Determining the Rate Equation of a Reaction

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Chemistry Plan

Determining the Rate Equation of a Reaction

Plan

The aim of this experiment is to determine the rate equation for the reaction between sodium thiosulphate and hydrochloric acid using the order of the reaction with respect to each of the reagents.

HCl (aq) + Na2S2O3 (aq)    →    2NaCl (aq) + SO2 (g) + S (s)

When Hydrochloric acid is added to sodium thiosulphate, a solid ppt of sulphur is formed. I will use this to judge when the reaction has ended.

The rate equation for this reaction is

R = k [HCl]x [Na2S2O3]y

In this equation:

R = the rate of reaction

k = the rate constant

x and y = order of reaction with respect to HCl and Na2S2O3 (e.g. 0, 1, 2 etc…)

[HCl] = the concentration of HCl

[Na2S2O3] = the concentration of Na2S2O3

My experiment will enable me to calculate x and y through plotting the volume against the reciprocal of the time it takes for the ppt to form.

Preliminary work:

The demonstration done by my teacher show that 50 cm3 of thiosulphate and 20 cm3 of deionised water mixed with 5cm3 of hydrochloric acid took 14.93 seconds to form the ppt. I will use this as a guide to decide which set of concentrations I will use to get a reasonable time which I can plot.

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When I implement my plan, I will plot my results. Using the shape of the graph, I will then deduce the order of the reaction. The following graphs show the shapes I should get for each order of a reaction. I am not changing the volume of hydrochloric acid because I want to test the effect of changing the concentration of the sodium thiosulphate to be able to find out the order of the reaction with respect to the sodium thiosulphate.

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