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edexcel chem synoptic question: help watch

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    Ethanoic acid is only about 1% dissociated whereas HCl is completely dissociated. By considering what happens to ions as they are formed in aqueous solution, suggest why deltaH(neut) for ethanoic acid/NaOH is slightly less exothermic than the value for HCl/NaOH, in spite of the very different degrees of dissociation of the acids.

    i haven't got a clue! (or a markscheme)

    help!
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    (Original post by scarlet ibis)
    Ethanoic acid is only about 1% dissociated whereas HCl is completely dissociated. By considering what happens to ions as they are formed in aqueous solution, suggest why deltaH(neut) for ethanoic acid/NaOH is slightly less exothermic than the value for HCl/NaOH, in spite of the very different degrees of dissociation of the acids.

    i haven't got a clue! (or a markscheme)

    help!
    Because of the equilibrium?
    When H+ is reacted, the position of equilibrim continues shifting to the left, giving more H+?
    Or the hydration enthalpy of the ethanoate ions and hydrogen ions provides some energy?
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    when weak acids are neutralised, some of the heat energy is then used to further dissociate more ethanoic acid molecules to H+

    Strong acids are completely dissociated so there is no need for any enerygy to be used to dissociate the ions so all the heat of neutralisation is released

    any probs, just ask
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    (Original post by nilkanth1)
    when weak acids are neutralised, some of the heat energy is then used to further dissociate more ethanoic acid molecules to H+

    Strong acids are completely dissociated so there is no need for any enerygy to be used to dissociate the ions so all the heat of neutralisation is released

    any probs, just ask
    you are not answering the question
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    (Original post by scarlet ibis)
    Ethanoic acid is only about 1% dissociated whereas HCl is completely dissociated. By considering what happens to ions as they are formed in aqueous solution, suggest why deltaH(neut) for ethanoic acid/NaOH is slightly less exothermic than the value for HCl/NaOH, in spite of the very different degrees of dissociation of the acids.

    i haven't got a clue! (or a markscheme)

    help!
    Some of the energy given out during neutralisation is required to dissociate ethanoic acid.
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    am still confused.

    maybe its to do with eqlbm
    when HCl splits into ions in water

    HCl <===> H+(aq) + Cl-(aq)

    as more H+ ions are formed, the equilibrium is driven to left by the increase in H+ concentration

    or not?

    Some of the energy given out during neutralisation is required to dissociate ethanoic acid.
    but if the ethanoic acid is only1% dissociated surely deltaH would be 1% of the HCl value

    when weak acids are neutralised, some of the heat energy is then used to further dissociate more ethanoic acid molecules to H+
    this makes sense, but i haven't got any notes on this. can anyone else help?

    has anyone got a markscheme?
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    (Original post by scarlet ibis)
    am still confused.

    maybe its to do with eqlbm
    when HCl splits into ions in water

    HCl <===> H+(aq) + Cl-(aq)

    as more H+ ions are formed, the equilibrium is driven to left by the increase in H+ concentration

    or not?

    but if the ethanoic acid is only1% dissociated surely deltaH would be 1% of the HCl value


    this makes sense, but i haven't got any notes on this. can anyone else help?

    has anyone got a markscheme?
    Is the dissociation of ethanoic acid exothermic too?
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    (Original post by scarlet ibis)
    am still confused.

    maybe its to do with eqlbm
    when HCl splits into ions in water

    HCl <===> H+(aq) + Cl-(aq)

    as more H+ ions are formed, the equilibrium is driven to left by the increase in H+ concentration

    or not?

    but if the ethanoic acid is only1% dissociated surely deltaH would be 1% of the HCl value


    this makes sense, but i haven't got any notes on this. can anyone else help?

    has anyone got a markscheme?
    what paper?
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    jan 04 so the most recent aargh!
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    (Original post by scarlet ibis)
    jan 04 so the most recent aargh!
    so you dont need to worry
    the similar question is less likely to appear again in only six months
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    true! but i still want to work it out!
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    oh well come back to that one...

    in the meantime, heres another question, from the same paper (anyone have the markscheme?)

    Water is an important solvent

    a) Use a Hess's law cycle to illustrate the energy changes that may determine whether or not an ionic compound dissolves in water.
    i have drawn a hess cycle, see attachment, what does everyone else reckon?
    if the lattice enthalpy is smaller than the hydration enthalpy, the compund will dissolve, with the excess energy given out as heat. if it is larger, it may not dissolve. if the deficiency is small, it may take the extra enargy from the surroundings- endo thermic, or it may not dissolve.

    b) Explain why propan 1,2,3 tri-ol mixes with water in all proportions, but propane is virtually insoluble in water.
    I think this could be to do with the polar OH groups in the tri-ol being able to form hydrogen bonds with the water, whereas the propane cannot form these bonds, so will be insoluble.

    what does everyone else think?
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    (Original post by scarlet ibis)
    oh well come back to that one...

    in the meantime, heres another question, from the same paper (anyone have the markscheme?)

