Chemical Equilibrium

Many reactions do not go to completion. Instead, they reach a state where the forward and reverse reactions proceed at equal rates. This is chemical equilibrium -- a dynamic state where concentrations remain constant even though reactions are still occurring in both directions.

Reversible Reactions

A reversible reaction can proceed in both directions:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

At equilibrium:

• Rate of forward reaction = Rate of reverse reaction
• Concentrations of reactants and products remain constant (not necessarily equal)

Equilibrium Constant (Kc)

For the general reaction: aA + bB ⇌ cC + dD

Kc = [C]^c [D]^d / [A]^a [B]^b

• Large Kc (>>1): Products are favoured
• Small Kc (<1): Reactants are favoured

Kp (For Gaseous Equilibria)

Kp = Kc (RT)^(delta n)

where delta n = (moles of gaseous products) - (moles of gaseous reactants).

Worked Example

For N₂ + 3H₂ ⇌ 2NH₃, at equilibrium: [N₂] = 0.5 M, [H₂] = 0.3 M, [NH₃] = 0.2 M. Calculate Kc.

Solution:

• Kc = [NH₃]² / ([N₂][H₂]³)
• Kc = (0.2)² / (0.5 x (0.3)³)
• Kc = 0.04 / (0.5 x 0.027)
• Kc = 0.04 / 0.0135
• Kc = 2.96

Le Chatelier's Principle

When a system at equilibrium is disturbed, it shifts to counteract the disturbance.

| Change | Equilibrium Shifts | |--------|-------------------| | Increase reactant concentration | Toward products (right) | | Increase product concentration | Toward reactants (left) | | Increase temperature (exothermic) | Toward reactants (left) | | Increase temperature (endothermic) | Toward products (right) | | Increase pressure (gas phase) | Toward side with fewer gas moles | | Add catalyst | No shift (reaches equilibrium faster) |

Important Notes

• A catalyst speeds up both forward and reverse reactions equally -- it does not change the equilibrium position or Kc.
• Pure solids and liquids are not included in the Kc expression.

Nepal Connection

The Haber process (N₂ + 3H₂ ⇌ 2NH₃) produces ammonia for fertilizers. Nepal's agriculture benefits from understanding equilibrium conditions that maximize ammonia yield: high pressure, moderate temperature, and iron catalyst.

Key Takeaways

• At equilibrium, forward and reverse rates are equal
• Kc is calculated from equilibrium concentrations
• Le Chatelier's principle predicts how equilibrium shifts when conditions change
• Catalysts speed up equilibrium but do not shift it