Core Practical: Investigating the Effect of Concentration on Rate

This lesson covers the Core Practical for investigating the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid (the disappearing cross experiment) as required by the Edexcel GCSE Combined Science specification (1SC0). You must know the method, variables, how to process results and how to evaluate the experiment.

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Aim

To investigate how changing the concentration of sodium thiosulfate affects the rate of reaction with hydrochloric acid.

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Background

When sodium thiosulfate solution reacts with hydrochloric acid, a yellow precipitate of sulfur is produced, which makes the solution go cloudy.

Equation

sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g) + S(s)

The sulfur produced is an insoluble yellow solid that forms a suspension, making the solution progressively more opaque. By timing how long it takes for the solution to become opaque enough to obscure a cross drawn on paper underneath the flask, we can measure the rate.

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Apparatus