CHEMICAL EQUILIBRIUM
For any chemical reaction, when the rate of forward reaction equals the rate of reverse
reaction, equilibrium is reached, and the concentrations of all reactants and products reach a
steady level.
A+B AB
Rate for = kfor [A][B]
Rate rev = krev [AB]
At equilibrium,
Rate for = Rate rev or
kfor [A][B] = krev [AB] or kfor / krev = K, the equilibrium constant = [AB] / [A][B]
Only aqueous and gaseous substances are included in the formula, liquids and solids are
omitted.
If there are reaction coefficients,
aA + bB cAB ,
K = [AB]c / [A]a[B]b , where [A],[B]and [AB] are equilibrium concentrations.
K for the reverse reaction is 1/K
A large value of K (>1) indicates mostly products are formed as the reaction moves to the right.
A small value of K (<1) indicates mostly reactants remain as the reaction stays to the left.
The reaction quotient Q is a constant that indicates the constant that can be calculated when
initial concentrations of reactants and substances are given. It can be compared to K to indicate
the direction of the reaction.
When K>Q , reaction proceeds in the forward direction or to the right
When K=Q , the system is at equilibrium
When K<Q , the reaction proceeds in the reverse direction or to the left
, Strategies for solving equilibrium Problems: (For these problems, make the usual assumption
that x is <5% of the initial concentration, and test the assumption after solving for x. If it is
greater than 5% use quadratic formula.
Facts given in the problem Strategy
1. Given equilibrium Use the equilibrium concentrations to find K
concentrations of all reactants
and products
2. Given K and equilibrium Find equilibrium concentration of products
concentrations of reactants using K, if there is more than one product, you
may have to use x
3. Given initial concentrations of Write the balanced equation
all reactants and products and Step 1: Set up a first table using the initial
K concentrations of reactants only to find
product concentrations in the forward
reaction; identify and use limiting reactants if
amounts are not stoichiometric; you don’t
need to use x in this step, just find the
concentrations stoichiometrically from the
equation and coefficients. To this, add the
initial product concentrations given. You will
have the new concentrations of all reactants
and products to be used in the next step
Step 2: Set up an ICE table using the new
concentrations from step 1
Assume this is the reverse reaction and use (-x)
in the products side and (+x) in the reactants
side. (Remember to use the coefficients 2x, 3x
etc. from the equation). Find the end
concentrations and solve for x using K.
Determine the equilibrium concentrations.
4. Given initial concentrations of Set the product initial concentration to zero
reactant (one reactant only in and set up ICE table with (-x) on the reactant
reaction) and K side and (+x) on the product side.
Solve for x using K
Determine equilibrium concentrations of
products and reactants.
For any chemical reaction, when the rate of forward reaction equals the rate of reverse
reaction, equilibrium is reached, and the concentrations of all reactants and products reach a
steady level.
A+B AB
Rate for = kfor [A][B]
Rate rev = krev [AB]
At equilibrium,
Rate for = Rate rev or
kfor [A][B] = krev [AB] or kfor / krev = K, the equilibrium constant = [AB] / [A][B]
Only aqueous and gaseous substances are included in the formula, liquids and solids are
omitted.
If there are reaction coefficients,
aA + bB cAB ,
K = [AB]c / [A]a[B]b , where [A],[B]and [AB] are equilibrium concentrations.
K for the reverse reaction is 1/K
A large value of K (>1) indicates mostly products are formed as the reaction moves to the right.
A small value of K (<1) indicates mostly reactants remain as the reaction stays to the left.
The reaction quotient Q is a constant that indicates the constant that can be calculated when
initial concentrations of reactants and substances are given. It can be compared to K to indicate
the direction of the reaction.
When K>Q , reaction proceeds in the forward direction or to the right
When K=Q , the system is at equilibrium
When K<Q , the reaction proceeds in the reverse direction or to the left
, Strategies for solving equilibrium Problems: (For these problems, make the usual assumption
that x is <5% of the initial concentration, and test the assumption after solving for x. If it is
greater than 5% use quadratic formula.
Facts given in the problem Strategy
1. Given equilibrium Use the equilibrium concentrations to find K
concentrations of all reactants
and products
2. Given K and equilibrium Find equilibrium concentration of products
concentrations of reactants using K, if there is more than one product, you
may have to use x
3. Given initial concentrations of Write the balanced equation
all reactants and products and Step 1: Set up a first table using the initial
K concentrations of reactants only to find
product concentrations in the forward
reaction; identify and use limiting reactants if
amounts are not stoichiometric; you don’t
need to use x in this step, just find the
concentrations stoichiometrically from the
equation and coefficients. To this, add the
initial product concentrations given. You will
have the new concentrations of all reactants
and products to be used in the next step
Step 2: Set up an ICE table using the new
concentrations from step 1
Assume this is the reverse reaction and use (-x)
in the products side and (+x) in the reactants
side. (Remember to use the coefficients 2x, 3x
etc. from the equation). Find the end
concentrations and solve for x using K.
Determine the equilibrium concentrations.
4. Given initial concentrations of Set the product initial concentration to zero
reactant (one reactant only in and set up ICE table with (-x) on the reactant
reaction) and K side and (+x) on the product side.
Solve for x using K
Determine equilibrium concentrations of
products and reactants.
