Forwards, backwards, left or right? – Equilibria Part 1

There were a few questions on equilibria in this year’s May exams. As a teacher I felt that students didn’t perform very well on these questions (I have no evidence for this apart from conversations with a very small number of students who sat the exams) so thought that this month may be an opportunity to help reinforce and summarise some of the concepts covered.

First of all, what do we mean by equilibrium? A reaction that is at equilibrium doesn’t reach completion. Reactants turn into products, but also products turn into reactants. The reaction is reversible and does not have an arrow:

but the two fish hook signs:

Initially, the rate of reactants turning into products is greater than products to reactants. The reaction where reactants turn to products sometimes referred to as the reaction to the ‘right’ or ‘forwards reaction’ with products turning into reactants termed the reaction to the ‘left’ or ‘backwards reaction’.

So if the rate of reactants turning into products is faster than the rate of products into reactants we may say the reaction is proceeding in the direction of the right hand side or in the forwards direction.

What will happen in this reaction is that over time, the rate of forwards reaction decreases and the rate of backwards reaction increases. Fewer reactants will be turned into products and more products turned into reactants.

Eventually, the rate of forwards reaction equals the rate of backwards reaction. At this point, equilibrium is reached. It is important to understand that at equilibrium, a reaction does not stop or pause or become fixed or locked – it is dynamic and continues to proceed or react.

However, as the rate of forwards and backwards reaction is equal, the concentration of products and reactants remains unchanged so it seems as if the reaction has finished or stopped.

An equilibrium does not occur when there are 50% products and 50% reactants (it could do by chance, but in reality, this is unlikely). One may find that at equilibrium, a reaction has 78% reactants and 22% products (those numbers have been picked out of the air).

As a chemist (more likely an industrial chemist) your task would be to do something to the reaction to increase the proportion of products (for example, you may wish to increase the % or product from 22% to 80%. This would also mean that the amount of reactants drop from 78% to 20%).

This is achieved using Le Chateliers principle… but more on this in the next post.

Do you have any questions related to the concepts covered below? If you do, please feel free to post them below as I would love to hear from you.

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