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SIMZZ
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#41
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#41
(Original post by kingpong)
Not necessarily. I'm not positive, I had to have a bit of a think when I did it. Working out equilibrium shifts from temperature changes always hurts my underused brain.
yeah the reaction was exothermic, because lowering temperatures favor exothermic reactions. Thats what i wrote.
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britney
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#42
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#42
(Original post by buzfvar_1)
Is YIELD affected by a CATALYST?
I re-sat this paper, it was a lot harder than past papers, got 71/90 on the last one think ill be def getting lower than that on this one.
In answer to yur question, i wrote that:-
catalyst increase both the forward and backward reaction to the the same amount, however it means that equilibrium is reached quicker, therefore more yield will be produced in a shorter time so the yield increases. I only know this as this was on a past paper question and that was the answer so i think that i got it right!
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kingpong
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#43
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#43
(Original post by britney)
I re-sat this paper, it was a lot harder than past papers, got 71/90 on the last one think ill be def getting lower than that on this one.
In answer to yur question, i wrote that:-
catalyst increase both the forward and backward reaction to the the same amount, however it means that equilibrium is reached quicker, therefore more yield will be produced in a shorter time so the yield increases. I only know this as this was on a past paper question and that was the answer so i think that i got it right!
AFAIK that's incorrect. Yield refers to the % of product, ie. the position of the equilibria. Yield only refers to the final proportion of products in a reaction, not how fast they are produced over a given time. What you're talking about there is rate of reaction. The presence of a catalyst doesn't effect the position of an equilibria (as you said), it only speeds up the rate it is reached.
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SIMZZ
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#44
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#44
(Original post by britney)
I re-sat this paper, it was a lot harder than past papers, got 71/90 on the last one think ill be def getting lower than that on this one.
In answer to yur question, i wrote that:-
catalyst increase both the forward and backward reaction to the the same amount, however it means that equilibrium is reached quicker, therefore more yield will be produced in a shorter time so the yield increases. I only know this as this was on a past paper question and that was the answer so i think that i got it right!
Thats is so wrong.. this is the text straight out the AQA collins book.. A catalyst does not alter the equilibrium position: the equilibrium CONCENTRATIONS REMAIN THE SAME!!. The effect of adding a catalyst is to increase the rate of forwards and backwards reactions to the same extent, so its takes a shorter time to reach equilibrium. So no..it does not increase the yield!
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britney
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#45
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#45
i guess we will have to see. but if a catalyst makes equilibrium be reached quicker, remembering the question that was asked i think you will find that it would result in an increase in yield of h2.
But i could be wrong and if i am i apologise.
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kingpong
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#46
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#46
(Original post by britney)
i guess we will have to see. but if a catalyst makes equilibrium be reached quicker, remembering the question that was asked i think you will find that it would result in an increase in yield of h2.
But i could be wrong and if i am i apologise.
No need to apologise.

% yield pertains to equilibruim position, right? Presence of catalyst has NO effect on eqm position, so how can it alter % yield? It isn't how quickly the product is formed, it's what proportion of the eqm it makes up.
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scottyoneill
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#47
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#47
it was +ve...think about it like this

if you apply heat to an exothermic reaction i.e. burning methane, then the more heat, the greater the rate of reaction....so therefore if lower heat favours it then endothermic...

or think about it this way..

le chatelier states that equilibrium will oppose changes, so, decreasing temperature, the equilibrium will shift to try and raise pressure....so therefore, forward reaction is raising temp i.e. taking in heat...

so its endothermic...

endothermic has +ve enthalpy change
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ShOcKzZ
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#48
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#48
If you're talking about that question in the exam, it was an exothermic reaction (-ve)

This is because the temperature was lowered, and the yield was increased.
If the temperature had been increased..and yield increased then it would be endothermic.

Table on pg 20 of the little AQA collins book.
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