Chemical Equilibrium

Chapter 15 Notes

Section 15-1

A ---> B

B ---> A

kf[A] = kr[B]


kf = [B] = constant kr [A]

N2(g) + 3H2(g) <---> 2NH3(g)

  • Mass-Action Expression - expresses the relationship between the concentration of the reactants and products at equilibrium.

  • The brackets represent the concentration of either the reactants or products

Kc = equilibrium constant

Kc = [products]coefficients [reactants]coefficients so Kc = [NH3]2 [N2][H2]3

  • The equilibrium constant depends only on the stoichiometry of the equation.

Equilibrium Constant in terms of pressure


Where K = equilibrium constant and p = pressure

aA + bB ---> dD + eE

Kp = PDd PEe -------- PAa PBb

Where PAa = partial pressure of A and so on and the small case superscripts are the coefficients of from the balanced equation.
Kc and Kp are different values, however, you can convert from one to another.

Kp = Kc(RT)/\ n


Kp = equilibrium constant of partial pressure in atm
Kc = equilibrium constant
R = gas constant of 0.0821 L.atm / mol.K
T = temperature in Kelvin
/\n = change in moles of gas in the chemical equation (sum of the cofficients of the gaseous products minus the sum of the coefficients of the gaseous reactants)

Size of equilibrium constants can be large or small
  • value is over 1 = favors products
  • value less than 1 = favors reactants

  • The concentration of a pure liquid or solid = the density / its molar mass
    • the density of a pure solid or liquid at any given temperature is constant
  • When pure liquids/solids are involved in heterogeneous equilibrium, their values are not included in the equilibrium constant expression, however, they must still be present in the reaction to reach equilibrium.

Applications of Equilibrium Constants

K allows us to identify what is in our mixture at equilibrium
  • if K is small, the mixture contains mostly reactants
  • if K is large, the mixture contains mostly products
We can also predict (1) the direction in which a reaction will proceed to reach equilibrium and (2) calculate the concentration of reactants and products once equilibrium is reached.
  • can calculate a reaction quotient (Q) and compare to Kc
Kc = Q (only at equilibrium)
Kc > Q (substances on the left will react to form substances on the right)
Kc < Q (substances on the right will react to form substances on the left)

Quadratic equation

_________ x = -b \/b2 - 4ac ---------------- 2a

Le Chatelier's Principle
  • If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium to counteract the stress.
  1. Concentration
    • reactants or products
    • increase reactant ---> shifts to the right
    • increase products ---> shifts to the left
  2. Pressure & Volume
    • increase in pressure means a decrease in volume ---> equilibrium will shift in the direction that yields the smallest number of gas molecules
    • decrease in pressure means an increase in volume ---> equilibrium will shift in the directino that yields the largest number of gas molecules
    • (the number of molecules is proportional to the coefficients from the balanced equation)
    • if the number of molecules is equal, there will not be a shift
  3. Temperature
    • must know if the reaction is endothermic or exothermic
    • heat as a reactant - endothermic - equilibrium shifts to the right
    • heat as a product - exothermic - equilibrium shifts to the left
  4. Catalyst
    • substance that increases the speed of a chemical reaction but does not chemically change the reaction
    • increases both the forward and reverse reaction
    • causes the reaction to reach equilibrium quicker, does not cause a shift - it does not add a stress to the system