What is equilibrium?What is equilibrium? Chemical reaction stops BEFORE the limiting reactant is consumed. All of the reactants are not COMPLETELY converted to products. o Amount of reactant converting to product = amount of product converting back to reactant Represented in a chemical reaction by arrows pointing in BOTH directions. (  ) Reactant and product concentrations are STABLE

Ex. H 2 + O 2  H 2 O

Chemical Reactions at Equilibrium  Big Picture/Forest  Reaction looks like it is STOPPED.  Ex. Helicopter view of traffic jam.  Small Scale/Trees  Reaction is moving, “dynamic” balance between forward/reverse reactions.  Ex. Road view of traffic jam.

Chemical Reactions at Equilibrium (cont.) Rate of forward = Rate of backward Equilibrium constant expression: o Ratio of the reactant concentration and product concentration. o Concentration MUST be in molarity (M) o A + B  C + D o K C = [products] [C][D] [reactants] [A][B]

Equilibrium Constant (K C )Equilibrium Constant (K C )  constant value for each chemical equation at equilibrium  numerical value of ratio  The equilibrium constant allows us to …. 1) Predict the direction in which a reaction mixture will proceed to achieve equilibrium. 2) Calculate the concentrations of reactants and products once equilibrium has been reached.

 aA + bB ↔ cC + dD  at equilibrium at a given temperature  K varies with temperature and is UNITLESS.

Example (a): Write the equilibrium expression for… PCl 5 (g)  PCl 3 (g) + Cl 2 (g) NOTE: [ ] denotes concentration.

Example (b): Write the equilibrium expression for… 4NH 3 (g) + 5O 2 (g)  4NO (g) + 6H 2 O (g)

Now you try……Now you try……  1) 2NO 2  N 2 O 4  2) 2SO 3  2SO 2 + O 2  3) N 2 + 3H 2  2NH 3

Ex 1: A mixture was found to contain the following concentrations: M PCl 3, M Cl 2 and M PCl 5. Write the equilibrium expression and calculate K. PCl 5 ↔ PCl 3 + Cl 2

How do we use a value like k=0.53 to determine a reaction’s direction??? When k >> 1, PRODUCTS are favored. When k << 1, REACTANTS are favored. Think about this statement using ………… o K C = [products] [reactants]

 Equilibrium Worksheet