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    Basically what it says inthe title... Why isn't it constantly endo or constantly exo?

    My book doesn't say much about this.. It just gives an energy level diagram showing that the heat of formation of NaCl is exothermic whilst that of C2H2 is endothermic - and I have no idea why that is!

    Any help would be much appreciated. Thanks
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    Sometimes energy is given out when a compound is formed, and sometimes energy must be put in in order for it to form.
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    It's all to do with the stability of what is being formed relative to the elements they are formed from. So you would expect quite a variety of values, positive and negative.
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    (Original post by EierVonSatan)
    It's all to do with the stability of what is being formed relative to the elements they are formed from. So you would expect quite a variety of values, positive and negative.
    Thanks for your reply.

    Does this mean however that there is no way to predict whether the enthalpy of formation of a particular substance would be endo or exo? (as opposed to other enthalpies [like the first ionisation energy] where you could reason out logically that the value would be +ve...)

    Thanks again though!
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    (Original post by VanDerWaal)
    Thanks for your reply.

    Does this mean however that there is no way to predict whether the enthalpy of formation of a particular substance would be endo or exo? (as opposed to other enthalpies [like the first ionisation energy] where you could reason out logically that the value would be +ve...)

    Thanks again though!
    Ha, I wish it was that simple

    The simple answer is no not really, they're experimetally derived numbers. There are a few generalisations though - salts are pretty much always exothermic
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    (Original post by EierVonSatan)
    Ha, I wish it was that simple

    The simple answer is no not really, they're experimetally derived numbers. There are a few generalisations though - salts are pretty much always exothermic
    Oh I see ...

    Thanks yet again
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    A related question has popped into my head.

    If you have two elements. What is it that makes the two elements react together? In other words, why do elements react together? From what I've read so far, it's just because elements want to gain a full outer shell of Electrons, as there is a certain stability associated with this.

    But is the more fundamental reason something to do with a change in Entropy? I tried reading "Why chemical reactions happen" by OUP, but I only understood the first 3 chapters
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    (Original post by KiraMayz)
    A related question has popped into my head.

    If you have two elements. What is it that makes the two elements react together? In other words, why do elements react together? From what I've read so far, it's just because elements want to gain a full outer shell of Electrons, as there is a certain stability associated with this.

    But is the more fundamental reason something to do with a change in Entropy? I tried reading "Why chemical reactions happen" by OUP, but I only understood the first 3 chapters
    Yeah, sort of. Whether something will happen is related to the Gibbs free energy G. Things react to lower the overall energy of the system i.e. more stable.

    Both enthalpy and entropy play a role as described by the equation: \Delta G = \Delta H - T\Delta S

    A process is favourable when ΔG < 0, so the more exothermic and the higher the gain in entropy the more favourable the process
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    ah. Kinda makes sense. I'm currently reading my way through a comprehensive A-level Chemistry text book. Haven't got to the thermodynamics chapter yet. Haha ^^
 
 
 
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