1.4.2

Calorimetry

Test yourself

Calorimetry

Calorimetry is used to physically measure changes in enthalpy.

Bomb calorimetry

• Bomb calorimetry uses a machine called a bomb calorimeter to measure enthalpy changes of combustion.
• This process involves burning a sample of a compound in a sealed vessel and measuring the temperature change.
• Often the calorimeter will just determine the temperature change in the vessel and you will have to calculate the enthalpy change of combustion.

Inaccuracy

• Bomb calorimetry can be inaccurate due to:
• Heat lost to the surroundings.
• Any incomplete combustion that may take place.
• Loss of some reactant that evaporates before it combusts.

Calculating enthalpy changes

• The equation to calculate enthalpy changes from temperature changes is:
• q = m × c × ΔT
• q is the heat energy.
• m is the mass of the sample.
• c is the specific heat capacity.
• ΔT is the temperature change.
• If the pressure is constant, q = ΔcH

Calculating enthalpy changes

• We have calculated q, the energy given off to the surroundings (the enthalpy change).
• The units of q are Joules.
• To determine the enthalpy change of combustion, we must convert Joules into the unit of enthalpy change, Joules per mole.
• Calculate the number of moles:
• Moles = mass ÷ Mr
• ΔH = $\frac{q}{moles}$

More Calorimetry

Calorimetry is a very useful technique to determine enthalpy changes.

Different types of calorimetry

• Calorimetry can also be used to measure other enthalpy changes.
• We have considered the combustion reaction that gives off heat.
• We can also measure any reaction that can be done in a sealed vessel.
• For example, you can measure the enthalpy change of neutralisation of an acid/base reaction.
• These reactions take place in a solution that can be inside the vessel.

Accurate temperature changes

• You might think that the best measurements to record are the initial and final temperatures of your calorimeter.
• This is not the case.
• This is because heat is always being lost from the calorimeter so the final and initial temperatures are inaccurate.
• Instead, measure the temperature loss after the reaction is complete and extrapolate a line to find the true value.
• This is shown clearly in the image on the next slide.

Accurate temperature changes

• Line extrapolation is shown above.

Example Calculations - Calorimetry Experiment

Below are some example calculations based on the calorimetry experiment.

Combustion of cyclohexane

• Cyclohexane fuel is burned completely in a calorimeter.
• There are 200 g of water in the calorimeter.
• There are 0.5 moles of cyclohexane burnt.
• The temperature of the water was raised from 298 K to 368 K.

The calculation

• The calculation:
• q = mcΔT
• q = 200 g x 4.18 Jg-1K-1 × 70 K
• q = 58520 Joules
• Enthalpy change of combustion = q ÷ moles
• ΔH = −58520 J ÷ 0.5 moles
• Note the minus sign added. This is because we know the reaction is exothermic since the water's temperature was increased.
• ΔH = −117040 Jmol-1
• ΔH = −117.04 kJmol-1
• Note the final units of kJmol-1 as this is more standard.

Neutralisation reaction

• Calculate the heat lost/gained during the reaction between H2SO4(aq) and NaOH(aq):
• 20 cm3 of the acid is added to an insulate container.
• 30 cm3 of the base is then added.
• The temperature change is recorded to be 40 K.
• Assume the density of the solutions to be the same as water, 1 gcm-3.
• Assume the specific heat capacity is the same as water's, 4.18 Jg-1K-1.

The calculation

• Because we have assumed that the density is the same as water, we can calculate the mass of the solution as:
• 1 cm3 = 1 g
• (20 + 30) cm3 = 50 g
• The heat change:
• q = mcΔT
• q = (50) g x 4.18 Jg-1K-1 x 40 K
• q = 8360 Joules
• q = 8.36 kJ