6.7 Reaction Rate: is the number of atoms, ions, or molecules that react in a given time to form products

Activation energy is the minimum energy the colliding particles must have in order for a reaction to occur.  It is a barrier that reactants must cross to form products.

If activation energy is very high and the collisions between particles are not energetic enough, the reaction could take a very long time to occur.  

Other factors however can be employed to speed up the reaction rate:

Temperature: Heat speeds up reactions by making the reactant particles move and collide faster and more frequently.  Lowering the temp will slow a reaction down.

Catalysts: these are substances which will speed up reactions by lowering the energy of activation.  Catalysts remain unchanged in the reaction.  They are written above the arrow to show that they are neither reactants nor products.  Catalysts can speed up reactions that would normally occur at much slower rates, but they CAN NOT make a reaction occur if it would not normally occur.

        Enzymes are the catalysts in living organisms.

Concentration: Increasing the concentration of reactants will speed up the reaction rate.

Particle Size and Surface Area: Larger surface area = faster reaction.  Homogeneous mixtures react more rapidly that heterogeneous mixtures.

6.8 Reversible Reactions

In very exorgonic reactions, reactants are used up to become products.  When reactions are only slightly exorgonic we’ll have a reversible reaction.: the forward reaction and the reverse reaction occur at the same time, so either side can be looked upon as being reactants or products.

Eventually the reactants will reach a state in which the forward reaction and the reverse reaction are occurring at the same rate.  This is Chemical Equilibrium.

Equilibrium Position: this refers to the relative amounts of reactants and products present at equilibrium.  It tells us whether the reactants or products are favored in a reversible reaction.

If, at equilibrium, the product is more abundant than the reactant, the reaction favors the products.

If, at equilibrium, the reactant is more abundant than the product, the reaction favors the reactants.Catalysts do not affect the equilibrium position.  They only decrease the amount of time it takes to reach equilibrium.

The equilibrium position can be shifted.

Le Chatlier's principle : If a system in dynamic equilibrium experiences a change in conditions, the system changes to relieve the stress.        

2 ways to shift the equilibrium position are:

Changing the concentration of reactants and products: adding more reactant will push the reaction in the direction of the products.  Adding more product will push the reaction in the direction of the reactants.

Changes in temp: increasing temp shifts the reaction toward the reactants (because heat is a product, and adding a product pushes the reaction in the direction of the reactants.)  Removing heat will cause more product to be formed.

Equilibrium Constant (K_eq) : relates the relative amounts of reactants and products in a chemical reaction at equilibrium.