If there is a decrease in product, shouldn't equilibrium favour the forward rate of reaction, according to Le Chatelier's principle?

So why does the forward rate of reaction decrease?

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1 Answer

There is a subtle difference between "favouring" the forward rate of reaction, and an overall increase in the rate of reaction. The words "favour" and "net" are relative terms, while "overall" refers to absolute terms.

When product is removed from the environment, yes - Le Chatelier's principle is correct and the reaction will favour the forward reaction to partially compensate for the loss of product (by engaging in a net forward reaction).

However, when we say that the forward reaction is relatively favoured, we should not be lulled into thinking that the absolute forward rate of reaction increases. Rather, it only relatively increases.

That is, while the forward reaction is favoured (i.e.: it is faster than the backward rate of reaction), it does not necessarily mean it has gotten faster overall. A possibility is that the backward reaction rate decreased (so that the forward reaction rate is relatively higher.

In fact, when we remove product:

the concentration decreases, which:
decreases the rate of reaction for the backward reaction
this results in a net (relative) forward reaction, which:
consumes reactants, causing the absolute forward reaction rate to decrease, while:
the backward reaction rate increases (due to new products being formed) from it's new low point, until:
the forward and back reaction rates equilibrate (meet up and match) at a new equilibrium where the rate of the forward reaction and the rate of the backward reaction are equal, but at some lower level now (compared to the beginning).

This should intuitively make sense as we have removed some particles from the system -- this should make equilibrium rates (back and forth) lower than previously, as there is a lower chance for particle collision.

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