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    (Original post by popnit)
    Yep I got 1.93 but I don't understand the explanation part.

    For part c, I have been doing the RHS-LHS rule. How do you know which one is positive and which one is negative?

    Thanks btw!
    It's okay well with the conventional cell diagram, the one with the most positive E is always on the RHS (with some exceptions but they're not important) and the one with the least positive/most negative E is always on the LHS.

    So RHS = +E = a reduction reaction
    and LHS = -E = an oxidation reaction

    Oh sorry, I didn't look at the question properly!! Okay so Mg is on the LHS so it is being oxidised:

    Mg <-> Mg2+ +2e-

    By decreasing the concentration of Mg2+, can you see that the equilibrium above would shift to the RHS to produce more Mg2+ to oppose this change in the equilibrium. Shifting to the RHS will therefore also result in more electrons being lost and this means the electrode potential of Mg is made more negative. So say it goes from being -2.37 to -2.40 for example.

    Now the overall emf = (Fe electrode potential) - (Mg electrode potential)
    = (-0.44) - (-2.40)
    = +1.96V

    So it increases, I hope that makes sense haha
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    (Original post by pjanoo)
    It's okay well with the conventional cell diagram, the one with the most positive E is always on the RHS (with some exceptions but they're not important) and the one with the least positive/most negative E is always on the LHS.

    So RHS = +E = a reduction reaction
    and LHS = -E = an oxidation reaction

    Oh sorry, I didn't look at the question properly!! Okay so Mg is on the LHS so it is being oxidised:

    Mg <-> Mg2+ +2e-

    By decreasing the concentration of Mg2+, can you see that the equilibrium above would shift to the RHS to produce more Mg2+ to oppose this change in the equilibrium. Shifting to the RHS will therefore also result in more electrons being lost and this means the electrode potential of Mg is made more negative. So say it goes from being -2.37 to -2.40 for example.

    Now the overall emf = (Fe electrode potential) - (Mg electrode potential)
    = (-0.44) - (-2.40)
    = +1.96V

    So it increases, I hope that makes sense haha
    Yep that's absolutely fantastic.
    Just one more thing though, if RHS= +ve and LHS= -ve then surely RHS-LHS= +ve - -ve?
    So why is it better to use the RHS- LHS formula..?
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    (Original post by Dirtybit)
    Nah just alkaline and acidic Fuel cells.
    I thought we just had to know hyrdogen fuel cell equation?
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    (Original post by popnit)
    Yep that's absolutely fantastic.
    Just one more thing though, if RHS= +ve and LHS= -ve then surely RHS-LHS= +ve - -ve?
    So why is it better to use the RHS- LHS formula..?
    Oh haha no solid reason really, just when I did some homework on this topic a while ago I did (+ve) - (-ve) on one cell diagram and it wasn't right so my teacher just said to use (RHS) - (LHS) from now on because that would have given me the right answer instead somehow!
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    (Original post by pjanoo)
    Oh haha no solid reason really, just when I did some homework on this topic a while ago I did (+ve) - (-ve) on one cell diagram and it wasn't right so my teacher just said to use (RHS) - (LHS) from now on because that would have given me the right answer instead somehow!
    Ahhhh! Okay. Thanks so much for taking the time out to explain that, much appreciated
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    (Original post by popnit)
    Ahhhh! Okay. Thanks so much for taking the time out to explain that, much appreciated
    No problem at all and best of luck!!
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    (Original post by pjanoo)
    No problem at all and best of luck!!
    You too! I'll probably have to ask you something before the exam anyway, hopeless at this unit!
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    Hey guys I'm doing the Jan 10 paper and it asks why the theoretical val of lattice dissociation would be greater for AgCl than for AgBr, saying that cl is smaller therefore attracted more strongly.

    I thought the argument for Br being a bigger ion meant it is easier to be distorted by the Ag so has some added covalent character, don't understand why this argument isn't applicable to this question?
    (As in the textbook it says that the Selenide is a big ion and therefore plenty of electrons to distort so I assumed with Br having an extra shell this would be the answer? So very confused, if anyone has an idea please quote me!

    thanks for the help!
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    (Original post by popnit)
    You too! I'll probably have to ask you something before the exam anyway, hopeless at this unit!
    Haha, I was the same with CHEM4 so please don't hesitate to ask I know how helpful it can be!!
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    (Original post by Radiohead_1)
    I thought we just had to know hyrdogen fuel cell equation?
    Yeah thats right but there is an Acidic Hydrogen-oxygen fuel cell and there is an Alkaline one too. It's the same overall equations just two different sets of half equations where it's either alkaline or it's acidic.
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    (Original post by Puffy14)
    Hey guys I'm doing the Jan 10 paper and it asks why the theoretical val of lattice dissociation would be greater for AgCl than for AgBr, saying that cl is smaller therefore attracted more strongly.

