Introducing Dynamic Equilibria
Things change when reversible reactions are carried out under ‘closed’ conditions – meaning no substances are added to the reaction mixture and no substances can escape from it. Heat however, can be given off or absorbed.
In a reversible reaction, you have the forward reaction (the reaction going from left to right) and the back reaction (the opposite of the forward reaction) happening at the same time. Both rates of reactions will become equal and this point is the dynamic equilibrium. It is dynamic in a sense that the reactions are still continuing, and equilibrium in a sense that the total amounts of the various things present are now constant. In other words:
A + 2B <-> C + D
When you have a reaction like the above, A + 2B (forward reaction) is reacting to produce C + D (back reaction). At the same time, C + D is reacting to produce A +2B. In the end, you have equal amounts of products and reactants. Another way to think of is, is to imagining walking down an elevator that goes up, making sure you’re walking at the same speed as the elevator. You would be going down, but everytime you take one step down, the elevator goes one step up. In the end, you remain where you are.
So how would you produce more of substance C in a reversible reaction such as the above? You can do this by altering the position of the equilibrium by either:
- Changing the pressure
- Changing the temperature
- Increasing/Decreasing the concentrations of substances present
- Adding a catalyst
If a dynamic equilibrium is disturbed by changing the conditions, the reaction moves to counteract the change.
In other words, the reaction will either go more towards the ‘forward’ direction or the ‘back’ direction in an attempt to ‘adapt’ to the conditions.
A + 2B <-> C + D
Changing the Concentration
What happens when more of A is added? If you add more A, the reaction will want to remove it. This can only be done by reacting more A to 2B, and in the end, this gives us more C and D. The conditions, in this case, favour the forward reaction.
Changing the Pressure
When you increase the pressure, you bring molecules closer together. Increasing the pressure will always help the reaction go in the direction which produces the smaller number of molecules. In this case, we have 3 molecules on the left (one A and 2 Bs), whereas, we only have 2 molecules on the right (one C and one D). The reaction can only reduce the pressure by producing few molecules. This can only be done by producing more C and D.
Changing the Temperature
Suppose the forward reaction was exothermic
A + 2B <-> C + D ∆H = -100 kJ mol-1
This would mean that the back reaction would be endothermic by the same amount (-100 kJ mol-1).
Suppose the temperature was decreased, the reaction would respond by increasing the temperature back up again. This can only be done by producing more C and D because the forward reaction is exothermic. Increasing the temperature will of course, have the opposite effect.
Adding a Catalyst
Adding a catalyst speeds up the forward and back reactions by the same proportion. This means that there is no change in the position of the equilibrium.