# The Enthalpy Change of the Thermal Decomposition of Calcium Carbonate

Results:

For CaCO3: T1 = 17

T2 = 19

DT= 02

using 2.57g of CaCO3

For CaO: T1 = 18

T2 = 27

DT= 09

using 1.39g of CaO

Analysis:

In order to determine the enthalpy change for the thermal decomposition of calcium carbonate, we must work out the enthalpy changes for both the reactions of calcium carbonate and calcium oxide with hydrochloric acid.

For CaCO3:

Temperature change = 2ºC

To find the enthalpy change of a reaction, we must first work out the amount of energy taken in by the reaction. This is done by using the following formula:

E=DT x mass surroundings x

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E=DT x mass surroundings x specific heat capacity of surroundings

We get:

E= (-9) x 51 x 4.2

= -1927.8J

Then, finding amount of CaO we used, we use the following formula:

No moles = Mass ¸Mr

Getting:

No moles = 1.39 ¸56.08

= 0.025 mol.

Then, having converted the energy intake into kJ (-1.9278 kJ), we put the data into this formula:

DH= (E ¸ 1000) ¸ No moles

Getting:

DH = (-1927.8 ¸ 1000) ¸0.025

= -1.9278 ¸0.025

= -77.112

» -77 kJ mol-1

Now we have found both measured enthalpy changes, we can work out the enthalpy change for the thermal decomposition of calcium carbonate. This is done by following Hess's Law, which states that "The enthalpy change in a reaction is the same, independent of the number of steps taken, provided the reactants and products end up in the same states that they would ordinarily be". It is this principle that is behind this experiment - since we are dissolving both chemicals in HCl, and starting with solid calcium carbonate, calcium oxide and carbon dioxide, we can work out the enthalpy change using this law. This is best done by