    I thought the argument for Br being a bigger ion meant it is easier to be distorted by the Ag so has some added covalent character, don't understand why this argument isn't applicable to this question?
    (As in the textbook it says that the Selenide is a big ion and therefore plenty of electrons to distort so I assumed with Br having an extra shell this would be the answer? So very confused, if anyone has an idea please quote me!

    thanks for the help!
    I think it's because the question is concerned with the theoretical value and so the theoretical model of a lattice, which takes a lattice to be made up of perfectly equal spherical ions with 100% ionic bonding.

    As you've said, Br is the bigger negative ion and so it wouldn't fit the theoretical model of a lattice as well as the smaller Cl ion would and so it would give a a smaller value.

    Is your problem that they didn't accept your argument in the mark scheme? Maybe I'm just getting tired haha but could you rephrase your last bit
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    (Original post by Dirtybit)
    Nah just alkaline and acidic Fuel cells.
    Can you give me the equations of the alkaline and acidic fuel cells? All I have got is Hydrogen Fuel Cell? Thanks man
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    any good ways to learn all the definitions?
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    (Original post by ayshapatel)
    any good ways to learn all the definitions?

    yes - learn them and write them out over and over again until you get them right
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    (Original post by crc290)
    Well maybe cockup is the wrong word. I just know that there is no mention in the specification of having to combine three half equations. It wasn't especially hard but obviously a lot of people got really confused so the mark scheme allowed a wide range of answers and you only lose a mark for getting the ratio itself wrong
    Oh I see, makes sense. Was a bit weird but I got it in the end simply because I saw it was only 1 mark and was like "This has to be easy!" It's sometimes easy to overcomplicate things when we haven't seen them before.
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    (Original post by ayshapatel)
    any good ways to learn all the definitions?
    What I did is I link all definitions together? Kinda weird but it goes like this.

    Enthalpy of atomisation, formation of 1 mole of gaseous atoms from its element in standard states under conditions

    Then this 1 mole of gaseous atom can undergo first ionisation energy or first electron affinity which measures the enthalpy change when 1 mole of gaseous atom is converted to 1 mole of gaseous ion each with either single positive charge(Ionisation) or negative charge(Electron affinity)

    Then, the gaseous ions can link to the next definitions:
    Lattice formation enthalpy: enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions.
    Lattice dissociation enthalpy: enthalpy change when 1 mole of solid ionic compound is separated into its gaseous ions.

    Then, enthalpy of hydration: enthalpy change when water molecules surround 1 mole of gaseous ion

    ..You get the point haha. Just form linkage between definitions This way also help you draw Born/Haber Cycle because the definitions fall in order.

    Formation<-Start->Atomisation -> First ionisation ->Affinity-> lattice formation/dissociation enthalpy

    hope this help man
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    (Original post by pjanoo)
    I think it's because the question is concerned with the theoretical value and so the theoretical model of a lattice, which takes a lattice to be made up of perfectly equal spherical ions with 100% ionic bonding.

    As you've said, Br is the bigger negative ion and so it wouldn't fit the theoretical model of a lattice as well as the smaller Cl ion would and so it would give a a smaller value.

    Is your problem that they didn't accept your argument in the mark scheme? Maybe I'm just getting tired haha but could you rephrase your last bit
    ooohh okay awesome, my head was too into this whole covalent character that i didnt realise it's just concerned with theoretical (not taking into account adding covalent character, thanks you so much!
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    (Original post by tr12)
    What table? :confused:
    Sorry, I did two pp today and got confused. The way you'd know Fe3+ is more positive in this case is because on the diagram they give you it's on the right hand side and also it says in the writing above the diagram 'the electrode on the right-hand side is positive'
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    (Original post by Puffy14)
    ooohh okay awesome, my head was too into this whole covalent character that i didnt realise it's just concerned with theoretical (not taking into account adding covalent character, thanks you so much!
    No problem
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    Could someome give me some mark scheme answers for describing structure.... and mpts of compounds?
 
 
 
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