9. // R3.2 Equilibrium

Dynamic Equilibrium

“steady state“, not stopped reaction

• Macroscopic properties don’t change

• Microscopically, forward and reverse processes occur simultaneously at the same rate

• The amounts of reactants and products
  remain the same

• Only in closed system!

The Equilibrium Law // The Equilibrium Constant

 aA + bB  ⇋  cC + dD

At a given temperature the ratio of the concentration of the products raised to the power of their molar coefficients to the concentration of the reactants raised to the power of their molar coefficients is a constant, K_c.

Common equilibrium reaction

Dissossication reactions: H₂CO₃+H₂O ⇋ HCO₃^- + H₃O

CO₂ (g) ⇋ CO₂ (aq)

CO₂(aq)+ H₂O (l) ⇋ H₂CO₃

Ionic product constant for water, K_w

10^-14 =[H+][OH-]

Equilibrium Position

• Position of the equilibrium

  • K_c < 1 reactants side / towards left

  • K_c = 1 equal amounts of reagents and products

  • K_c > 1 products side / towards right

Closed system

Neither matter nor energy can be lost or gained from the system.

Macroscopic properties remain constant.

If the system is open, some of the products from the reaction could escape and equilibrium would never be reached .

CEC - Reverse the reaction

Kc^-1

CEC - Halve Coefficiants

√Kc

CEC - Double the coefficients

Kc^2

Reaction Quotient, Q

If the reaction is NOT in equilibrium, the reaction quotient can be used to find out the direction of the reaction

• Q < K_c The reaction will produce more products / forward reaction is favored

• Q = K_c The reaction is in EQUILIBRIUM

• Q > K_cThe reaction will produce more reagents / reverse reaction is favored

Le Châtelier's principle

If a change is made to a system that is in equilibrium, the balance between the forward and reverse reactions will shift to offset this change and return the system to equilibrium.

Factors affecting the position of equilibrium

• Concentration of product or reactant

• Pressure

• Temperature

• Adding a catalyst

• Heterogeneous equilibria

Heterogeneous equilibria

Chemical systems at equilibrium involving substances in multiple distinct phases (e.g., solid-gas, solid-liquid). The equilibrium constant ( or ) expression excludes pure solids and liquids because their concentrations remain constant

Homologous equilibria

Reactions with very large or slow K

Practically irreversible.

• K >> 1, = forward reaction is favoured so strongly that the extent of the reverse reaction becomes negligible.

• K << 1 = only the reverse reaction will be observed while the forward reaction will not proceed to any noticeable extent.