2 A sample of ammonium bromide has been accidentally mixed with another salt. A student suggests that the percentage purity of the ammonium bromide can be determined by measuring the change in temperature when a solution is prepared. The enthalpy change when the sample dissolves in water, ΔH, can then be calculated and compared with the literature value for ammonium bromide. You will carry out the student's experiment. FA 4 is ammonium bromide, NH4Br, mixed with another salt.

1. (a)Method • Support the cup in the 250 cm³ beaker. • Use the 25 cm³ measuring cylinder to transfer 25.0 cm³ of distilled water into the cup. • Place the thermometer in the water and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of the water in the space for results. • Weigh the container with FA 4. Record the mass. • Add all of FA 4 to the water in the cup. • Stir the mixture gently so that FA 4 dissolves. • Measure and record the minimum temperature reached. • Calculate and record the change in temperature. • Weigh the container with any residual FA 4. Record the mass. • Calculate and record the mass of FA 4 added to the cup. Results I II III IV [4][4]
2. (b)(i)Calculate the energy change, in J, for the reaction. energy change = .......................................... J [1][1]
3. (b)(ii)When one mole of ammonium bromide dissolves in water the enthalpy change, ΔH, is +16.8 kJ mol⁻¹. Use this value to calculate the amount, in mol, of ammonium bromide in the 25.0 cm³ of solution FA 4 used in your experiment. Assume that the enthalpy change when the other salt dissolves in water is 0 kJ mol⁻¹. amount of NH4Br in 25.0 cm³ = .......................................... mol [1][1]
4. (b)(iii)Calculate the mass, in g, of ammonium bromide in your sample. (If you were unable to calculate the amount of ammonium bromide in (b)(ii), then assume it is 2.06 x 10⁻² mol. This may not be the correct answer.) mass of NH4Br in sample = .......................................... g [1][1]
5. (b)(iv)Hence calculate the percentage by mass of ammonium bromide in your sample. % by mass of NH4Br = .......................................... % [1][1]
6. (c)Another student states that this method is not accurate as the enthalpy change when the other salt dissolves in water could be exothermic. Suggest how the calculated value in (b)(iv) would change if the other salt dissolves in water exothermically. Explain your answer. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [1][1]
7. (d)The maximum uncertainty in a single thermometer reading is ±0.5°C. Calculate the maximum percentage error in your temperature change recorded in (a). Show your working. maximum percentage error = .......................................... % [1][1]