How is the acid-catalysed iodine–propanone reaction quenched?

By adding the sample to excess sodium hydrogencarbonate, which neutralises the acid catalyst.

Write the equation for the titration of iodine with thiosulfate.

I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

What indicator is used in an iodine titration, and when is it added?

Starch, added near the end-point; blue-black to colourless.

What is measured by a colorimeter?

The absorbance of light by a coloured solution, proportional to concentration.

How is the filter colour chosen for colorimetry?

Use the colour that the solution absorbs most strongly (the complementary colour).

What is a calibration curve in colorimetry?

A graph of absorbance against concentration for solutions of known concentration.

Why is mass change unsuitable for following a reaction that evolves hydrogen?

Hydrogen is so light that the change in mass is too small to measure accurately.

Name two methods of following a reaction that produces a gas.

Measuring the volume of gas in a gas syringe, and measuring the loss of mass on a balance.

What is the initial-rate method?

Separate experiments with different initial concentrations of one reagent, comparing their initial rates to find the order.

What is measured in a clock reaction, and what is proportional to the rate?

The time t for a sudden colour change; the rate is proportional to 1/t.

Why is a clock reaction only an approximation?

It gives an average rate over the time to the colour change, not the rate at t = 0.

What colour change signals the end of an iodine clock?

The solution suddenly turns blue-black as iodine and starch react.

What does a straight-line concentration–time graph show?

The reaction is zero order with respect to that reactant; the gradient is the rate.

Experimental methods for rate dataEdexcel A-Level Chemistry: Flashcards

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Why must samples be quenched in a titrimetric method?

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Why must samples be quenched in a titrimetric method?: To stop the reaction so the composition is fixed at the moment of sampling.
How is the acid-catalysed iodine–propanone reaction quenched?: By adding the sample to excess sodium hydrogencarbonate, which neutralises the acid catalyst.
Write the equation for the titration of iodine with thiosulfate.: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
What indicator is used in an iodine titration, and when is it added?: Starch, added near the end-point; blue-black to colourless.
What is measured by a colorimeter?: The absorbance of light by a coloured solution, proportional to concentration.
How is the filter colour chosen for colorimetry?: Use the colour that the solution absorbs most strongly (the complementary colour).
What is a calibration curve in colorimetry?: A graph of absorbance against concentration for solutions of known concentration.
Why is mass change unsuitable for following a reaction that evolves hydrogen?: Hydrogen is so light that the change in mass is too small to measure accurately.
Name two methods of following a reaction that produces a gas.: Measuring the volume of gas in a gas syringe, and measuring the loss of mass on a balance.
What is the initial-rate method?: Separate experiments with different initial concentrations of one reagent, comparing their initial rates to find the order.
What is measured in a clock reaction, and what is proportional to the rate?: The time t for a sudden colour change; the rate is proportional to 1/t.
Why is a clock reaction only an approximation?: It gives an average rate over the time to the colour change, not the rate at t = 0.
What colour change signals the end of an iodine clock?: The solution suddenly turns blue-black as iodine and starch react.
What does a straight-line concentration–time graph show?: The reaction is zero order with respect to that reactant; the gradient is the rate.

Exam questions on Experimental methods for rate data

A student investigates the rate of reaction between magnesium ribbon and an excess of dilute hydrochloric acid at constant temperature: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The student wants to follow the reaction continuously from the moment the magnesium is added.
Explain why measuring the mass lost by the reaction mixture would be an unsuitable way of following this reaction.2 marks
A student follows the reaction between bromine and methanoic acid, Br₂(aq) + HCOOH(aq) → 2Br⁻(aq) + 2H⁺(aq) + CO₂(g), at constant temperature. Bromine solution is orange-brown and the products are colourless. The student uses a colorimeter.
Describe how the student could use the absorbance readings to decide whether the reaction is first order or zero order with respect to bromine.2 marks
In an iodine clock reaction, hydrogen peroxide oxidises iodide ions in acid solution: H₂O₂(aq) + 2I⁻(aq) + 2H⁺(aq) → I₂(aq) + 2H₂O(l). A small, fixed amount of sodium thiosulfate and some starch are added to each mixture. The thiosulfate reduces the iodine back to iodide ions as soon as it forms, until the thiosulfate has been used up, when the solution suddenly turns blue-black. The student varies only the concentration of iodide ions, keeping the total volume constant. When [I⁻] = 0.010 mol dm⁻³ the colour change takes 80 s, when [I⁻] = 0.020 mol dm⁻³ it takes 40 s and when [I⁻] = 0.030 mol dm⁻³ it takes 27 s.
Explain why 1/t can be used as a measure of the initial rate, and deduce the order of reaction with respect to iodide ions.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).