22 (A410U10-1) Examiner only 11. Fuel cells are an electrochemical method of obtaining energy from fuels. (a) Give one advantage and one disadvantage of the use of fuel cells. [2] Advantage ..................................................................................................................................................................................... Disadvantage .............................................................................................................................................................................. (b) Hydrogen gas is a common fuel used in fuel cells. Hydrogen can be produced in the electrolysis of sodium chloride solution. The reaction occurring is shown below. © WJEC CBAC Ltd. 2NaCI(aq) H2(g) CI2(g) 2H2O(l) 2NaOH(aq) + + + (i) Electrolysis was undertaken on 250 cm3 of a solution of sodium chloride of concentration 5.00 mol dm–3 until 2856 cm3 of hydrogen gas was produced at a temperature of 320 K and a pressure of 1.14 × 105 Pa. Calculate the concentration of the sodium hydroxide present at the end of the process. [4] Concentration = ………............................………………… mol dm–3 23 (A410U10-1) Examiner only © WJEC CBAC Ltd. Turn over. (ii) The chlorine gas produced in the reaction was stored under a pressure of 5.05 × 105 Pa at a temperature of 283 K. Calculate the volume of the chlorine gas under these conditions. [2] Volume = ………............................………………… cm3 (iii) At the end of the electrolysis process the solution contained a base and a salt. A student suggests that this could therefore be used as a buffer. Is the student correct? Explain your answer. [3] (A410U10-1) 24 (c) When an electrical discharge is passed through hydrogen gas, the atoms emit light and other frequencies of electromagnetic radiation. These form the emission spectrum shown below. © WJEC CBAC Ltd. 91 nm 122 nm 365 nm 650 nm 820 nm 1875 nm 1282 nm 1094 nm 102 nm ultraviolet infrared Wavelength visible region (A410U10-1) Turn over. 25 Examiner only Explain why this pattern of sharp lines forms and how it can be used to find the ionisation energy of hydrogen. You should also calculate the ionisation energy of hydrogen in kJ mol–1. [6 QER] Ionisation energy = ………............................………………… kJ mol–1 © WJEC CBAC Ltd. 26 (A410U10-1) Examiner only © WJEC CBAC Ltd. (d) Another fuel that can be used in fuel cells is methanol. The equation for the combustion of gaseous methanol at a temperature of 120°C is given below. 2CH3OH(g) 4H2O(g) ∆H = –1286 kJ mol –1 3O2(g) 2CO2(g) + + (i) Calculate the bond energy of the C―O bond in methanol. [3] Bond energy = ………............................………………… kJ mol–1 Bond Bond energy / kJ mol–1 414 498 464 803 C H O O O H C O (A410U10-1) © WJEC CBAC Ltd. Turn over. (ii) Give a reason why the value calculated in part (i) may not precisely match the actual energy of this C―O bond. [1] (iii) The standard enthalpy change of combustion of methanol has a value of –715 kJ mol–1. Give reasons why this value is very different from the value associated with the equation. [3] 24 27 Examiner only