OCR A Jan 2011 Paper 5 Q6

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16 The dissociation of water is a reversible reaction. H2O(l) H+(aq) + OH(aq) The ionic product of water, Kw, measures the extent of dissociation of water. Kw varies with temperature. Therefore, it is always important to quote the temperature at which measurements are being taken. Fig. 6.1 shows the variation of Kw between 0 C and 60 C. 8.01014 7.01014 6.01014 5.01014 KW / mol2 dm6 4.01014 3.01014 2.01014 1.01014 0.11014 10 20 30 40 50 60 temperature / C (a) (i) Write the expression for Kw. Fig. 6.1[1] (ii) Calculate the OH(aq) concentration in an aqueous solution of hydrochloric acid with a pH of 4.37 at 25 C. Give your answer to two significant figures. OH concentration =mol dm3 [2]OCR 2011<br />
 (b) (i) Using Fig. 6.1, explain whether the dissociation of water is an exothermic or endothermic process. 17[1] (ii) Determine the pH of pure water at body temperature, 37 C. pH =[3] (iii) Many experimental measurements use published data, such as Kw, measured at 25 C. Often these measurements have been taken at different temperatures, especially in experimental work carried out at body temperature. What is the consequence of this for published scientific work?[1]OCR 2011 Turn over<br />
 (c) The reverse reaction of the dissociation of water is called neutralisation. 18 Plan an experiment that a student could carry out to measure the enthalpy change of neutralisation. In your answer you should explain how the enthalpy change of neutralisation could be calculated from the experimental results.[6]OCR 2011<br />
 19 (d) When dissolved in water, the enthalpy change of solution of the salt potassium fluoride, KF, is 15 kJ mol1. The salt rubidium fluoride, RbF, has an enthalpy change of solution in water of 24 kJ mol1. Suggest reasons for the difference between the enthalpy changes of solution of KF and RbF.[4] (e) A student hurt his ankle whilst playing football. The physiotherapist applied a cold pack to soothe the pain. The cold pack is made of two separated compartments, one containing ammonium nitrate crystals, NH4NO3, the other containing water. The pack is activated by breaking the barrier between the two compartments. The crystals dissolve spontaneously in the water causing the temperature of the pack to drop. Explain why ammonium nitrate in the cold pack dissolves spontaneously in water even though this process is endothermic.[2]OCR 2011 [Total: 20] Turn over<br />

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