Chemical Equilibrium Chapter 18

Consider a glass of water… Evaporation

Consider a glass of water… Now, put a lid on it….

Consider a glass of water… Evaporation continues, but condensation also occurs...

Consider a glass of water… The rates equalize, and the system reaches equilibrium.

Chemical Equilibrium Consider the following reaction(s): H 2 O (liquid)  H 2 0 (gas) H 2 O (gas)  H 2 O (liquid) H 2 O (liquid)  H 2 O (gas) Equilibrium Symbol

Chemical equilibrium occurs when opposing reactions are proceeding at equal rates. The rate at which the products are formed from the reactants equals the rate at which the reactants are formed from the products. For equilibrium to occur, neither reactant nor products can escape from the system.

N 2 + 3H 2  2NH KCal Forward Reaction

N 2 + 3H 2  2NH KCal Reverse Reaction

Reversible Reactions REVERSIBLE REACTIONS do not go to completion and can occur in either direction: aA + bB  cC + dD

CHEMICAL EQUILIBRIUM exists when the forward & reverse reactions occur at exactly the same rate Reactant Product Time (reaction progress ) EQUILIBRIUM concentration

At equilibrium: If there are more products than reactants, the products are said to be favored. If there are more reactants than products, the reactants are said to be favored.

The Equilibrium Constant, K c For the reaction: aA + bB  cC + dD at equilibrium, the constant, k c or k eq : K c is a measure of the extent to which a reaction occurs; it varies with temperature.

A few notes about K c … K c is completely unrelated to rate constant, k “k” is generically used for “constant” in science In K c, the “c” is for “concentration” Note that we plug in molarities, or concentrations to solve for K c There are other equilibrium constants that are studied in IB Chemistry (K a, K b, K w, K p,K sp )

Example (a): Write the equilibrium expression for… PCl 5  PCl 3 + Cl 2

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

Ex: One liter of the equilibrium mixture from example (a) was found to contain mol PCl 3, mol Cl 2 and mol PCl 5. Calculate K c. PCl 5  PCl 3 + Cl 2

What does K c =0.53 mean to me??? When K c >> 1, most reactants will be converted to products. When K c << 1, most reactants will remain unreacted.

The equilibrium constant allows us to …. Predict the direction in which a reaction mixture will proceed to achieve equilibrium. Calculate the concentrations of reactants and products once equilibrium has been reached.

Reaction Quotient (Q) Reaction Quotient (Q) is calculated the same as K c, but the concentrations are not necessarily equilibrium concentrations. Comparing Q with K c enables us to predict the direction in which a rxn will occur to a greater extent when a rxn is NOT at equilibrium.

Comparing Q to K c When Q < K c : When Q = K c : When Q > K c : Forward rxn predominates – “reaction proceeds to the right”(until equil. is reached) System is at equilibrium Reverse reaction predominates – “reaction proceeds to the left” (until equilibrium is reached)

Ex: H 2 (g) + I 2 (g)  2HI(g) K c for this reaction at 450  C is 49. If 0.22 mol I 2, 0.22 mol H 2, and 0.66 mol HI are put into a 1.00-L container, would the system be at equilibrium? If not, what must occur to establish equilibrium. Q < K c ; therefore forward reaction predominates until equilibrium is reached.

Ex: PCl 3 (g) + Cl 2 (g)  PCl 5 (g) K c =1.9 In a system at equilibrium in a 1.00 L container, we find 0.25 mol PCl 5, and 0.16 mol PCl 3. What equilibrium concentration of Cl 2 must be present?