Year 10 Science · Unit 2 · Lesson 18

Investigating Reaction Rates

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Learning Goals

Compare the thiosulfate clock reaction and gas volume methods using scientific criteria.
Analyse real temperature-rate data to calculate rate changes and write a CER conclusion.
Interpret a data table using collision theory to explain observed trends.

Warm Up

Compare two

Complete the table to compare the Na₂S₂O₃ clock reaction method and the gas volume method for measuring reaction rate.

Feature	Na₂S₂O₃ clock reaction	Gas volume method
What is measured
Equipment needed
Main advantage
Main limitation
Safety consideration

Your Turn

Experimental data

A student investigates how temperature affects the rate of the sodium thiosulfate clock reaction (Na₂S₂O₃ + HCl → sulfur precipitate + NaCl + SO₂ + H₂O). She records the time until the X mark is no longer visible through the cloudy solution at five temperatures. The results are shown below.

Temperature (°C)	20	30	40	50	60
Time to obscure X (s)	95	52	29	17	11
Rate (1/time, s⁻¹ × 100)	1.05

(a) Calculate the missing rate values (Rate = 1/time × 100, round to 2 decimal places). Then describe the overall trend in the data.

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(b) Write a CER conclusion for this investigation. Your claim must be specific, your evidence must include at least two data values, and your reasoning must refer to collision theory.

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Wrap Up

In one sentence, explain why a shorter time to obscure the X mark means a faster reaction rate in the thiosulfate clock experiment.