1 This question is about mean bond enthalpies and their use in the calculation of enthalpy changes.

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1 1 This question is out men ond enthlpies nd their use in the lultion of enthlpy hnges. Define men ond enthlpy s pplied to hlorine. Explin why the enthlpy of tomistion of hlorine is extly hlf the men ond enthlpy of hlorine. The men ond enthlpy for hlorine is 242 kj mol 1 nd tht for fluorine is 158 kj mol 1. The stndrd enthlpy of formtion of ClF(g) is 56 kj mol 1. i Write n eqution, inluding stte symols, for the retion tht hs n enthlpy hnge equl to the stndrd enthlpy of formtion of gseous ClF. Clulte vlue for the ond enthlpy of the Cl F ond. i Clulte the enthlpy of formtion of gseous hlorine trifluoride, ClF 3 (g). Use the ond enthlpy vlue tht you otined in prt. (If you hve een unle to otin n nswer to prt, you my ssume tht the Cl F ond enthlpy is 223 kj mol 1. This is not the orret vlue.) iv Explin why the enthlpy of formtion of ClF 3 (g) tht you lulted in prt i is likely to e different from dt ook vlue. This resoure sheet my hve een hnged from the originl 1

2 d Suggest why vlue for the N Cl ond enthlpy is not found in ny dt ook. 2 This tle ontins some vlues of lttie dissoition enthlpies. AQA Chemistry, 2013, Unit 5 (Question 1) Compound MgCl 2 CCl 2 MgO Lttie dissoition enthlpy/kj mol Write n eqution, inluding stte symols, for the retion tht hs n enthlpy hnge equl to the lttie dissoition enthlpy of mgnesium hloride. Explin why the lttie dissoition enthlpy of mgnesium hloride is greter thn tht of lium hloride. Explin why the lttie dissoition enthlpy of mgnesium oxide is greter thn tht of mgnesium hloride. d When mgnesium hloride dissolves in wter, the enthlpy of solution is 155 kj mol 1. The enthlpy of hydrtion of hloride ions is 364 kj mol 1. Clulte the enthlpy of hydrtion of mgnesium ions. This resoure sheet my hve een hnged from the originl 2

3 e Energy is relesed when mgnesium ion is hydrted euse mgnesium ions ttrt wter moleules. Explin why mgnesium ions ttrt wter moleules. You my use lelled digrm to illustrte your nswer. f Suggest why vlue for the enthlpy of solution of mgnesium oxide is not found in ny dt ooks. AQA Chemistry, 2013, Unit 5 (Question 2) 3 The feslity of physil or hemil hnge depends on the lne etween the thermodynmi quntities of enthlpy hnge (ΔH), entropy hnge (ΔS), nd temperture (T). Suggest how these quntities n e used to predit whether hnge is fesile. Explin why the evportion of wter is spontneous even though this hnge is endothermi. In your nswer, refer to the hnge in the rrngement of wter moleules nd the entropy hnge. (4 mrks) This resoure sheet my hve een hnged from the originl 3

4 This tle ontins some thermodynmi dt for hydrogen, oxygen, nd wter. S ϴ /J K 1 mol 1 Δ f H ϴ /kj mol 1 H 2 (g) O 2 (g) H 2 O(g) H 2 O(I) 70 i Clulte the temperture ove whih the retion etween hydrogen nd oxygen to form gseous wter is not fesile. (4 mrks) Stte wht would hppen to smple of gseous wter tht ws heted to temperture higher thn tht of your nswer to prt i. Give reson for your nswer. Answer Reson d When hydrogen is used s fuel, more het energy n e otined if the gseous wter formed is ondensed into liquid wter. Use the entropy dt from the tle in prt () to lulte the enthlpy hnge when one mole of gseous wter is ondensed t 373 K. Assume tht the free-energy hnge for this ondenstion is zero. AQA Chemistry, 2013, Unit 5 (Question 3) This resoure sheet my hve een hnged from the originl 4

5 4 When potssium nitrte (KNO 3 ) dissolves in wter the vlue of the enthlpy hnge ΔH 34.9 kj mol 1 nd the vlue of the entropy hnge ΔS 117 J K 1 mol 1. Write n eqution, inluding stte symols, for the proess tht ours when potssium nitrte dissolves in wter. Suggest why the entropy hnge for this proess is positive. Clulte the temperture t whih the free-energy hnge, ΔG, for this proess is zero. d i Dedue wht hppens to the vlue of ΔG when potssium nitrte dissolves in wter t temperture lower thn your nswer to prt (). Wht does this new vlue of ΔG suggest out the dissolving of potssium nitrte t this lower temperture? 5 Thermodynmis n e used to investigte the hnges tht our when sustnes suh s lium fluoride dissolve in wter. Give the mening of eh of the following terms: i enthlpy of lttie formtion for lium fluoride, AQA Chemistry, 2011, Unit 5 (Question 2) enthlpy of hydrtion for fluoride ions. This resoure sheet my hve een hnged from the originl 5

6 Explin the intertions etween wter moleules nd fluoride ions when the fluoride ions eome hydrted. Consider the following dt. ΔH ϴ /kj mol 1 Enthlpy of lttie formtion for CF 2 Enthlpy of hydrtion for C 2 ions Enthlpy of hydrtion for F ions Use these dt to lulte vlue for the enthlpy of solution for CF 2. AQA Chemistry, 2011, Unit 5 (Question 1) This resoure sheet my hve een hnged from the originl 6