Section 8.1—Equilibrium What is equilibrium?

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

Collision Theory & Reaction Rate Recall that molecules MUST collide with proper energy and orientation before they can react!!! The more molecules there are, the more often these successful collisions will occur—Reaction rate will be faster. At first the concentration of reactants is high—the forward reaction has a high reaction rate As reactants start to turn into products, the reaction rate lowers (fewer molecules) Reaction rate Time Forward reaction

Collision Theory & Reaction Rate But in reversible reactions, once there are products the reverse reaction can occur as well. 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 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.

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

Dynamic Equilibrium Dynamic Equilibrium – The reaction continues to proceed in both directions, 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!