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Rate of reactions

This question refers to question 1.a.ii from this paper:

I wrote:

The reaction has reached equilibrium, so the rates of the forward and backward reactions are equal and constant, so the rate of reaction is constant.

However, the markscheme only accepts 'the rate of reaction is zero' and 'all the magnesium has reacted' (or a similar explanation), with nothing about equilibrium at all. I don't really understand this because from what I understood, all reactions are reversible to a certain extent so surely the rate of reaction isn't really zero at all (even though it will be close) and hence the mark scheme is wrong?
Original post by Chlorophile
This question refers to question 1.a.ii from this paper:

I wrote:

The reaction has reached equilibrium, so the rates of the forward and backward reactions are equal and constant, so the rate of reaction is constant.

However, the markscheme only accepts 'the rate of reaction is zero' and 'all the magnesium has reacted' (or a similar explanation), with nothing about equilibrium at all. I don't really understand this because from what I understood, all reactions are reversible to a certain extent so surely the rate of reaction isn't really zero at all (even though it will be close) and hence the mark scheme is wrong?


You are told that there is excess acid. This ensures that ALL of the magnesium has reacted. When no more gas is produced it means that the magnesium has all gone and hence the rate of the reaction is zero.

This is not an equilibrium reaction, it goes to completion and all of the magnesium gets used up...
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Chlorophile
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Original post by charco
You are told that there is excess acid. This ensures that ALL of the magnesium has reacted. When no more gas is produced it means that the magnesium has all gone and hence the rate of the reaction is zero.

This is not an equilibrium reaction, it goes to completion and all of the magnesium gets used up...


I thought all reactions were in dynamic equilibrium to a lesser or higher extent?
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Borek
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Original post by Chlorophile
I thought all reactions were in dynamic equilibrium to a lesser or higher extent?


And on some general level you are right. But on less general level, when equilibrium lies so far to the right we can't even think about detecting reactants in the mixture, we treat the reaction as irreversible.
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Chlorophile
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Original post by Borek
And on some general level you are right. But on less general level, when equilibrium lies so far to the right we can't even think about detecting reactants in the mixture, we treat the reaction as irreversible.


Alright, fair enough. Thanks!
Original post by Chlorophile
I thought all reactions were in dynamic equilibrium to a lesser or higher extent?


If one of the components for the reverse reaction is removed as soon as it is formed it is impossible for the reaction to reach equilibrium. There are simply no reactants available for the reverse reaction.

In this case the hydrogen gas leaves the reaction mixture and the reaction goes to completion ...

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