1. Typical Reaction: reactants  products
Actually, most reactions are REVERSIBLE:
Reactants  products
Reactants  products
Reactants > products

2. When a reaction is occurring:
Rate of forward rxn > rate of reverse rxn
product will be formed (reaction favors products)
Rate of reverse rxn > rate of forward rxn
Product will decompose to reactants (reaction favors reactants)
Rate of forward rxn = rate of reverse rxn
Product is being formed at same rate as being decomposed……Rxn is at EQUILIBRIUM

3. Some Equilibrium Tidbits:
For equilibrium to occur, the system must be CLOSED (neither products nor reactants can enter or leave the system)
Equilibrium can be approached from either side of the reaction equation
At equilibrium, the concentration of the reactants and products do not change

4. At equilibrium, OBSERVABLE PROPERTIES become constant
Colour change - colour intensity has stabilized
Mass of solid formed remains the same
Gaseous reaction – pressure stabilizes

5. Physical Equilibrium: equilibrium which has occurred during a physical change (eg. Ice water)
Chemical Equilibrium: equilibrium which has occurred during a chemical change

6. HOMOGENEOUS REACTION EQUILIBRIUM: products and reactants are all in same phase (gases or in solution)
eg. N2 (g) + 3H2 (g)  2NH3 (g)
HETEROGENEOUS REACTION EQUILIBRIUM: products and reactants are not all in same phase
eg. CaCO3 (s) > CaO (s) + CO2 (g)

7. Equilibrium can be altered by changes in:
Concentration of reactants or products
Temperature
Volume and pressure (if gases)
BUT NOT BY A CATALYST
A CATALYST AFFECTS BOTH THE FORWARD AND THE REVERSE RATE…THE SYSTEM WILL REACH EQUILIBRIUM FASTER, BUT EQUILIBRIUM IS STILL THE SAME!!