Equilibrium

Watch this for insights into chemical equilibrium in industry needed for Higher Chemistry (breathe in, and hold…)

Have you ever tried yoga? It is all about finding a balance – the point at which your body and your mind are perfectly weighted. Allowing opposing forces to work against each other to keep stable and in control. It is challenging but it is powerful.

Nature seeks balance too – an equilibrium between the forces that are at work. Many would argue that we have upset this balance and that our impact on the natural world needs to be reduced.

Balance is something that chemical reactions try to find too, and by understanding it, and manipulating it, we can make it work to our benefit. Let me show you what I mean…

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Most chemical reactions do not go to completion, for some there will be a very high concentration of products compared to the reactants. For others there will be a noticeable concentration of reactants still remaining. This result is dependent on what we call the equilibrium.

We’ll use the Haber Process as an example. One mole of nitrogen reacts with three moles of hydrogen to form two moles of ammonia. You will be aware of this reaction and its importance in the process of making fertiliser and the huge impact on the agriculture industry.

This reaction is a reversible reaction shown by the arrows going both ways in the chemical equation. At the start there will be 100% reactants and 0% products, but as reaction proceeds the percentage of reactants decrease while the products increase until they reach a point where the concentration remains constant. At this point the rate of the forward reaction is equal to the rate of the reverse reaction. While it may look like the reaction has finished, the forward reaction and the reverse reaction are constantly occurring indefinitely. A balance has been struck.

Henry Louis Le Chatelier studied the equilibrium and what happened when it was disturbed. He stated the principle that ‘if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.’

In industry, an aim is to have a high percentage yield of product so that the reaction is effective and produces good profit for the company. There are three ways that you need to be aware of in how we can manipulate a reaction equilibrium to make this happen.

First of all there is concentration. Referring to the Haber process, if we were to remove the ammonia being produced then the equilibrium would shift towards the right, towards the products, increasing the yield of product being made. Or if we added more hydrogen, the equilibrium would again shift to the right to counteract this and make more product.

Secondly there is temperature. The Haber Process is run at about 400 to 450oC. This temperature is a compromise between shifting the equilibrium to the right and the rate of the reaction. When increasing the temperature of a reaction at equilibrium, the equilibrium will shift towards the endothermic direction. The forward reaction for the Haber process is exothermic, so an increase in temperature would favour the reverse reaction. The opposite is true; however, we can’t have too low a temperature to favour the exothermic forward reaction as the rate would be too slow. Therefore, a balance has to be made.

Finally, there is pressure. This only refers to substances in the gaseous state and will only affect an equilibrium where there are different number of gaseous moles on each side of the reaction. With the Haber process there is a total of 4 moles on the left and 2 moles on the right. The equilibrium counteracts a high pressure by shifting to the side where there are fewer molecules to relieve the stress. In this case, a high pressure would shift the equilibrium to the right increasing the product yield of ammonia.

There are times in life to find a balance but there are also times to distort the balance.

Understanding the chemical equilibrium comes through practising with different reactions and changing the conditions – understand what changes the balance, and why.

And if you get the chance, give yoga a shot. It might just help with your balance as well as your chemistry.

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