Application of Hess's Law

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Chemistry                                                               Nick Curum

Application of Hess’s Law

Hess’s Law states, that the enthalpy change for a reaction is independent of the route by which the reaction if achieved, provided that the physical states of the reactants and products, and their temperature and pressure, are the same in each case.

  • ‘Organic Chemistry, Energetics, Kinetics and Equilibrium’ by Nelson books.

Objective

When sodium hydrogen carbonate is heated it decomposes to form sodium carbonate, water and carbon dioxide.  

2NaHCO3 (s)                   Na2CO3(s) + H2O(l) + CO2(g)

My objective is to calculate the enthalpy change that occurs during this process and to determine the amount of energy that is gained and/or lost within the chemical system. The enthalpy change for the decomposition of sodium hydrogen carbonate cannot be directly measured.  This is because the excess energy from an external source like a Bunsen burner flame disrupts the result in the change in temperature, which makes it difficult to determine an accurate result when calculating enthalpy.

        If I apply Hess’s Law, where it states ‘that the enthalpy change for a reaction is independent of the route by which the reaction is achieved’ then I must find a method, which allows me to carry out the same reaction but by a different means other than decomposition.

I know that both sodium hydrogen carbonate and sodium carbonate will react with dilute hydrochloric acid:

NaHCO3(s) + HCl(aq)                    NaCl(aq) + H2O(l) + CO2(g)

Na2CO3(s) + 2HCl(aq)                    2NaCl(aq) + H2O(l) +CO2(g)

Sodium Carbonate reacts with dilute hydrochloric acid to form sodium chloride, water and carbon dioxide.  If I compare the reactions of the decomposition of sodium hydrogen carbonate, the reaction of sodium hydrogen carbonate with hydrochloric acid and the reaction of sodium carbonate with hydrochloric acid then a pattern begins to emerge.

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ΔHr

2NaHCO3 (s)                   Na2CO3(s) + H2O(l) + CO2(g)

        ΔH1        2HCl                                                   2HCl                 ΔH2

                        2NaCl(aq) + H2O(l) +CO2(g)

As an alternative to directly calculating the enthalpy change during the decomposition of sodium hydrogen carbonate, represented above as ΔHr, a different approach is required and according to Hess’s Law, ΔHr could be calculated by following a different route, in this case calculating the enthalpy changes that occur during the reactions of ΔH1 (NaHCO3(s) + HCl(aq))  and ΔH2 (Na2CO3(s) + 2HCl(aq)) would allow me to evaluate a reasonable estimate to the enthalpy of ΔHr using the formula below.

ΔHr = ΔH1 - ΔH2

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