ACER 10/26 Lecture #17: Equilibrium

What is K_eq? How is it calculated for the following equation?

aA (aq) + bB (aq) ⇌ cC (aq) + dD (aq)

K_eq is the equilibrium constant.

K_eq = [products]/[reactants] = [C]^c[D]^d/[A]^a[B]^b

• The square brackets [] indicate concentration.

How is K_eq calculated for gases?

K_eq = P_cP_d/P_aP_b

What is Q and when is it used? How is it calculated?

Q is the reaction quotient, and is used when the reaction is not at equilibrium. It is calculated the same way as K_eq

By comparing Q and K_eq, we can…

Explain each

we can determine the direction in which the reaction will shift to reach equilibrium

• If Q is less than K_eq, then the product concentrations are lower than equilibrium product concentrations and the reactant concentrations are higher than equilibrium.

• If Q is equal to K_eq, then the reaction is at equilibrium.

• If Q is greater than K_eq, then the product concentrations are higher than equilibrium product concentrations and the reactant concentrations are lower than equilibrium.

When calculating K_eq and Q, concentration is used to…

Click hint after

Concentrations are used to approximate activities (which includes other factors like kinetic energy, etc…).

For gases the activities are approximated by partial pressures, and for solids or liquids it is = 1.

What is not included from the reaction formula in K_eq and Q?

Solids and liquids are excluded.

Describe the formula used to calculate ∆G when a reaction is taking place under non-standard conditions.

Click hint after.

What is ∆G if the reaction is at equilibrium? What does this mean?

Describe the formula used to calculate ∆G° when the reaction is at equilibrium.

∆G = ∆G° + RTln(Q)

• R is the gas constant, R = 8.314 J/mol.K

• T is the temperature

  • Equilibrium constants are very temperature dependent.

• Q is the reaction quotient

At equilibrium, ∆G = 0, meaning there is no longer a driving force and the reaction is no longer spontaneous.

∆G° = -RTlnK

How does the sign of Gibbs Free Energy determine the direction of the reaction?

If ∆G > 0, then we favor the reactants (form more reactants).

If ∆G < 0, then we favor the products (form more products).

If ∆G° = 0, then K is…

Click hint after.

K = 1

∆G° = 0

How does K change when you reverse a reaction?

How does K change when you combine reactions?

When reversing the reaction, K is flipped.

• So if the equilibrium constant for A ⇌ B is [B]/[A], then the equilibrium constant for B ⇌ A is [A]/[B].

When combining reactions, K is the product of the equilibrium constants of each combined reaction.

• If A ⇌ B (with K₁) and B ⇌ C (with K₂) are combined, the net reaction is A ⇌ C and the new equilibrium constant is K = K₁ x K₂

What is “x is small” approximation and when is it used?

It is used if K is much smaller than the concentrations.

This approximation will assume that any change in concentration is so negligible compared to the actual full value, so x = 0.

What is Le Châtelier's Principle?

Describe 4 different scenarios where a system is disturbed and how the equilibrium quotient shifts to restore equilibrium.

Hint available.

Le Châtelier's Principle states that if a system at equilibrium is disturbed, the system will proceed in the opposite direction of the disturbance in order to return to equilibrium.

• If reactants are added, EQ shifts towards the products.

• If products are added, EQ shifts towards the reactants.

• If heat is added, EQ shifts towards the side with lower heat energy (in order to consume the heat). For endothermic reactions, heat is a reactant so EQ shifts towards products. For exothermic reactions, heat is a product, so EQ shifts towards reactants.

• For gases, when volume is decreased, EQ will shift towards the side with fewer moles of gas.