# To determine the enthalpy change of the thermal decomposition of calcium carbonate.

To determine the enthalpy change of the thermal decomposition of calcium carbonate.

Results:

For CaCO3: T1 = 17

T2 = 19

ΔT= 02

using 2.57g of CaCO3

For CaO:        T1 = 18

T2 = 27

ΔT= 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=ΔT x mass surroundings x specific heat capacity of surroundings

For this calculation, we will assume that the specific heat capacity of HCl is identical to that of water, and that the shc of water is 4.2J/ºC/g. We used 51cm3 of HCl, so the mass of this is taken to be 51g, as 1cm3 of water weighs 1g (and we are assuming that HCl(aq) has the same density as water). So, putting this data into the equation, we get:

E= (-2) x 51 x 4.2

= -428.4J

Then, in order to find the enthalpy change for this reaction, this value should be converted into kJ, and divided by the number of moles of the substance (in this case, calcium carbonate). To find the number of moles used, we divide the mass used by the relative atomic mass of the substance. So, we get:

No moles = Mass ÷ Mr

= 2.57 ÷ 100.08

= 0.026 mol.

Then we convert our energy into kJ rather than joules, getting 0.4284 kJ, and dividing this by the amount of substance we used. So, this comes out as:

ΔH= (E ÷ 1000) ÷ No moles

= (-428.4 ÷ 1000) ÷0.026

= -0.4284 ÷ 0.026

= -16.4769

-16 kJ mol-1

For CaO:

Temperature change = 9ºC

We have to follow exactly the same process and calculations as with calcium carbonate, only we used a different amount of CaO, and the temperature change was different.

So, using the formula

E=ΔT 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

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