Chem 2 Equilibrium

Sanoj Summer Class

Chemical Equilibrium

A dynamic process in which concentrations of reactants and products remain constant over time

• Rate of reaction in the forward direction matches its rate in the reverse direction (Rate f = Rate r)

Equilibrium Constant (K)

The numerical value of the equilibrium constant expression of a reversible chemical reaction at a specific temperature.

Equilibrium Constant Expression

Ratio of equilibrium concentrations or partial pressures of products to reactants, each term raised to a power equal to the coefficient of that substance in the balanced chemical equation.

Law of Mass Action

The principle relating the balanced chemical equation of a reversible reaction to its mass action expression (or equilibrium constant expression)

Mass Action Expression

Equivalent to equilibrium constant expression, but applied to reaction mixtures that may or may not be at equilibrium.

Equilibrium Constants

Kc: [X] = Concentration units of moles/liter

• Used when given concentrations or majority of molecules are in aq phases

Kp: Px= units of partial pressure

• used when given pressure (atm) or when majority of molecules are in g phase

Interpreting K Values

Very large K: Favors Formation of Products (mostly forward reaction)

Very small K: Favors reactants; not much product formed at equilibrium

Intermediate Value of K: comparable amounts of products and reactants at equilibrium (favors equilibrium)

Equilibrium in the Gas Phase (Ideal Gas Law: PV=nRT or P=(n/V)RT

Kp = Kc(RT)^delta n

delta n = number of gaseous products minus the number of moles of gaseous reactants

R= gas constant (0.08206)

T = Temperature (K)

K for Reverse Reaction

K Forward = 1/K Reverse

Equation Multiplied by a number

The rate (K1) is raised to the power of the number that the reaction is multiplied by.

Ex:

Reaction 1: A + B < — > 2C K1 = [C]² / [A][B]

Reaction 2: ½ A + ½ B < — > C K2 = (K1)^1/2

Combining K Values

1) Cancel out or simplify any molecules that appear on both sides of the reaction.

2) Add the remaining molecules from the two reactions to create a 3rd reaction.

3) Create an expression from the 3rd reaction

or

1) Multiply the K1 x K2

2) Cancel out or simplify any matching molecules that appear on both the numerator and denominator sides of the fraction.

3) Write out your new K3.

Reaction Quotient (Q)

Numerical value of the mass action expression for any values of concentrations (or partial pressures) of the reactants and products.

At Equilibrium: Q = K

Q < K: Reactants are favored (forward Reaction, shift right)

Q > K: Products are favored (Reverse Reaction, Shift Left)

Homogenous Equilibria

Involve reactants and products in the same phase

• H2O(g) + CO(g) < — > H2(g) + CO2(g)

Heterogeneous Equilibria

Involve reactants and products in more than one phase

• CaO(s) + CO2(g) < — > CaCO3(s)

• Only Gaseous or Aqueous molecules are put into Q or K expressions. (amount of solid product doesn’t affect the equilibrium)

Le Chatelier’s Principle

“A system at equilibrium responds to stress in such a way that it relieves that stress.”

• Factors that will change the relative rates of forward/ reverse reactions or change the value of Q compared to K will cause a shift in the position of equilibrium.

Response to adding a reactant or removing a product

consume that reactant and produce more product; shifting the equilibrium to the right

Response to Removing a reactant or adding a product

Produce the reactant and consume the product; shifting the equilibrium to the left

Response to compressing the reaction mixture (decreasing volume & increasing pressure)

reduce moles of gas; shift the equilibrium to the right

Response to expanding the reaction mixture (increasing the volume and decreasing the pressure)

Increase the Moles of gas; shift the equilibrium to the left

Endothermic Response to Temperature Change

Increase in Temp: Shift to for more product

Decrease in Temp: Shift to form more reactant

Exothermic Response to Temperature Change

Increase in temp: Shift to form more reactant

Decrease in Temp: Shift to form more product

Catalysts and Equilibrium Systems

Systems reach equilibrium faster

No Change in K or postion of equilibrium

RICE Table

R: balanced chemical Reaction

I: Initial concentrations (concentrations, pressures)

C: Changes as the system moves to equilibrium

E: Equilibrium values

Equilibrium and Thermodynamics

The magnitude of ΔGrxn indicates how far a system is from equilibrium.

The thermodynamic view of equilibrium and the relationship between ΔGrxn and Q are described by ΔGrxn = ΔG°rxn + RTlnQ Where = ΔG°rxn is the change in free energy under standard conditions.

Changing K with changing Temperature

Combining ΔG°rxn = ΔH°rxn- TΔS°rxn and lnK = (-ΔG°rxn)/RT

to get:

• lnK = -ΔH°rxn/R (1/T) + ΔS°rxn/R