The hydrolysis of urea, (NH₂)₂CO, Mr = 60, is shown by the equation. The reaction is catalysed by the enzyme urease, when present in low concentrations. (NH₂)₂CO + 2H₂O → 2NH₄⁺ + CO₃²⁻ The formation of the NH₄⁺ and CO₃²⁻ ions increases the conductivity of the solution. The increase in conductivity over time can be measured and the rate of hydrolysis can be determined from this. A student plans to carry out this reaction using a 50 cm³ sample of 5.00 × 10⁻³ mol dm⁻³ urea solution.

1. (a)(i)The 5.00 × 10⁻³ mol dm⁻³ urea solution cannot be accurately prepared by dissolving urea in 50.0 cm³ of water. A more concentrated solution is first prepared. This is then diluted to make the solution of the required concentration. Suggest why diluting a more concentrated solution is a more accurate method of preparing this solution than by dissolving urea in 50.0 cm³ of water.[1]
2. (a)(ii)Describe how you would accurately prepare exactly 100 cm³ of 5.00 × 10⁻³ mol dm⁻³ urea solution, starting with 3.00 g of solid urea. Your method should use more than one volumetric flask, including one of capacity 1000 cm³, and other suitable equipment.[3]
3. (b)(i)A student added a small amount of urease to a 50.0 cm³ sample of the solution made in (a)(ii). The conductivity of the solution was measured. The student obtained the results shown in the table. Conductivity is measured in µS cm⁻¹. | time, t /s | conductivity / µS cm⁻¹ | |:---|:---| | 0 | 0 | | 30 | 55 | | 60 | 110 | | 90 | 160 | | 120 | 215 | | 150 | – | | 180 | 320 | | 210 | 375 | | 240 | 425 | | 270 | 480 | | 300 | 530 | Plot a graph on the grid to show the relationship between time t and conductivity. Use a cross (×) to plot each data point. Draw the straight line of best fit.[2]
4. (b)(ii)Use the graph to determine the gradient of the best-fit line. State the co-ordinates of both points you used in your calculation. Give your answer to three significant figures. co-ordinates 1 co-ordinates 2 gradient =[2]
5. (b)(iii)Use your graph to determine the conductivity of the solution at t = 150 s. conductivity at 150s =[1]
6. (c)(i)In another experiment, a student added a small amount of urease to 50.0 cm³ samples of urea of various concentrations and measured their conductivity until hydrolysis was complete. Sketch a graph on the axes below to show the variation in conductivity in one such experiment. Explain the shape of your graph as hydrolysis reaches completion. [Figure 2.0 - Axes for conductivity vs time][2]
7. (c)(ii)From their results, the student plotted the calibration graph, shown below, of final conductivity of solution against initial concentration of urea. [Figure 2.1 - Calibration graph: final conductivity vs initial concentration of urea] You are going to determine the average rate of hydrolysis over the first 150 s of the experiment in (b). Use the calibration graph to determine the initial concentration of urea that would give the same final conductivity as your answer to (b)(iii). (If you were unable to obtain an answer to (b)(iii) you should use the value of 350 µS cm⁻¹. This is not the correct value.) initial concentration of urea =[1]
8. (c)(iii)Use your answer to (c)(ii) to calculate the average rate of hydrolysis of urea during the first 150 s. Determine the units of the rate of hydrolysis. rate of hydrolysis = units =[2]
9. (d)The student repeats the experiment in (b) using a different low concentration of urea solution. The student measures the conductivity of the solution for 300 s. At 300 s, the student adds 1.0 cm³ of 0.05 mol dm⁻³ aqueous silver nitrate, AgNO₃(aq). The measured conductivity of the solution immediately increases. AgNO₃(aq) is known to prevent the enzyme urease from hydrolysing urea. Using the axes below, sketch a graph to show how the conductivity of the solution changes during this experiment. Extend the graph beyond the point at which AgNO₃(aq) is added at 300 s. [Figure 2.2 - Axes for conductivity vs time][3]
10. (e)The following information gives some of the hazards associated with the chemicals used in the procedure. | silver nitrate | Solutions equal to or more concentrated than 0.18 mol dm⁻³ are classified as corrosive and hazardous for the aquatic environment; solutions equal to or more concentrated than 0.06 mol dm⁻³ but less concentrated than 0.18 mol dm⁻³ are classified as moderate hazard and hazardous for the aquatic environment; solutions less concentrated than 0.06 mol dm⁻³ are classified as hazardous for the aquatic environment. | |:---|:---| | urea | All solutions are low hazard. | | urease | Concentrated solutions may produce allergic reactions. They can cause asthma and irritate the eyes, nose and skin. Dilute solutions are low hazard. | Describe one relevant precaution, other than eye protection and a lab coat, that should be taken to keep the risk associated with the chemicals used to a minimum. Explain your answer. precaution explanation[1]

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