Reversible Reactions: All reactions can reverse under suitable conditions, reaching equilibrium.
Equilibrium Defined: At equilibrium, concentrations of products and reactants remain constant, as the rate of the forward reaction equals the rate of the reverse reaction:
\text{rate}{\text{forward}} = \text{rate}{\text{reverse}}
Dynamic State: Chemical equilibrium is dynamic; reactions occur continuously, but no net change is observable at a macroscopic level.
The Equilibrium Constant
Considering a Reaction: For the reaction
2NO2(g) \rightleftharpoons 2NO(g) + O2(g)
At equilibrium:
\text{rate}{\text{forward}} = \text{rate}{\text{reverse}}
Expression for K: The equilibrium constant (K) is derived from the concentrations of reactants and products:
K = \frac{[NO]^2[O2]}{[NO2]^2}
K and the Extent of Reaction
Meaning of K: K reflects the ratio of product concentrations to reactant concentrations at equilibrium.
Small K value: Indicates the reaction yields little product, favoring reactants.
Large K value: Indicates the reaction produces predominantly products, favoring products.
The Reaction Quotient Q
Definition of Q: For the general reaction
aA + bB \rightleftharpoons cC + dD
Q = \frac{[C]^c[D]^d}{[A]^a[B]^b}
Equilibrium Comparison: At equilibrium,
Q = K
Direction of Reaction: The value of Q indicates the direction the reaction must shift to reach equilibrium. If:
Q < K: Shift to products
Q > K: Shift to reactants
Q = K: System is at equilibrium.
Heterogeneous Equilibrium
Definition: Involves reactants and/or products in different phases.
Concentration Exclusion: Pure solids and liquids are not included in equilibrium expressions.
Example: For
CaCO3(s) \rightleftharpoons CaO(s) + CO2(g)
Q = [CO_2]