AQA Jun 2015 Paper 5 Q3 (with explained solutions) |

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AQA Jun 2015 Paper 5 Q3

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do not write outside the box hydrogen can be manufactured from the reaction of steam with methane ch g h o g co g h g a table contains some enthalpy of formation and entropy data table substance h f kj mol s j k mol ch g h o g co g h g co g a i use data from table to calculate the enthalpy change h for the reaction of steam with methane marks a ii use data from table to calculate the entropy change s for the reaction of steam with methane marks wmp jun chem do not write outside the box b use your values of h and s from questions a i and a ii to calculate the temperature above which this reaction is feasible marks c the temperature used for this manufacture of hydrogen is usually about k suggest one reason other than changing the position of equilibrium why this temperature is used rather than the value that you calculated in question b mark question continues on the next page turn over wmp jun chem do not write outside the box d hydrogen can also be obtained by reaction of carbon monoxide with steam co g h o g co g h g h kj mol s j k mol d i explain using a calculation why this reaction should not occur at k marks d ii explain how the conditions for the reaction could be changed to allow this reaction to take place marks wmp jun chem

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mark scheme a level chemistry chem june marking guidance mark comments question a i h enthalpies formation products enthalpies formation reactants kj mol or correct cycle with enthalpy changes labelled scores only units not essential if ans in kj mol but penalise incorrect units units not essential but penalise incorrect units of a ii s entropies of products entropies reactants j k mol or kj k mol b when g or h t s t h s k c to speed up the rate of reaction or wtte mark scheme a level chemistry chem june m also scores m allow error carried forward from a i and a ii ignore unexplained change of sign from to allow units of k essential must be ve if values from a i and a ii lead to negative value in m allow m to m but do not allow negative temperature for m if negative value changed to positive for m allow m allow so that more molecules have energy greater than the activation energy if t in b allow answers such as to reduce energy cost to slow down reaction do not allow to increase rate of d i method g h t s g m kj mol g must be negative for the reaction to be feasible or g is positive so reaction is not feasible method for reaction to be feasible g must be negative or zero t when g h s k s is ve so g must be ve at temperatures above k at k d ii if the temperature is lowered ignore reference to catalyst and or pressure g will become more negative because the t s term will be less positive t s h mark scheme a level chemistry chem june if and not used can score m only but allow g x m j mol m units essential alternative mark scheme if t is calculated allow t reduced to k or lower m at this temperature the reaction becomes feasible because g m of

Question 1 2 3 4 5 6 7 8 9

Topics to revise

AQA > Physical chemistry > Thermodynamics >

5 Gibb’s Free Energy

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AQA > Physical chemistry > Equilibrium AS >

1 Dynamic equilibrium and factors affecting equilibrium

AQA > Inorganic chemistry > Periodicity AS >

1 Periodicity

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