    Water is an important solvent

    a) Use a Hess's law cycle to illustrate the energy changes that may determine whether or not an ionic compound dissolves in water.
    i have drawn a hess cycle, see attachment, what does everyone else reckon?
    if the lattice enthalpy is smaller than the hydration enthalpy, the compund will dissolve, with the excess energy given out as heat. if it is larger, it may not dissolve. if the deficiency is small, it may take the extra enargy from the surroundings- endo thermic, or it may not dissolve.

    b) Explain why propan 1,2,3 tri-ol mixes with water in all proportions, but propane is virtually insoluble in water.
    I think this could be to do with the polar OH groups in the tri-ol being able to form hydrogen bonds with the water, whereas the propane cannot form these bonds, so will be insoluble.

    what does everyone else think?
    a)Fine

    b)Yes. tri-ol can at most form 6 hydrogen bonds but propane can form none. H2O molecules can exist between the tri-ol molecules, so its soluble
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    thanx keisiuho!

    heres another question from the same paper, again. seeing as i havn't got a mark scheme 4 this one, i thought, i'd discuss them on here! sorry! still, its good revision 4 everyone!

    an Many salts crystallise from aqueous solution with water of crystallisation incorporated into their crystals. One such salt is copper(II) sulphate pentahydrate, CuSO4.5H2O. The crystals are blue.



    a) If CUSO4.5H2O crystals are heated, steam is evolved and the crystals become white, giving anhydrous copper(II) sulphate. Why do the crystals lose their colour?


    b) If a little water is added to cold anhydrous copper(II) sulphate, the blue colour is restored and the mixture becomes hot. Explain the origin of the heat.


    a) for this one, is the water ligands are still bonded by dative covalent bonds to the Cu2+ ion? If they are then it follows that if the water ligands are removed by heating, the 3d orbital is no longer split into two energy levels, so the electrons cannot absorb the visible light and move up to the higher energy level, so the compound is not coloured.

    b) energy is given out when the water ligands form dative bonds to the Cu2+ ion

    what do you reckon?
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    (Original post by scarlet ibis)
    thanx keisiuho!

    heres another question from the same paper, again. seeing as i havn't got a mark scheme 4 this one, i thought, i'd discuss them on here! sorry! still, its good revision 4 everyone!

    an Many salts crystallise from aqueous solution with water of crystallisation incorporated into their crystals. One such salt is copper(II) sulphate pentahydrate, CuSO4.5H2O. The crystals are blue.



    a) If CUSO4.5H2O crystals are heated, steam is evolved and the crystals become white, giving anhydrous copper(II) sulphate. Why do the crystals lose their colour?


    b) If a little water is added to cold anhydrous copper(II) sulphate, the blue colour is restored and the mixture becomes hot. Explain the origin of the heat.


    a) for this one, is the water ligands are still bonded by dative covalent bonds to the Cu2+ ion? If they are then it follows that if the water ligands are removed by heating, the 3d orbital is no longer split into two energy levels, so the electrons cannot absorb the visible light and move up to the higher energy level, so the compound is not coloured.

    b) energy is given out when the water ligands form dative bonds to the Cu2+ ion

    what do you reckon?
    a)yeah
    b)yes, bond formation always releases heat and bond breaking absorbs
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    yay
    thanx keisuiho, you've been really helpful thanx to everyone else who helped too. i've finished with chemistry revision for today so its back to physics. maybe i'll b online later to ask 4 more help with physics!
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    Essentially all neutralisation reactions are the same

    H+ + OH- ---> H2O

    So they should all have the same value ie. -57kj/mol

    However the dissociation of ethanoic acid:

    CH3COOH ----> CH3COO- + H+

    IS ENDOTHERMIC, because it is such a weak acid so some energy is required to ionise it. Therefore, the enthalpy of neut with ethanoic acid is only SLIGHTLY less than that with HCl.

    Hope this answers your q
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    (Original post by amzk)
    Essentially all neutralisation reactions are the same

    H+ + OH- ---> H2O

    So they should all have the same value ie. -57kj/mol

    However the dissociation of ethanoic acid:

    CH3COOH ----> CH3COO- + H+

    IS ENDOTHERMIC, because it is such a weak acid so some energy is required to ionise it. Therefore, the enthalpy of neut with ethanoic acid is only SLIGHTLY less than that with HCl.

    Hope this answers your q
    it seems to be asking why the values differ slightly but not greatly
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    (Original post by scarlet ibis)
    Ethanoic acid is only about 1% dissociated whereas HCl is completely dissociated. By considering what happens to ions as they are formed in aqueous solution, suggest why deltaH(neut) for ethanoic acid/NaOH is slightly less exothermic than the value for HCl/NaOH, in spite of the very different degrees of dissociation of the acids.

    i haven't got a clue! (or a markscheme)

    help!

    I think the other posts were getting at the answer but they weren't explaining it very clearly... I'll try and have a go. As you add NaOH to the ethanoic acid, the H+ ions are neutralised, so the equilibrium shifts and more ethanoic acid becomes dissociated and neutralised until all of it has been. The neutralisation is exothermic and eventually the same amount of energy is released for the neutralisation of ethanoic acid, as for that of HCl, because in both cases all the molecules have been dissociated and neutralised. However, for ethanoic acid to dissociate, some energy is required to break the bonds, meaning that the total energy released is a bit less for ethanoic acid than for HCl, but not as much as you might expect - due to the equilibrium shifting and all the ethanoic acid being split up.

    Hope this helps a bit?
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    (Original post by MadNatSci)
    I think the other posts were getting at the answer but they weren't explaining it very clearly... I'll try and have a go. As you add NaOH to the ethanoic acid, the H+ ions are neutralised, so the equilibrium shifts and more ethanoic acid becomes dissociated and neutralised until all of it has been. The neutralisation is exothermic and eventually the same amount of energy is released for the neutralisation of ethanoic acid, as for that of HCl, because in both cases all the molecules have been dissociated and neutralised. However, for ethanoic acid to dissociate, some energy is required to break the bonds, meaning that the total energy released is a bit less for ethanoic acid than for HCl, but not as much as you might expect - due to the equilibrium shifting and all the ethanoic acid being split up.

    Hope this helps a bit?
    oh yes
    that's what i thought
 
 
 
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