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    Hi,
    So I'll be sitting the C3 exam Monday, and in the spec I've got, it states that we should be able to
    "Explain why a moderately high temperature is chosen despite a higher yield being achieved at lower temperatures. Also explain why a higher pressure is not chosen although that would give a higher yield.''

    So, I know a) An iron catalyst is used
    b) There is a (realtively) high temp. used
    c) The pressure is lower at around 200 atm.

    But I don't know why these conditions are used.
    Anyone know?
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    A high temperature is used because it increases the rate of reaction (but as you've said, not too high as it lowers the yield).
    High pressure is used as it gives a higher yield, because high pressure favours the forward reaction (the haber process is a reversible reaction). Again, this condition is a compromise, as they cannot use too high a pressure because it would be too expensive to build a building that could withstand such pressure.
    An iron catalyst is used to speed up the rate of reaction. This also allow a lower temperature to be used.
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    (Original post by 3mma_gal96)
    Hi,
    So I'll be sitting the C3 exam Monday, and in the spec I've got, it states that we should be able to
    "Explain why a moderately high temperature is chosen despite a higher yield being achieved at lower temperatures. Also explain why a higher pressure is not chosen although that would give a higher yield.''

    So, I know a) An iron catalyst is used
    b) There is a (realtively) high temp. used
    c) The pressure is lower at around 200 atm.

    But I don't know why these conditions are used.
    Anyone know?
    An iron catalyst is used to speed up the rate of reaction, so the yield of ammonia can be obtained at a faster rate. As the forward reaction is exothermic, increasing the temperature would increase the rate of the reverse reaction, decreasing the yield of ammonia. However, using lower temperatures would mean that the rate of reaction would be very slow - so a compromise between these two facts is struck. A relatively high temperature is used so that the yield of ammonia is still relatively good, with the rate of reaction being relatively high too.
    When pressure is lowered, the position of equilibrium moves to the side with more gas molecules (to the left towards the reactants in this case). Hence, to produce more ammonia a higher pressure would be favoured. However, this involves compromise too! Maintaining a high pressure involves the input of lots of energy to compress the gases - so a pressure is used that is high enough to give a relatively high yield of ammonia, but not so high that lots of energy is needed to compress the gases and that the workforce are put in danger.

    I hope that is helpful
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    (Original post by izpenguin)
    A high temperature is used because it increases the rate of reaction (but as you've said, not too high as it lowers the yield).
    High pressure is used as it gives a higher yield, because high pressure favours the forward reaction (the haber process is a reversible reaction). Again, this condition is a compromise, as they cannot use too high a pressure because it would be too expensive to build a building that could withstand such pressure.
    An iron catalyst is used to speed up the rate of reaction. This also allow a lower temperature to be used.
    I'm sorry for basically repeating what you just said, I didn't realise that you posted until after I posted! Silly me :P
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    (Original post by zef1995)
    I'm sorry for basically repeating what you just said, I didn't realise that you posted until after I posted! Silly me :P
    ooops i did the same thing!!


    well at least now you have 3 similar accounts
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    (Original post by pinkcherrytart)
    ooops i did the same thing!!


    well at least now you have 3 similar accounts
    Haha, that's okay!

    That's very true, with our combined knowledge we should have covered everything
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    (Original post by zef1995)
    Haha, that's okay!

    That's very true, with our combined knowledge we should have covered everything
    Yeah, thank-you, makes much more sense now!
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    (Original post by 3mma_gal96)
    Yeah, thank-you, makes much more sense now!
    That's great to hear, glad we could be of help
 
 
 
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