaker, re-weigh the beaker + water and record the temperature of the water over a couple of minutes
• Quickly add the anhydrous copper(II) sulfate to the water and stir until dissolved. Record the highest temperature reached and, hence, the temperature change, ΔT.
• Repeat the test several times.

Part II

• Accurately weigh out approximately 7.8 g of copper(II) sulfate pentahydrate (Mr = 249.5)
• Measure out 50 ml water into a weighed polystyrene beaker, re-weigh the beaker + water and record the temperature of the water over a couple of minutes
• Quickly add the copper(II) sulfate pentahydrate to the water and stir until dissolved. Record the final temperature reached and, hence, the temperature change, ΔT.
• Repeat the test several times.

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)

Evaluation

The experiment has several inaccuracies that should be discussed:

• 1 heat losses to surroundings by conduction, convection and radiation
• 2 energy used to heat up polystyrene beaker
• 3 specific heat capacity of copper sulfate solution may be different from that of water
• 4 heat used to heat up thermometer
• 5 instrumental inaccuracies

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