Rates of Reaction – The Effect of Concentration Using the PASCO Colourimeter

The rate of a chemical reaction depends on how frequently particles collide with enough energy to react. One of the easiest ways to explore this relationship is with the reaction between sodium thiosulfate and hydrochloric acid, which produces a cloudy sulfur precipitate. Using a PASCO colourimeter, students can measure this change precisely, transforming a classic “disappearing cross” experiment into a fully quantitative study of reaction kinetics.

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The Experiment

Reaction:

$$\text{Na}_2\text{S}_2\text{O}_3(aq) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{SO}_2(g) + \text{S}(s) + \text{H}_2\text{O}(l)$$

Traditional method:
Students mix sodium thiosulfate and hydrochloric acid, and time how long it takes for a printed cross beneath the flask to disappear as sulfur forms.

With the PASCO colourimeter:

• The colourimeter measures light transmission through the solution at regular intervals.

• As sulfur forms, the solution becomes cloudy, reducing the amount of transmitted light.

• The data are recorded automatically in PASCO Capstone, producing a transmission vs time graph.

By repeating the experiment with different thiosulfate concentrations, students can plot reaction rate against concentration and determine the order of reaction with respect to thiosulfate.

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The Science

As concentration increases, more particles occupy the same volume, leading to more frequent collisions and a faster rate of reaction.

The colourimeter allows students to calculate initial reaction rates objectively, avoiding human error and giving a clear quantitative link between concentration and rate.

$$\text{Rate} = k [\text{Na}_2\text{S}_2\text{O}_3]^n$$

The graph of rate versus concentration reveals whether the reaction is first, second, or zero order with respect to thiosulfate.

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Skills Highlight

• Using PASCO colourimeters for real-time quantitative data

• Measuring reaction rate from absorbance or transmission curves

• Controlling variables: temperature, volume, and acid concentration

• Analysing graphs to interpret reaction order and rate laws

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Why It Works in Teaching

The experiment connects a familiar reaction with advanced data analysis. Students see the transition from a qualitative observation to precise measurement and mathematical modelling — exactly the kind of scientific thinking needed at GCSE and A-Level.