Dilute sulfuric acid, H₂SO₄(aq), can be electrolysed using platinum electrodes and a direct current. Hydrogen gas is produced at the cathode and oxygen gas is produced at the anode. The two gases are collected separately in burettes filled with dilute sulfuric acid placed over each electrode. [Figure showing an electrolysis setup with two inverted burettes (labelled burette 1 and burette 2) filled with dilute sulfuric acid, placed over platinum electrodes connected to a power supply. Burette 1 shows a 50.0 cm³ mark at the top and a 0.0 cm³ mark at the bottom, indicating gas collection. The electrodes are connected to positive and negative terminals.] reaction at electrode in burette 1: 2H⁺(aq) + 2e⁻ → H₂(g) reaction at electrode in burette 2: H₂O(l) → ½O₂(g) + 2H⁺(aq) + 2e⁻

1. (a)The production of hydrogen gas over time can be measured, and the data used to determine the charge of one mole of electrons, known as the Faraday constant, F. Process the results to calculate the volume of hydrogen gas produced, in cm³, and the charge passed, in coulombs, C. charge (C) = current (A) × time (s) The current was kept constant at 0.80 A. time/s reading on burette 1/cm³ volume of hydrogen gas produced/cm³ charge passed /C 0 46.20 0.00 50 41.20 100 36.20 150 31.45 200 25.80 250 20.80 300 16.40 350 11.45 400 6.80 450 1.50 [2][2]
2. (b)Plot a graph on the grid to show the relationship between volume of hydrogen gas produced and charge passed. Use a cross (x) to plot each data point. Draw the straight line of best fit. [Figure: Graph grid with y-axis 'volume of hydrogen gas produced/cm³' from 0 to 44.00, x-axis 'charge passed/C' from 0 to 360.][2]
3. (c)Do you think the results obtained in (a) are reliable? Explain your answer.[1]
4. (d)(i)The gradient of the line of best fit gives the volume of hydrogen gas produced per coulomb. Use the graph to determine the gradient of the line of best fit. State the co-ordinates of both points you used in your calculation. co-ordinates 1 ......................................... co-ordinates 2 ......................................... gradient = .............................................. cm³ C⁻¹ [2][2]
5. (d)(ii)Calculate the number of moles of hydrogen gas produced per coulomb. If you were unable to obtain an answer for (d)(i), you may use the value 0.148 cm³ C⁻¹, but this is not the correct answer. [The molar volume of gas = 24.0 dm³ at room temperature and pressure.] .............................................................................................................................. mol C⁻¹ [1][1]
6. (d)(iii)Use your answer to (ii) and the half-equation for the production of H₂(g) to calculate a numerical value for the Faraday constant (the charge of 1 mole of electrons). 2H⁺(aq) + 2e⁻ → H₂(g) .............................................................................................................................. C mol⁻¹ [1][1]
7. (e)(i)The graph below shows the relationship between volume of H₂(g) produced at the cathode and time, in a similar experiment. Draw a line on the graph to show the relationship between volume of O₂(g) produced at the anode and time in this experiment. [Figure: Graph grid with y-axis 'volume of gas produced /cm³' from 0 to 20.0, x-axis 'time/s' from 0 to 300. An existing line for H₂(g) is shown from (0,0) to (300, 18.0).][1]
8. (e)(ii)Suggest why the volume of O₂(g) measured in this experiment might be less than that shown by your drawn line. Assume that no gas is lost from leaks.[1]
9. (f)(i)In these experiments, the pressure of the gas inside the burette is assumed to be atmospheric pressure, P_atm. However, the presence of water vapour and the mass of the solution in the burette change the pressure of the gas to P_new. The expression below shows the relationship between P_new and P_atm: P_new = P_atm - 2.81 - (9.81 × height of solution in burette) Use the expression to sketch a graph on the axes below to show the relationship between P_new and the height of solution in the burette. [Figure: Graph grid with y-axis 'P_new' and x-axis 'height of solution'][1]
10. (f)(ii)State how P_new changes the value of the Faraday constant calculated at P_atm in (d)(iii). Explain your answer.[1]
11. (g)A student's teacher suggested it would be cheaper to use copper rather than platinum electrodes in the electrolysis of dilute sulfuric acid. half-equation E°/V 2H⁺(aq) + 2e⁻ ⇌ H₂(g) 0.00 Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) +0.34 ½O₂(g) + 2H⁺(aq) + 2e⁻ ⇌ H₂O(l) +1.23 Using the information in the table, suggest what effect, if any, the use of copper electrodes would have on the volume of gas produced at each electrode. Explain your answer. cathode .............................................................................................................................. ............................................................................................................................................. anode ................................................................................................................................. .............................................................................................................................................[3]

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.