Dynamic Equilibrium

The initial dissolution (the process of dissolving) of sodium chloride in water is thought to be the result of collisions among water molecules and ions that make up the crystals. Once ions have entered the dissolved state, collisions between water molecules and the remaining crystal continue. However, dissolved ions will now also collide with the crystal.

If both dissolving and crystallizing processes take place at the same rate, no observable changes would occur in either the concentration of the ions in solution or in the quantity of solid present. The system is in a state of dynamic equilibrium.

In a closed system, a phase change may establish an equilibrium, such as the evaporation/condensation equilibrium.

H2O(l) ↔ H2O(g)

http://www.google.ca/url?sa=i&rct=j&q=phase+equilibrium&source=images&cd=&cad=rja&docid=aXXVY7bSKWwiqM&tbnid=KhxZkZf1ochEsM:&ved=0CAUQjRw&url=http%3A%2F%2Fspiderman-bbq.blogspot.com%2F2010%2F03%2Fequilibrium.html&ei=okmKUafYMNTK4AOLiYG4Dw&bvm=bv.46226182,d.dmg&psig=AFQjCNGhBZ2MIAq91XHdhCYNi-SSiPbfdQ&ust=1368103703542498

Note: In an open container no equilibrium can be established because molecules that leave the surface of the liquid escape from the system and do not return to the liquid phase.

Chemical reaction equilibria are more complex than phase or solubility equilibria, due to the large variety of possible chemical reactions and the greater number of substances involved.Some chemical reactions proceed to completion, meaning that all of the reactants are converted to products in proportions dictated by a balanced chemical equation. Such reactions are called quantitative reactions.

open

closed

The equilibrium equation is

The percent reaction is the yield of product measured at equilibrium (actual product yield) compared with the maximum possible yield of product (theoretical product yield).

Classes of Chemical Reactions at Equilibrium

Stoichiometric calculations are straightforward when reactions proceed to completion. However, when reversible reactions achieve equilibrium before all of the reactants become products, the stoichiometry requires a little more thought.

I means initial concentrations of reactants and products (before reaction)

C stands for the change in the concentrations of reactants and products between the start and the point at which equilibrium is achieved

E stands for the concentrations of reactants and products at equilibrium