16.1.2 Measuring rates and half-lives Quiz: Pearson Edexcel Chemistry, Unit 16

The 20 questions

1. What is the half-life of a reaction?

   • The time taken for the concentration of a reactant to fall to half of its initial value
   • The time taken for the reaction mixture to reach a constant colour, showing that equilibrium has been reached
   • The time taken for the concentration of a product to double from its starting value
   • The time taken for the rate of reaction to fall to half of its value at the start of the reaction

2. For a first-order reaction, which statement about the half-life is correct?

   • It is inversely proportional to the square of the initial concentration
   • It increases steadily as the reactant is used up during the reaction
   • It is constant and does not depend on the initial concentration of the reactant
   • It doubles each time the initial concentration of the reactant is halved

3. Which technique is most suitable for following the rate of a reaction in which a coloured species is formed?

   • Weighing the reaction flask on a balance sensitive to changes of 0.01 g
   • Measuring the melting point of the reaction mixture at regular intervals
   • Measuring the boiling point of a sample taken from the reaction mixture at regular intervals
   • Colorimetry, measuring the absorbance of light passing through the reaction mixture over time

4. In the initial-rate method, what is changed between experiments?

   • The initial concentration of one reagent, with the other reagents kept at the same concentrations
   • The total volume of the reaction mixture, while each reagent concentration is kept at the same ratio
   • The initial concentration of every reagent, changed together by the same factor in each experiment
   • The temperature of the reaction, while every reagent concentration is kept the same in each run

5. What is the purpose of a clock reaction in rate experiments?

   • To calculate the activation energy directly from one run by measuring the temperature change at the endpoint
   • To measure the time taken for a fixed amount of product, or a colour change, to appear, giving an approximate initial rate
   • To identify whether a species acts as a catalyst by comparing its mass before and after the reaction is complete
   • To measure the mass of solid product left once the reaction has finished and the flask is fully dried out

6. Which expression defines the rate of a reaction?

   • The total amount of product formed divided by the total mass of the reactants
   • The change in concentration of a reactant or product divided by the time taken for that change
   • The number of collisions per second between all molecules in the mixture
   • The energy change of the reaction divided by the number of moles reacting

7. In the iodine-propanone titration, why is each sample cooled or diluted before titrating?

   • To increase the rate of the reaction so that the iodine is used up before the titration is carried out
   • To make the solution alkaline so that the starch indicator changes colour at the end of the reaction
   • To convert the iodine into iodide ions so that the titration can be carried out in acid conditions
   • To slow or stop the reaction so the titre reflects the concentration at the moment the sample was taken

8. A first-order reaction has a rate constant k = 0.0462 s-1. What is its half-life?

   • 30.0 s
   • 7.50 s
   • 15.0 s
   • 21.6 s

9. A first-order reaction has a half-life of 40 s. How long does it take for the concentration to fall from 0.80 to 0.10 mol dm-3?

   • 60 s
   • 120 s
   • 80 s
   • 160 s

10. A zero-order reaction has a concentration-time graph. Which shape and gradient describe it?

    • A straight line with a negative gradient whose magnitude equals the rate constant k
    • A straight line with a positive gradient that passes through the origin
    • An exponential decay curve with a constant half-life for every interval
    • A curve whose gradient doubles each time the concentration halves

11. On a concentration-time graph, how is the rate at a given time found?

    • Measuring the horizontal distance from that point along the curve to the time axis and dividing it by the total time
    • Drawing a tangent to the curve at that time and taking the gradient of the tangent, ignoring the sign
    • Dividing the total change in concentration over the whole reaction by the total time taken for the reaction to finish
    • Reading the concentration at that time from the graph and then dividing this value by two to find the rate

12. A first-order reaction has [A]0 = 0.200 mol dm-3 and k = 0.0100 s-1. What is [A] after 100 s?

    • 0.121 mol dm-3
    • 0.0736 mol dm-3
    • 0.100 mol dm-3
    • 0.0368 mol dm-3

13. A first-order reaction takes 60 s for the concentration to fall from 1.00 to 0.25 mol dm-3. What is its half-life?

    • 15 s
    • 20 s
    • 60 s
    • 30 s

14. A first-order reaction has a half-life of 25 s. What is its rate constant?

    • 0.693 s-1
    • 2.77 s-1
    • 0.0400 s-1
    • 0.0277 s-1

15. A student plots concentration against time and obtains a straight line with a gradient of -0.030 mol dm-3 s-1. What is the rate of reaction?

    • 0.030 mol dm-3 s-1
    • 33 mol dm-3 s-1
    • 0.30 mol dm-3 s-1
    • 0.0030 mol dm-3 s-1

16. The volume of gas from a reaction is plotted against time. What does the final plateau volume represent?

    • The volume of gas produced at the half-life of the reaction
    • The volume of gas that dissolves in the solvent during the reaction
    • The total volume of gas produced when the reaction is complete
    • The volume of gas produced in the first ten seconds of the reaction

17. A first-order reaction takes 50 s to halve the concentration. How long does it take to fall from 100% to 12.5% of its initial value?

    • 200 s
    • 150 s
    • 125 s
    • 100 s

18. A clock reaction gives a time for a fixed amount of product. Why is it only an approximation to the initial rate?

    • The endpoint always occurs at exactly 100% conversion, which the method cannot measure accurately in practice
    • The coloured product absorbs light strongly, so the endpoint cannot be seen clearly by eye in any experiment
    • It assumes the rate is roughly constant up to the clock point, so the time gives an average rate over that interval
    • The method needs a catalyst that changes the rate during each run, so the clock time can never be compared

19. A reaction takes 60 s for [A] to fall from 0.40 to 0.20 mol dm-3, and 120 s for it to fall from 0.20 to 0.10 mol dm-3. What is the order?

    • First order, because the half-life is constant for every interval of the reaction
    • Zero order, because the half-life gets shorter as the reaction proceeds
    • Third order, because the half-life is inversely proportional to the cube of the concentration
    • Second order, because the half-life doubles each time the concentration is halved

20. In a gas-evolving experiment the volume increases by 24 cm3 in 40 s. What is the average rate?

    • 960 cm3 s-1
    • 0.60 cm3 s-1
    • 1.67 cm3 s-1
    • 0.33 cm3 s-1

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