OCR A Jan 2012 Paper 5 Q2

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Lattice enthalpies can be calculated indirectly using BornHaber cycles. Table 2.1 shows enthalpy changes needed to calculate the lattice enthalpy of sodium oxide, Na2O. letter A B C D E F G enthalpy change 1st electron af nity of oxygen 2nd electron af nity of oxygen 1st ionisation energy of sodium atomisation of oxygen atomisation of sodium formation of sodium oxide lattice enthalpy of sodium oxide Table 2.1 energy /kJ mol1 141 +790 +496 +249 +108 414 (a) Define the term lattice enthalpy.[2]OCR 2012<br />
 (b) The BornHaber cycle below links the lattice enthalpy of sodium oxide with its enthalpy change of formation. (i) On the BornHaber cycle, write the correct letter from Table 2.1 in each box. 2Na+(g) + O2(g) 2Na+(g) + O(g) + e 2Na+(g) + O(g) + 2e 2Na(g) + O(g) 2Na(g) +1 2O2(g) 2Na(s) +1 2O2(g) (ii) Calculate the lattice enthalpy of sodium oxide, G. Na2O(s) [3] (c) Explain why it is difficult to predict whether the lattice enthalpy of magnesium sulfide would be more or less exothermic than the lattice enthalpy of sodium oxide. answer =kJ mol1 [2][3]OCR 2012 Turn over<br />
 (d) A student wanted to determine the lattice enthalpy of sodium carbonate, Na2CO3. Unfortunately this is very difficult to do using a similar BornHaber cycle to that used for sodium oxide in (b). (i) Suggest why this is very difficult.[1] (ii) The student thought that he could determine the lattice enthalpy of Na2CO3 using a BornHaber cycle that links lattice enthalpy with enthalpy change of solution. The enthalpy change of solution of Na2CO3 is exothermic.Sketch this BornHaber cycle, Explain how the lattice enthalpy of Na2CO3 could be calculated from the enthalpy changes in the cycle.[3] [Total: 14]OCR 2012<br />

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