Chemical Equilibrium

Chemical equilibrium is a dynamic equilibria in chemical reaction, which means the rate from reactants to products (forward rate) equals the rate from products to reactants (reverse rate).
Example:
2NO₂ßà N₂O₄
Forward reaction: 2NO₂à N₂O₄  Rate forward=K_f[N₂O₄]
Reverse reaction: N₂O₄à2NO₂  Rate reverse=Kr[NO₂]²

Chemical equilibrium occurs when  Rate forward equals Rate reverse.

Image from: Equilibrium.gif

Here are 3 important phenomenons and conditions to sustain a chemical equilibrium:

• At equilibrium, the concentration of products and reactions are unchanged.
• To keep the chemical equilibrium, neither products and reactants can escape from the system.
• The ratio of both forward and reverse reaction is a constant. 

The first one and second is easily understand. It just like we put the same weight object on each side of scale. If we change the weight of one side, then the scale will not balance. So, to keep the equilibrium, we need make sure the concentration of each substances unchanged. Keep reactants and products escape from system actually is keep them concentration unchanged. The ratio of forward and reverse reaction is decide by coefficients of reaction. Hence, for a certain reaction, the coefficient is certain, and the ratio is certain which is a constant.