18 (A410U10-1) 18 © WJEC CBAC Ltd. 10. Chlorine forms a range of oxides and oxyanions. Many of these are reactive species that can oxidise or chlorinate a range of elements and compounds. (a) Dichlorine heptoxide, Cl2O7, is a colourless liquid that can be formed by reaction of ozone with chlorine in the presence of ultraviolet light. Substance Standard enthalpy change of formation, ΔfH θ / kJ mol–1 Cl2(g) 0 O3(g) 142 7O3(g) ΔH θ = −199 kJ mol–1 3Cl2(g) 3Cl2O7(g) + (A410U10-1) 19 19 © WJEC CBAC Ltd. Examiner only Energy / kJ mol –1 Reaction progress Reaction progress – 250 – 200 –150 –100 – 50 0 100 150 200 250 50 Turn over. (i) The activation energy for this reaction is 18 kJ mol–1. Draw the energy profile for this reaction on the axes provided. [3] 20 (A410U10-1) 20 Examiner only © WJEC CBAC Ltd. (ii) The reaction is carried out at a temperature of 223 K and the rate constant is found to have a value of 2.24 × 103. The unit of the rate constant is not stated. Calculate the temperature required to double the rate of reaction. [4] T = ...................................................... K (iii) The rate equation for this reaction is first order with respect to each reactant and the rate is measured in units of mol dm–3 s–1. I. Write the rate equation. [1] II. Give the unit of the rate constant. [1] ..................................................................................................... (iv) Explain why the standard enthalpy of formation for chlorine gas is zero but that of ozone is not. [2] (A410U10-1) 21 21 Examiner only © WJEC CBAC Ltd. (v) Dichlorine heptoxide decomposes readily to form Cl2(g) and O2(g). Calculate the volume of gas produced when 2.70 g of Cl2O7 decomposes at a temperature of –12°C and 1 atm pressure. [4] Volume = ...................................................... dm3 2Cl2(g) 2Cl2O7(g) 7O2(g) + Turn over. 22 (A410U10-1) 22 Examiner only © WJEC CBAC Ltd. (b) Chlorine dioxide, ClO2, is an unstable oxide of chlorine. It is often stored in aqueous solution and in some countries solutions can only be transported if the concentration is lower than 0.30 g in 100 cm3 of water. Calculate the concentration of this solution in mol dm–3. [2] Concentration = ...................................................... mol dm–3 (c) Chlorine perchlorate, ClOClO3, is an oxide with the two chlorine atoms in different oxidation states. (i) If one chlorine atom has an oxidation state of +1, find the oxidation state of the other. [1] ............................................................ (ii) Predict the appearance of the molecular ion peaks seen in the mass spectrum of chlorine perchlorate. Give reasons for your answer. You should refer to both the positions and heights of the peaks. [4] (A410U10-1) 23 23 Examiner only © WJEC CBAC Ltd. (d) The chlorite ion,  , has a non-linear shape. Explain why this ion is not linear. [2] 24 O Cl O– Turn over.