Water and ice are at equilibrium (balanced) at 0 degrees Celsius. E.g. H2O (l) = H2O (s) [the equilibrium point is at any point between these two extremes]
The number of water molecules freezing is at the same rate as the number of water molecules melting.
When a reversible reaction takes place in a closed loop system (no reactants/ products lost), a state of equilibrium can be reached.
Reaching equilibrium-Iodine example:
- Iodine = soluble in potassium iodide
- The iodine is dissolved in hexane before it is dropped into the test tube with the potassium iodide (aq)
- 2 layers form: one layer of hexane + iodine, other layer with potassium iodide. The aqueous potassium iodide and organic solvent of hexane do not mix together
- Shake tube--> some of the iodine transfers to the potassium iodide, changing the colourless potassium iodide into a pale colour
- Shaking again--> more iodine transfers--> the colour gets darker
- Continue shaking--> the distribution of iodine is equal between the hexane and the potassium iodide--> equilibrium has been reached
Rules of equilibrium
- When it comes to I/GCSE Chemistry, at equilibrium, the concentration of reactant and products doesn’t change; - only the distribution of particles changes.
- Equilibrium state can occur from the products or the reactants. E.g. if you put the iodine in the hexane, it will eventually lead to equilibrium of both solutions; likewise, if you place the iodine in the potassium iodide, it will also lead to equilibrium in both solutions.
Refer to I/GCSE Chemistry, when the solution of iodine in hexane was shaken, the movement was in one direction- the iodine particles were all moving from the hexane to the potassium iodide in the forwards direction.
However, when the potassium iodide started to increase too much in iodine concentration, the iodine particles began to head in the backwards direction (back to the hexane).
The forwards reaction is originally faster, but it gradually starts to slow, and then the backwards reaction starts to take precedence.
The rate becomes equal when the backwards and forwards reaction continue BUT there is no overall change because each layer is gaining and losing iodine at the same rate. The reactions cancel each other out. This is known as dynamic equilibrium.
You got it now!