Section 8.1—Equilibrium What is equilibrium?

Reversible Reactions Reversible Reaction – A chemical reaction that can proceed in both direction (represented by a “  ”) Molecules A & B collide to produce molecules C & D But in some reactions, molecules C & D can collide and produce A & B again Reversible reactions are shown with a double arrow A + B C + D

Collision Theory & Reaction Rate At first the concentration of reactants is high—the forward reaction has a high reaction rate As reactants start to react away into products, the reaction rate lowers (fewer molecules Collision theory says that molecules must collide with right orientation & minimum energy to react (Chapter 2) The more molecules there are, the more often these successful collisions will occur—Reaction rate will be faster. Reaction rate Time Forward reaction

Collision Theory & Reaction Rate At first the concentration of products is very low, the reaction rate of the reverse reaction is low As products begin to form, the reverse reaction rate increases But in reversible reactions, one there are products the reverse reaction can occur as well. Reaction rate Time Reverse reaction

Putting Forward & Reverse Together Reaction rate Time Forward reaction Reaction rate Time Reverse reaction Reaction rate Time Forward reaction Reverse reaction Equilibrium is reached

Equilibrium Equilibrium – When the rate of the forward reaction equals the rate of the reverse reaction.

Establishing equilibrium It takes time to establish equilibrium Reactants  Products At first, there are only reactants present. Only the forward reaction is possible.

But once there are products as well, they can begin to reform reactants. The reverse reaction becomes possible. Establishing equilibrium It takes time to establish equilibrium Reactants  Products Once the rate of the forward and reverse process are equal, it is at equilibrium. When equilibrium is established, the number of products and reactants doesn’t change…but the reaction keeps going.

Dynamic Equilibrium Dynamic Equilibrium – The reaction continues to proceed in both direction, but at the same rate. The number of products and reactants no longer change, it may look as thought the reaction has stopped… But the reactions continues!