IB Chemistry home > Syllabus 2016 > Practical Scheme of Work > The enthalpy of reaction

Calorimetry can be used to find the energy change in chemical reactions. This is much easier to carry out in aqueous solutions.

Reaction enthalpy can be used in conjunction with Hess' law to determine the enthalpy change of reactions who's energy measurement would otherwise be unfeasible.

The decomposition of calcium carbonate

Calcium carbonate decomposes when heated to give calcium oxide and carbon dioxide.

CaCO3(s) CaO(s) + CO2(g)

It is not realistically possible to measure the enthalpy change of this reaction directly, but using the fact that both calcium carbonate and calcium oxide react directly with hydrochloric acid, a Hess' law cycle can be constructed to give the enthalpy change of decomposition of calcium carbonate.

CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)


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Procedure

Part 1 - reaction of calcium carbonate with hydrochloric acid

Part 2 - reaction of calcium oxide with hydrochloric acid


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Recording the data

Once again all quantitative and qualitative data must be recorded with inaccuracy and instrumental tolerance.

Raw data - part 1

Mass of calcium carbonate used: 4.10 g ± 0.01

Mass of polystyrene beaker and lid: 27.34 g ± 0.01

Initial temperature of acid: 21.5 ºC ± 0.5

Final temperature of reaction mixture: 27.0 ºC ± 0.5

Mass of beaker, lid and products: 81.23 g ± 0.01

Raw data - part 2

Mass of calcium oxide used: 1.90 g ± 0.01

Mass of polystyrene beaker and lid: 27.66 g ± 0.01

Initial temperature of acid: 21.0 ºC ± 0.5

Final temperature of reaction mixture: 41.5 ºC ± 0.5

Mass of beaker, lid and products: 83.71 g ± 0.01


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Data analysis

Calcium carbonate + hydrochloric acid

Moles of calcium carbonate = mass/Mr = 4.10/100 = 0.041 mol

Temperature change = 27.0 - 21.5 = 5.5 ºC

Mass of solution being heated = 81.23 - 27.34 = 53.89 g = 0.0539 kg

Energy change for calcium carbonate + hydrochloric acid = mcΔT = 0.0539 x 4.18 x 5.5 = -1.24 kJ (negative as it's exothermic)

Hence energy per mole = -1.24/0.041 = -30.2 kJ mol-1

Calcium oxide + hydrochloric acid

Moles of calcium oxide used = mass/Mr = 1.90/56 = 0.034 mol

Temperature change = 41.5 - 21.0 = 20.5 ºC

Mass of solution being heated = 83.71 - 27.66 = 56.05 g = 0.0561 kg

Energy change for calcium carbonate + hydrochloric acid = mcΔT = 0.0561 x 4.18 x 20.5 = -4.80 kJ (negative as it's exothermic)

Hence energy per mole = -4.80/0.034 = -141.3 kJ mol-1

Hess' cycle for the decomposition

ΔH1CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

ΔH2CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)

Enthalpy change of decomposition = ΔH1 - ΔH2

Enthalpy change of decomposition = -30.2 - -141.3 = +110.1 kJ


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Treatment of errors and inaccuracies

Calcium carbonate inaccuracies

Mass of calcium carbonate = 4.10 g ± 0.01 = 100 x 0.01/4.10 % = 0.24%

Temperature change = 5.5 ºC ± 1 = 100 x 1/5.5 % = 18.2 %

Mass of heated solution = 53.89 g ± 0.02 = 100 x 0.02/53.89 % = 0.04 %

Total percentage inaccuracy = 18.5 %

Calcium oxide

Mass of calcium oxide used = 1.90 g ± 0.01 = 100 x 0.01/1.90 % = 0.53 %

Temperature change = 20.5 ºC ± 1 = 100 x 1/20.5 % = 4.88 %

Mass of solution being heated = 56.05 g ± 0.02 = 100 x 0.02/56.1 % = 0.04%

Total percentage inaccuracy = 5.5 %

Summed percentage inaccuracy for both experiments = 18.5 + 5.5 = 24%

Hence the enthalpy change for decomposition of calcium carbonate = +110.1 kJ ± 26.4


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Conclusion and evaluation

The experiment has a high percentage inaccuracy with the major contributor being the temperature change for the reaction between calcium carbonate and hydrochloric acid.

The literature value for the decomposition enthalpy is +178 kJ, which is outside the error limits imposed by the instruments used.

This suggests that there are other systematic inaccuracies involved. Heat losses to the environment have not been considered, although the small change in temperature during the reaction should limit this.

One improvement could be to use a larger amount of calcium carbonate with the same volume of hydrochloric acid with a higher concentration. This would lead to a larger temperature change and reduce the percentage error.

Clearly the results above are based on one experiment and repetitions would be needed to reduce random error.


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