Chemical equilibria, Le Chatelier's principle and Kc revision notes

Chemical Equilibria

Dynamic Equilibrium

Dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products.

Le Chatelier's Principle

Le Chatelier's principle states that if a system at equilibrium is subjected to a change in concentration, pressure, or temperature, the system will adjust to counteract that change and restore a new equilibrium.

Effects of Changes:

• Concentration Changes: Increasing the concentration of reactants shifts the equilibrium to the right, favoring product formation. Decreasing reactants shifts it left.
• Pressure Changes: Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. Decreasing pressure shifts it towards the side with more moles.
• Temperature Changes: For exothermic reactions, increasing temperature shifts the equilibrium left (towards reactants). For endothermic reactions, it shifts right (towards products).

Catalysts

Catalysts speed up the attainment of equilibrium but do not change the position of equilibrium or the value of Kc.

Equilibrium Constant (Kc)

Kc is a measure of the ratio of concentrations of products to reactants at equilibrium. It is expressed as:

$$ K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b} $$

where [A], [B], [C], and [D] are the equilibrium concentrations of the reactants and products, and a, b, c, and d are their respective stoichiometric coefficients.

Calculating Kc

To calculate Kc, substitute the equilibrium concentrations into the Kc expression. The units of Kc depend on the reaction and can be determined from the expression.

Interpreting Kc

A large Kc (>>1) indicates that products are favored at equilibrium, while a small Kc (<<1) indicates that reactants are favored.