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Calorimetry

Chemistry: AQA A Level, topic 1.4.4

Calorimetry

Calorimetry is used to physically measure changes in enthalpy.

Bomb calorimetry

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

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

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

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 = qmoles\frac{q}{moles}

More Calorimetry

Calorimetry is a very useful technique to determine enthalpy changes.

Different types of calorimetry

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

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

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

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

  • 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

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

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

Test yourself on Calorimetry

  • Which of the following does bomb calorimetry measure?

  • Why are the initial and final temperatures in a calorimeter inaccurate?

  • We can only convert the mass of a solution to a volume if we know which of the following?

  • What is the only case where we can state that a reaction mixture has the same density as water or the same specific heat capacity as water?

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