Aim: What is equilibrium? DO NOW: State what a system at equilibrium means.

Reversible Reactions A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

Equilibria Equilibrium occurs when the forward and reverse reactions occur at the same rate; the concentration of the reactants and products remain constant. Can only occur in a closed system; reactants nor products can leave the system.

Phase equilibrium The rate at which a substance evaporates is equal to the rate which the substance condenses. Rate of melting = rate of freezing

Solution Equilibrium A saturated solution is at equilibrium; the rate of dissolving and recrystallization are equal.

Chemical Equilibrium Chemical equilibrium is when the rates of the forward and reverse reactions are equal in a chemical reaction.

Chemical Equilibrium The forward reaction will continue to slow as the reverse reaction proceeds until equilibrium is met. That is chemical equilibrium.

Question Which is a property of a reaction that has reached equilibrium? a.The amount of products is greater than the amount of reactants. b.The amount of products equals the amount of reactants c.The rate of the forward reaction is greater than the rate if the reverse reaction d.The rate of the forward reaction is equal to the rate of the reverse reaction and the reactions continue going on.

Le Chatelier’s Principle If a stress is applied to a system in dynamic equilibrium, the system changes in a way to relieve the stress.

Change in Concentration Addition of concentration on the reactant side would cause an increase of product; equilibrium is said to “shift to the right.” Addition of concentration on the product side would cause an increase of reactant; equilibrium is said to “shift to the left.”

Change in Temperature: Endothermic Reaction When heat is added, the equilibrium shifts to the right. When heat is removed (cooled), the equilibrium shifts to the left. H 2 + I 2 + heat   2HI

Change in Temperature: Exothermic Reaction When heat is added, equilibrium shifts to the left. When heat is removed (cooled), the equilibrium shifts to the right. N 2 + 3H 2   2NH kJ

Change of Pressure Change of pressure has only an effect on gases. If the pressure on an equilibrium is increases, the reaction is shifted in the direction of less gas molecules (smaller number of gas molecules). N 2 (g) + 3H 2 (g)   2NH 3 (g) When there is more pressure, the reaction shifts to the right.

Summary Add to left, shift to right Add to right, shift to left Remove from left, shift to left Remove from right, shift to right If increase pressure, shift to the side with less gas molecules

1.In which reaction will the point of equilibrium shift to the left when the pressure in the system is increased? a.C(s) + O 2 (g)   CO 2 (g) b.CaCO 3 (s)   CaO(s) + CO 2 (g) c.2Mg(s) + O 2 (g)   2MgO(s) d.2H 2 (g) + O 2 (g)   2H 2 O(g)

2. Given the equilibrium reaction at constant pressure: 2HBr(g) kJ   H 2 (g) + Br 2 (g) When the temperature is increased, the equilibrium will shift to the a.right, and the concentration of HBr(g) will decrease b.Right, and the concentration of HBr(g) will increase c.Left, and the concentration of HBr(g) will decrease d.Left, and the concentration of HBr(g) will increase

3. Given the reaction at equilibrium: N 2 (g) + O 2 (g)   2NO(g) As the concentration of N 2 (g) increases, the concentration of O 2 (g) will a.Decrease b.Increase c.Remain the same