The time taken for the concentration of a reactant to fall to half its value.

What does a constant half-life show?

The reaction is first order with respect to that reactant.

Which technique suits a reaction that gives off a gas?

Collect the gas in a gas syringe and record its volume against time (or measure the loss in mass).

Which technique suits a reaction involving a coloured reactant?

Colorimetry, with the absorbance proportional to concentration.

Why must a sample be quenched before titrating?

To stop the reaction, so the titre gives the concentration at the time the sample was removed.

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

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

What is the equation for the titration of iodine with thiosulfate?

I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

What is the role of starch in the titration?

Indicator: blue-black colour with iodine, which disappears at the end-point.

What order is iodine in the iodination of propanone?

Zero order. A graph of iodine concentration against time is a straight line.

Why is 1/t a measure of rate in a clock reaction?

The same amount of reaction happens before the colour change in every experiment, so rate is proportional to 1/t.

What is the role of thiosulfate in the iodine clock reaction?

It reacts with iodine as soon as it forms, delaying the blue-black colour until the thiosulfate has been used up.

How is the rate found from a concentration–time graph?

It is the gradient of a tangent drawn at that point on the curve.

Half-life and experimental techniquesEdexcel International A Level Chemistry: Flashcards

Card 1 of 120 of 12 known

Question

Define half-life.

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

Define half-life.: The time taken for the concentration of a reactant to fall to half its value.
What does a constant half-life show?: The reaction is first order with respect to that reactant.
Which technique suits a reaction that gives off a gas?: Collect the gas in a gas syringe and record its volume against time (or measure the loss in mass).
Which technique suits a reaction involving a coloured reactant?: Colorimetry, with the absorbance proportional to concentration.
Why must a sample be quenched before titrating?: To stop the reaction, so the titre gives the concentration at the time the sample was removed.
How is the acid-catalysed iodine–propanone reaction quenched?: By adding the sample to excess sodium hydrogencarbonate solution, which neutralises the acid catalyst.
What is the equation for the titration of iodine with thiosulfate?: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
What is the role of starch in the titration?: Indicator: blue-black colour with iodine, which disappears at the end-point.
What order is iodine in the iodination of propanone?: Zero order. A graph of iodine concentration against time is a straight line.
Why is 1/t a measure of rate in a clock reaction?: The same amount of reaction happens before the colour change in every experiment, so rate is proportional to 1/t.
What is the role of thiosulfate in the iodine clock reaction?: It reacts with iodine as soon as it forms, delaying the blue-black colour until the thiosulfate has been used up.
How is the rate found from a concentration–time graph?: It is the gradient of a tangent drawn at that point on the curve.

Exam questions on Half-life and experimental techniques

A student follows the reaction between propanone and iodine in the presence of an acid catalyst: CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + HI(aq). Iodine solution is brown; the other species are colourless.
Explain why the volume of sodium thiosulfate solution needed to react with a fixed volume of the quenched mixture is proportional to the concentration of iodine, and state the role of starch in the titration.2 marks
The concentration of a reactant Q in solution is measured at intervals at constant temperature. It is 0.640 mol dm⁻³ at 0 s, 0.320 mol dm⁻³ at 90 s, 0.160 mol dm⁻³ at 180 s and 0.080 mol dm⁻³ at 270 s, and the points lie on a smooth curve.
Deduce the order of reaction with respect to Q, and use your answer to find the time taken for the concentration to fall from 0.320 mol dm⁻³ to 0.040 mol dm⁻³.2 marks
A student investigates the reaction H₂O₂(aq) + 2I⁻(aq) + 2H⁺(aq) → I₂(aq) + 2H₂O(l) by an iodine clock method. A small, fixed amount of sodium thiosulfate solution and some starch are added to every mixture. The time t for the blue-black colour of the iodine–starch complex to appear is measured. In experiment 1, [I⁻] = 0.040 mol dm⁻³ and t = 80 s. In experiment 2, [I⁻] = 0.080 mol dm⁻³ and t = 40 s. In experiment 3, [I⁻] = 0.020 mol dm⁻³ and t = 160 s. In all three experiments the concentrations of H₂O₂ and H⁺, the total volume and the temperature are the same.
Explain how this clock reaction works and why 1/t can be used as a measure of the initial rate.3 marks

See the full worksheet

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).