Equilibrium

Definition: Reversible reactions

Reactions that proceed in both forward and backward reactions

(concentration-time graph) Dynamic Equilibrium

(rate-time graph) Dynamic Equilibrium

Definition: Dynamic Equilibrium

When a reversible reaction in a closed system reaches dynamic chemical equilibrium, reaction continues to occur. Rates of the forward and backward reactions are equal and there is no net change in the concentrations of the reactants and products.

Definition: Equilibrium Constant, K_c

Ratio of the product of the concentrations of products raised to the appropriate powers in the product of the concentrations of reactants raised to the appropriate powers at equilibrium at a given temperature.

Formula: K_c

Formula: K_c units

In an aqueous solution where water is used as a solvent, K_c expression does not include …

the concentration of water

K_c does NOT include ____.

solids

Definition: Partial Pressure of a gas

Pressure which the gas would exert if it alone occupies the same volume as mixture in the container

K_p can only be applied if the equilibrium has at least one ____

gas

Formula: K_p

Formula: Partial Pressure

Formula: Relationship between K_c and K_p

When does K_c = K_p?

Only when the number of moles of gaseous reactants is equal to the number of moles of gaseous products in the equation i.e. Δn = 0

Formula: Δn

aA + bB → cC + dD

Δn = (c + d) - (a + b)

K_c is proportional to K_p at ____

constant temperature

Memorise: What does K_c measure?

Extent to which a reversible reaction takes place

Memorise: K_c > 1

• Reaction proceeds from left to right

• Formation of products are favoured → equilibrium lies to the right

• When K_c > 1000, reaction is effectively complete

Memorise: K_c < 1

• Reaction proceeds from right to left

• Formation of reactants are favoured → equilibrium lies to the left

• When K_c < 0.001, reaction is negligible

K_c and K_p are dependent on _____ only

temperature

Are K_c and K_p affected by changes in concentration or pressure?

No

Formula: Drawing ICE Table

Q) (i) HI dissociates into H₂ and I₂ in a reversible reaction. 0.2 mole of gaseous HI was placed in a 200 cm³ flask. At equilibrium, it was found that 20% of HI had dissociated. What is K_c?

(ii) In another experiment at the same temperature, 1.0 mol of HI in a 1 dm³ vessel was allowed to dissociate. Calculate the percentage dissociation of HI.

(i)

(ii)

Formula: Percentage Dissociation

Formula: Degree of Dissociation

Definition: Le Chatelier’s Principle

When a system at dynamic equilibrium is subjected to a change which disturbs the equilibrium, the system will react in a way so as to counteract the effect of the change to re-establish equilibrium.

Memorise: What happens to equilibrium when concentration of reactants [A] is increased?

• By LCP, the equilibrium position will shift right to decrease concentration of reactant until new equilibrium is attained.

• New equilibrium mixture will contain more products, more reactant [A] and less reactant [B].

• No change in K_c, k_f and k_b.

Memorise: What happens to equilibrium when concentration of reactants [A] is decreased?

• By LCP, the equilibrium position will shift left to increase concentration of reactant until new equilibrium is attained.

• New equilibrium mixture will contain less products, less reactant [A] and more reactant [B].

• No change in K_c, k_f and k_b.

Memorise: What happens to equilibrium when concentration of products [C] is increased?

• By LCP, the equilibrium position will shift left to decrease the concentration of products until a new equilibrium is attained.

• New equilibrium mixture will contain less products, less reactant [B] and more reactant [A].

• No change in K_c, k_f and k_b.

Memorise: What happens to equilibrium when concentration of products [C] is decreased?

• By LCP, the equilibrium position will shift right to increase the concentration of products until a new equilibrium is attained.

• New equilibrium mixture will contain more products, less reactants [A] and [B].

• No change in K_c, k_f and k_b.

Graph for 2SO₂ + O₂ → 2SO₃ where SO₂ is added.

• SO₂ added.

• By LCP, the equilibrium position will shift right to decrease concentration of reactant until new equilibrium is attained.

• New equilibrium mixture will contain more products, more reactant [A] and less reactant [B].

• No change in K_c, k_f and k_b.

Pressure changes only affect ____ reactions.

gaseous

Definition: Pressure

Force exerted by the gas particles on the wall of its container

Memorise: What happens to equilibrium when pressure increases?

• By LCP, the equilibrium shifts [side with less moles] to decrease the number of moles of gas, hence, decreasing pressure.

• K_c, k_f and k_b not affected.

Memorise: What happens to equilibrium when pressure decreases?

• By LCP, the equilibrium shifts [side with more moles] to increase the number of moles of gas, hence, increasing pressure.

• K_c, k_f and k_b not affected.

Pressure has no effect on equilibrium position and composition of equilibrium mixture when…

Total no. of moles of gaseous reactants = Total no. of moles of gaseous products

Memorise: What happens to equilibrium when temperature increases?

#EQUILIBRIUM SHIFTS TOWARDS ENDOTHERMIC REACTION TO ABSORB HEAT.

~ Endothermic

• By LCP, the equilibrium position shifts right towards endothermic reaction to absorb heat.

• K_c increases.

~ Exothermic

• By LCP, the equilibrium position shifts left towards endothermic reaction to absorb heat.

• K_c decreases.

Memorise: What happens to equilibrium when temperature decreases?

#EQUILIBRIUM SHIFTS TOWARDS EXOTHERMIC REACTION TO RELEASE HEAT.

~ Endothermic

• By LCP, the equilibrium position shifts left towards exothermic reaction to release heat.

• K_c decreases.

~ Exothermic

• By LCP, the equilibrium position shifts right towards exothermic reaction to release heat.

• K_c increases.

Catalysts have no effect on …

K_c or K_p

Yield of Product

Memorise: What happens to equilibrium when catalyst is added?

• Catalysts increase the rate of forward and backward reaction to the same extent, hence, position of equilibrium remains unchanged.

• Catalysts increase the rate of reaction so that dynamic equilibrium is established faster.

Haber Process Equation for ammonia manufacturing

N_2(g) + 3H_2(g) — 2NH_3(g)

ΔH = -92 kJ mol^-1

Common operating conditions for Haber Process

1. High pressure of 250 atm

2. Moderate temperature of 450

3. Iron catalyst

4. Molar ratio of N₂:H₂ is 1:3

Definition: ΔG, Gibbs Free Energy

ΔG is a measure of the spontaneity of a reaction

Memorise: ΔG < 0

Spontaneous

Memorise: ΔG > 0

Non-spontaneous

Memorise: ΔG = 0

System is at equilibrium

Definition: Q, Reaction Quotient

Ratio of product of the concentrations of products raised to the appropriate powers to the product of the concentrations of reactants raised to the appropriate powers at a given temperature

Q = K_c when…

system is at equilibrium

Formula: Q (reaction quotient)

Formula: Relationship between K_c and k_f/k_b

Limitations of LCP

When more than 1 factor is changing