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Equilibrium Basics
When you have a system made up of a bunch of molecules, those molecules sometimes combine. A chemical reaction sometimes starts at one point and moves to another (forward reaction). Sometimes , reaction may want to go through a reverse chemical reaction and become the original molecules again ( reverse or backward reaction).
Put those two ideas together and you have equilibrium:
1. Two reactants combine to make a product.
- Products like to break apart and turn back into the reactants.
There is a point where those two reactions happen and you can’t tell that any reactions are happening. That’s the point when the reaction looks like it is finished. In reality, some of the molecules are turning into products and some are turning back into reactants. That’s what equilibrium really is. There is no pressure greater in one direction over another.
Equilibrium always happens at the same point in the reaction no matter where you start. So, if you start with all of substance A, it will break up and become B and C. Eventually, B and C will start recombining to become A. Those reactions happen until they reach equilibrium. They reach equilibrium at the same point whether you start with all A, all B/C, or half A and half B/C. It doesn’t matter. There is one special point where the forward and reverse reactions cancel each other out.
Equilibrium is reached by itself with no outside forces acting on the system. If you put two substances in a mixture, they can combine and react by themselves. Eventually, they will reach equilibrium. Scientists say that equilibrium happens through spontaneous processes. They happen on their own.
A system “at equilibrium” appears to have no charge (neutral). All the pluses and minuses cancel each other out and give a total charge of “0”. Scientists use the letter “K” to add up all of the actions and conditions in a reaction. That “K” is the equilibrium constant.
The Equilibrium Constant
All chemical equilibrium systems obey the law of mass action.
- Where Kc = Products
- Reactants
For the general equilibrium equation
aA + bB ↔ cC + dD
A value called the equilibrium constant can be assigned to the chemical system by measuring, at equilibrium, the concentrations of all the species.
The equilibrium constant in the above reaction will be given by
- K = [C]c [D]d
- [A]-a [B]-b
The law of mass action can be extended to systems of any number or reactants and products. If K is much larger than unity, the reaction is said to lie on the right. If K is much smaller than one, then the equilibrium lies to the left.