Calorimetry“heat measuring”

Energy changes can be calculated by measuring temperature changes in a

calorimetre.

Calorimetres

At their simplest a polystyrene cup and lid.

Polystyrene is an insulator and cuts heat losses to the environment.

A thermometre measures heat changes during a reaction.

Data is substituted into the formula;

Energy = M C ΔTwhere;

M = mass

C = specific heat capacity

Δt = change in temperature.

Mass• NB The mass used is not the mass of the

solid reactants.• It is that of the solution in which the reaction

takes place. • The mass of a solution can be calculated

using the formula;• Density = mass / volume.• Assume that the solution has the same

density as pure water.• 1 g/cm3 at 298 K.• Eg; 25 cm3 will have a mass of 25 x 1 = 25 g

Specific heat capacity

• The energy, in joules, required to raise the temperature of 1g of a substance by 1K.

• Units J/g/K

• Alternatively the energy, in kilojoules, required to raise the temperature of 1kg of a substance by 1K.

• Units kJ/kg/K

• For water it is 4.2 J/g/K (or 4.2 kJ/kg/K)

Eg; To raise 1g of water by 1oC requires;

4.18 Joules

So to make 5g hotter by 1 oC needs;

5 x 4.18 = 20.9 Joules

Why are our winters (usually) mild?

Our British climate is maritime, and water needs more energy to heat up than does land.

So maritime climates are milder than continental.

Other substances have very different specific heat capacities.

Eg Copper c = 0.385 J/g/K

Copper requires a lot less energy to heat up, which explains why it is used for boilers and piping.

Change in temperature

• Δt can be simply calculated by subtracting the highest temperature reached during a reaction from the initial temperature.

• Δt = T2 – T1

• But this assumes that all heat is released (or taken up) simultaneously.

• In practice the temperature change occurs more gradually.

• Δt is better estimated by monitoring the temperature over a period of time, plotting a graph and extrapolating.

Energy change into ΔH

• To convert the energy change, calculated from m c Δt, into an enthalpy change divide by the number of moles of the reactant that is not in excess.

• ΔH = mcΔt / n • Units; kJ/mol • NB Signs! If the temperature rises the

reaction is exothermic and ΔH is –ve.• If the temperature falls it is

endothermic and ΔH is +ve

Oxygen enters

Sample burnt in combustion chamber

Waste gases removed by suction pump through a copper coil.

Stirrer evens out water temperature.

Water absorbs the heat released.

Flame calorimetre

Bomb calorimetre

Firing leads are used to ignite a sample.

The sample burns in pure oxygen at 25atm.

An insulated water jacket surrounds

the chamber.

After each test the machine is calibrated electrically to find the energy required to generate the same Δt.