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CHEM 5 JUNE 12 Section A Unofficial Mark Scheme

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    (Original post by s4n4iqb4l)
    Can anyone remember the potassium titration?
    itwas something like work out percentage by mass....
    i got 75% i think...but it just seemed so high...probably wrong??
    I got 130% using their 3:7 ratio so i wouldn't hedge your bets on your answer being too high :')
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    (Original post by Scott.M)
    I got 130% using their 3:7 ratio so i wouldn't hedge your bets on your answer being too high :')


    Yeh it seems to be the question everyone is in uproar about, well one of many....:'P
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    (Original post by s4n4iqb4l)
    Yeh it seems to be the question everyone is in uproar about, well one of many....:'P
    did u write ur value down, i ran out of time, decided to use their values, got something similar to you, 30 seconds left thought it was wrong so i left it out, but i swear thats impossible, and then i was told to close my booklet by the invigilators. I know everyones complaining about the exam, but tbh, i do think most of the stuff couldve been done, if solid revision was done, though that was not the case with me, but surely this question should be emitter, WHAT SLY GAME ARE AQA TRYING TO PLAY
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    (Original post by paul272)
    did u write ur value down, i ran out of time, decided to use their values, got something similar to you, 30 seconds left thought it was wrong so i left it out, but i swear thats impossible, and then i was told to close my booklet by the invigilators. I know everyones complaining about the exam, but tbh, i do think most of the stuff couldve been done, if solid revision was done, though that was not the case with me, but surely this question should be emitter, WHAT SLY GAME ARE AQA TRYING TO PLAY
    Yeh it was the same with me, i added the 2 bottom equations together amd that gave the ratio wjen combinee with the autocatalysis one...ah well whats done is done i suppose, and haha, as for trying to understand why AQA like to mess with us....we will never kmoe!!
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    know*
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    (Original post by TheBigDog)
    This is Section A, obviously loads/most are wrong because I thought this was a ridiculously hard exam, and a lot of it unrelated. I still don’t think boundaries will change too much because AQA aren’t like that, 70 for an A i reckon. Everyone edit and let’s try and come up with a proper mark scheme

    Question 1 is 9 marks

    1a) Why is Phosporous Oxide stored in water? – Reacts violently in air
    b) Why P4O10 not P2O5? – P4O10 is it in its most stable form under standard conditions
    c) Equation = P4O10 + 6H2O = 4H3PO4
    pH = 0
    d) equation for MgO + H3PO4 – No idea
    e)Why can you use excess MgO? Because H3PO4 is a strong acid
    f) Why is NaOH an environmental risk? Strong alkaline harms fish?

    Question 2 is 7 marks
    2a) Equation – G = H – TS
    b)Gradient = 0.1 Units = J K mol-1 Symbol = S
    c) Why is significance of temperatures below the temp line – delta G is negative so reactions are feasible
    d) Why does curve slope? – Change of state, freezing.

    Question 3 is 10 marks
    3a) delta H =
    b)Delta S =
    c) T = 700 K? Can’t remember the other 2 but they gave this temperature.
    d) why would NO2 not form? – Not 700K
    e) N2 + O2 = NO2
    f) same number of moles so small entropy so small TdeltaS

    Question 4 is 12 marks
    4a) Define enthalpy of electron affinity – Enthalpy change when 1 mole of electrons added to an isolated atom in the gas phase
    b) Why is F- more electronegative – more shells so more and further from nucleus
    c) Why does F- have more negative enthalpy of hydration? No idea
    d) Calculation – can’t remember I got 900 something I think
    e) Why is it always feasible – delta G always negative because entropy positive and enthalpy change is negative?
    f) Can’t remember but it was 2 marks

    Question 5 (a disgrace) was 17 marks
    5a) Weakest oxidising agent – CdO(OH) because it was most negative.
    b) emf = 1.1? can’t remember
    c) Equation – cant remember
    d) Carbon anode – conduct electricity
    e) Essential property of porous cover? – Inert
    f) Why leakage? Ions used up
    g) +3 and reverse equation
    h) Environmental problem? More waste/to landfill
    Two ethanol (wtf) fuel cell questions
    I) Carbon neutral as no net emission into atmosphere as ethanol comes from plants that photosynthesise.

    Question 6 was 10 marks
    6a) Coordinate bond
    b) O atom acts as Lewis base and donates electrons
    Metal ion acts as Lewis Acid and accepts electrons.
    c) 180 degrees
    d) ratio 3:5 and calculation – no idea

    Someone else do section B??
    liking some of those answers.
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    (Original post by blueqq)
    have u got the paper?
    lol nope sorry!
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    (Original post by Polioz)
    isnt that the same thing as conducting electricity ?
    uh yeah i guess so lol
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    that was a hard test
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    i got -56 kJmol-1 for delta H
    and -70.5JK-1mol-1 for delta S
    which gives you a temp. value of 794.32, so 794K (3sf)


    As for the rest of the paper, :eek:
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    (Original post by TheBigDog)
    The oxidising agent is always on the left, because its being reduced.
    it was asking for the best reducing agent in a roundabout way
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    Hey guys
    Take some time out....it only took me 6 minutes to call the AQA people and complain.....
    This is the number 0161 957 3264........ jst tell whoever picks up the phone that you want to talk to someone who deals with A-level Chemistry.......and then jst tell them your worries........
    The more people calling, the more they are likely to realise how frickiing hard tht paper was and the more chances of low grade boundaries...........
    Your future is in your hands do call them............
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    (Original post by simaframe)
    Did anyone else get Zn for the weakest oxidising agent or am I the only one?
    I got the cr(OH)..<something like that then i went back and changed it to Zn after realising the weakest oxidising agent must be the most negative on the left, and they didnt put them in + to - order (mean trick!!)
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    (Original post by aljabiraa)
    it was asking for the best reducing agent in a roundabout way
    Do you reckon? i'm not sure they were
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    The paper has been uploaded on the other thread!
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    1. d) 3MgO + 2H3PO4 = Mg3(PO4)2 + 3H2O

    Think that's right?
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    (Original post by ruby.hanmer)
    1. d) 3MgO + 2H3PO4 = Mg3(PO4)2 + 3H2O

    Think that's right?
    yeah i got that as well.
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    (Original post by TheBigDog)
    Do you reckon? i'm not sure they were
    as long as the cadmium equation is most negative, then CdO(OH) is correct since Cd3+ needs to gain electrons to form Cd metal
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    I thought the reason you can use excess MgO was that it forms Mg(OH)2 . Which is not partially soluble in water, so unlikely to affect living organisms. Just a suggestion. PLease can someone post the link to the paper. cant find it anywhere.
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    Could someone post the paper?? then i could help... as my memory is useless (not helpful for exams :cry2: )

    i got around 75% ish using there values, but i didn't know what i was doing... and then aqua suggest its 138%...

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Updated: June 20, 2012
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