AQA Jun 2015 Paper 4 Q4 (with explained solutions) |

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AQA Jun 2015 Paper 4 Q4

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do not write outside the box water dissociates slightly according to the equation h o i h aq oh aq the ionic product of water kw is given by the expression kw h oh kw varies with temperature as shown in table table temperature c kw mol dm a explain why the expression for kw does not include the concentration of water marks b explain why the value of kw increases as the temperature increases marks wmp jun chem c calculate the ph of pure water at c give your answer to decimal places do not write outside the box marks d calculate the ph of mol dm aqueous naoh at c give your answer to decimal places marks turn over for the next question turn over wmp jun chem

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mark scheme a level chemistry chem june marking guidance mark comments not partially dissociates allow changes by only a very small amount allow to oppose change only if increase t mentioned of question a m m h o is very high compared with h and oh or very few h and oh ions or only very slightly dissociates or equilibrium lies far to the left h o is effectively constant or is incorporated into the constant k dissociation or breaking bonds is endothermic equilibrium moves to rhs at higher t to absorb heat or to lower t or oppose increase in t b c marked with d d marked with c h kw or if wrong method no marks ph h ph kw oh or if wrong method no marks mark scheme a level chemistry chem june correct ph answer scores using alternative kw x gives ph which scores final answer must have dp correct ph answer scores if use alternative kw x again do not penalise repeat error so ph scores if use alternative kw x not as a repeat error ph scores if ae in kw value made in part c is repeated here do not penalise again final answer must have dp but if dp penalised in c allow more than dp here but not fewer of

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Topics to revise

AQA > Physical chemistry > Equilibrium AS >

1 Dynamic equilibrium and factors affecting equilibrium

AQA > Physical chemistry > Acid base equilibria >

1 Bronsted-Lowry theory and pH

AQA > Physical chemistry > Thermodynamics >

5 Gibb’s Free Energy

AQA > Physical chemistry > Atomic structure AS >

4 Electronic arrangement

AQA > Physical chemistry > Acid base equilibria >

2 Ionic product of water Kw

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