# An Experiment to Determine the Enthalpy Change for the Decomposition of Calcium Carbonate.

Shee Wah Wan

An Experiment to Determine the Enthalpy Change for the Decomposition of Calcium Carbonate

Aim: to determine the enthalpy change for the decomposition of calcium carbonate

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

Problem: this experiment cannot be done because many reactions have enthalpy changes that cannot be found directly from a single experiment due to many factors which includes the fact that you cannot measure the amount heat that has been put in from the Bunsen burner.

So in order to determine the enthalpy change for the decomposition of calcium carbonate Hess’s Law needs to be used because it provides a method for the indirect determination of the enthalpy changes.

Hess’s Law is an extension of the First Law of Thermodynamics.

Hess’s Law states that, if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

So I will measure the temperature changes when calcium oxide and calcium carbonate reacts with hydrochloric acid solution. Then I can use a Hess’ Law cycle to calculate the enthalpy changes using the formula:

Heat produced = mass of solution x change in temperature x specific heat capacity of

water

This equation will help me to calculate the enthalpy changes of each experiment.

I then drew a Hess’ Law cycle to illustrate the enthalpy values.

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

H?

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

R2                + 2HCl acid

+ 2HCl acid                     R1

CaCl2(aq)   +      CO2(g)      +      H2O(l)

From here I can clearly see two different routes in which I will conduct my experiments from (route 1 and route 2).

ROUTE 1

Diagram for Route 1

Apparatus Needed

The apparatus, equipment and chemicals that is needed for this experiment is:

1. 100cm³ measuring cylinder
2. 250cm³ beaker
3. Polystyrene beaker and lid
4. A weighing bottle
5. Thermometer

Substances Needed

1. Calcium Carbonate (CaCO3)
2. Hydrochloric acid-HCl (2.0 mol dmˉ ³)

The amount of substance I chose to use was 5.0 grams of calcium carbonate because from the equation

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

The stoichiometry (stoichiometry is simply the molar ratio between any of the chemicals) of the equation which is only concerning the CaCO3 (calcium carbonate) and HCl (hydrochloric acid) is 1:2 ratio.

The Mr of CaCO3 (calcium carbonate) is 100 so I can say 1 mole of CaCO3 (calcium carbonate) is 100 grams but this amount is too big too use so I want to scale it down to 0.05moles which is 5grams of CaCO3 (calcium carbonate) which is an appropriate quantity to use.

So because there is a 1:2 ratio between CaCO3 (calcium carbonate) and the HCl (hydrochloric acid) that means I want 0.1 moles of HCl (hydrochloric acid) so double the amount of CaCO3 (calcium carbonate).

There are 2 moles of HCl (hydrochloric acid) in 1000cm³ so there are 0.1moles of HCl (hydrochloric acid) in 50.0cm³. So I will use 50.0cm³ of HCl (hydrochloric acid).

So in this experiment I would want to use 5.0 grams of CaCO3 (calcium carbonate) and 50.0cm³ of HCl (hydrochloric acid) but I want the HCl (hydrochloric acid) to be in excess so I would want 70.0cm³ of HCl (hydrochloric acid).

Method for Route 1

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

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