Determination of the Enthalpy Change of a Reaction

Determination of the Enthalpy Change of a Reaction

Determine the enthalpy change of the thermal decomposition of calcium carbonate by an indirect method based on Hess' law.

Determination of the Enthalpy Change of a Reaction

Determine the enthalpy change of the thermal decomposition of calcium carbonate by an indirect method based on Hess' law.

Using the proposed method of obtaining results, these values were gathered: Reaction 1: CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

GRAPH

¼ in both cases represents the mean of the data.

Using the equation for enthalpy change: H = mcT

Where: m = Mass of liquid to which heat is transferred to (g)

c = Specific heat capacity of aqueous solution (taken as water = 4.18
J.g-1.K-1)

T = Temperature change (oK)

We can thus determine the enthalpy changes of reaction 1 and reaction
2 using the mean (¼) of the data obtained.

Reaction 1: H = 50 x 4.18 x -2.12

H = -443.08

This value is for 2.51g of calcium carbonate, not 100.1g which is its molecular weight.

Therefore: H = -443.08 x (100.1 / 2.51) = -17670.2 J.mol-1.

H = -17.67 kJ.mol-1.

Reaction 2: H = 50 x 4.18 x -10.3

H = -2152.7

This value is for 1.37g of calcium oxide, not 56.1g which is its relative molecular mass.

Therefore: H = -2152.7 x (56.1 / 1.37) = -88150.7 J.mol-1.

H = -88.15 kJ.mol-1.

Hess' law states that: 1"The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided initial and final conditions are the same."

This means that therefore the enthalpy change of a reaction can be measured by the calculation of 2 other reactions which relate directly to the reactants used in the first reaction and provided the same reaction conditions are used, the results will not be affected.

We have the problem set by the experiment: to determine the enthalpy change of the thermal decomposition of calcium carbonate. This is difficult because we cannot accurately measure how much thermal energy is taken from the surroundings and provided via thermal energy from a
Bunsen flame into the reactants, due to its endothermic nature.
Therefore using the enthalpy changes obtained in reaction 1 and reaction 2 we can set up a Hess cycle:

Thus using Hess' law we can calculate the enthalpy change of reaction
3.

Reaction 3: H = Reaction 1 - Reaction 2

H = -17.67 - (-88.15) = +70.48 kJ.mol-1.

Comparing the value +70.