Equilibrium Constant

Rucete ✏ Chemistry In a Nutshell

1. What Is Chemical Equilibrium?

• Equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction.
• At this point, concentrations of reactants and products remain constant (but not necessarily equal).

2. Equilibrium Constant Expressions

General Reaction:

aA + bB ⇌ cC + dD

Equilibrium Constant (K):

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

• Kc: uses molar concentrations
• Kp: uses partial pressures
• Ka, Kb, Ksp: for acids/bases and solubility
  🔹 Pure solids and liquids are excluded from the expression!

3. Types of Equilibria

• Homogeneous Equilibrium: All substances in the same phase (e.g., all gases or all aqueous).
• Heterogeneous Equilibrium: Substances in different phases (e.g., solid and gas).

4. Interpreting the Value of K

K Value	Interpretation
K > 1	Products favored (reaction lies to the right)
K < 1	Reactants favored (reaction lies to the left)
K = 1	Reactants and products at similar concentrations

5. Relationship Between Kc and Kp

Kp = Kc(RT)^Δn

• R = 0.0821 L·atm/mol·K
• T = Temperature in Kelvin
• Δn = mol of gas products − mol of gas reactants

6. Reaction Quotient (Q)

Used to predict direction of shift toward equilibrium.

• Same expression as K, but uses current concentrations.

Comparison	Prediction
Q < K	Reaction shifts right (toward products)
Q > K	Reaction shifts left (toward reactants)
Q = K	System is at equilibrium

7. Solubility Product (Ksp)

For dissolution of sparingly soluble salts:

$$\text{For }{A}_x{B}_y(s)\rightleftharpoons x{A}^{+}+y{B}^{-}$$
$$K_{sp}=[A^{+}{]}^x[B^{-}{]}^y$$

In a Nutshell

• K > 1 → Products dominate
• K < 1 → Reactants dominate
• Q vs K tells which way the reaction will go