At temperatures above 5°C, the benzenediazonium ion, C₆H₅N₂⁺, reacts with water as shown. C₆H₅N₂⁺(aq) + H₂O(l) → C₆H₅OH(aq) + N₂(g) + H⁺(aq) A student investigates this reaction by measuring the volume of nitrogen gas produced at regular time intervals. The diagram shows the experimental set-up used to investigate this reaction. [Figure showing a conical flask with C₆H₅N₂⁺(aq) connected to a gas syringe]

1. (a)The student finds that the reaction is very slow, so decides to investigate the reaction at 30°C. Complete the diagram to show how the student could investigate this reaction at a constant 30°C.[2]
2. (b)(i)The student prepared a solution of C₆H₅N₂⁺(aq) at 5°C. A 200.0 cm³ sample of this solution was placed in a conical flask. The apparatus was allowed to equilibrate at 30°C. The gas syringe was then connected, a stop-clock was started and readings of time and gas volume were taken. When the decomposition of the C₆H₅N₂⁺ ion was complete (as shown by no more gas production) the final volume of N₂(g) produced, Vfinal, was 72 cm³. Show by calculation that when the stop-clock was started the concentration of C₆H₅N₂⁺(aq) was 0.0150 mol dm⁻³. [The molar volume of gas under room conditions is 24.0 dm³.] concentration of C₆H₅N₂⁺(aq) = ........................................... mol dm⁻³[2]
3. (b)(ii)The volume of nitrogen produced is proportional to the amount of C₆H₅N₂⁺ that reacts. As C₆H₅N₂⁺ reacts, its concentration in the solution falls. Using the axes shown, sketch a graph to show the change in volume of N₂(g) produced with the change in concentration of C₆H₅N₂⁺(aq). Label the axes [C₆H₅N₂⁺(aq)]/mol dm⁻³ and volume of N₂(g)/cm³, putting the independent variable on the x-axis. Include on the axes the maximum values for concentration and volume where the lines on the axes are shown. [Figure showing blank axes for plotting][2]
4. (c)The results the student obtained are shown in the table. To calculate the concentration of C₆H₅N₂⁺(aq) the student used the following equation. [C₆H₅N₂⁺(aq)] = 0.0150 × (1 – V/Vfinal) V = volume of N₂(g) recorded at a specified time. Vfinal = final volume of N₂(g) at complete decomposition of C₆H₅N₂⁺. Vfinal is 72 cm³. Complete columns C and D to three significant figures. You may use the space below for any working. [Table with columns A (time/minutes), B (volume of N₂(g), V/cm³), C (V/Vfinal), D ([C₆H₅N₂⁺(aq)]/mol dm⁻³)][2]
5. (d)Plot a graph on the grid to show the relationship between [C₆H₅N₂⁺(aq)] and time. Use a cross (×) to plot each data point. Draw a line of best fit. [Grid for plotting provided][2]
6. (e)Draw a tangent at time = 0.0 on your graph and use the tangent to determine the initial rate of reaction. State the units of the initial rate of reaction. State the co-ordinates of both points you used in your calculation. co-ordinates 1 .................................................. co-ordinates 2 .................................................. initial rate of reaction = ................................................... units = ...................................................[4]
7. (f)Use your graph to complete the table and calculate the values of two half-lives. For the first half-life, concentrations have been chosen for you. [Table with columns concentration 1, time 1, concentration 2, time 2, t1/2][3]
8. (g)State and explain what the values of the two half-lives suggest about the order of reaction with respect to [C₆H₅N₂⁺(aq)].[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.