Chemical Equilibrium

Learning Outcomes

• Understand and explain chemical equilibrium.
• Write expressions for calculating the equilibrium constant, ( K ).
• Calculate and compare reaction quotient ( Q ) and ( K ).
• Predict relative amounts of reactants and products based on equilibrium constant ( K ).

Key Concepts

Chemical Equilibrium

• Describes the state where the rate of the forward reaction equals the rate of the reverse reaction.
• Can be reached starting with either all reactants, all products, or a combination of both.
• At equilibrium, concentrations of reactants and products remain constant, not necessarily equal.

The Reaction Process

• Initial Reaction: Only forward reaction occurs as no product is present.
• Dynamic Equilibrium: Rates of forward and reverse reactions are equal, though reactions continue to occur.
• Equilibrium is independent of initial concentrations but dependent on the specific reaction and temperature.

Conditions Required for Equilibrium

1. Closed system (no entry or exit of substances).
2. Dynamic process with ongoing forward and reverse reactions.
3. Equal rates of forward and reverse reactions.
4. Constant amounts of reactants and products, but not necessarily equal.

Types of Equilibrium

• Chemical Equilibrium: Between reactants and products.
• Phase Equilibrium: Between two states of a substance.
• Solution Equilibrium: Between a solute in a saturated solution.

Equilibrium Constant ( (K_{eq}) )_

• Expressed as the ratio of product concentrations to reactant concentrations, each raised to the power of their coefficients.
• ( K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b} )
• ( K ) is constant at a given temperature, specific to a reaction.
• Greater than 1: Products favored.
• Less than 1: Reactants favored.
• Equal to 1: Reactants and products are in similar concentrations._

Reaction Quotient ( (Q) )

• Used to determine if a reaction has reached equilibrium.
• Calculated identically as ( K ).
• Q = K: System is at equilibrium.
• Q < K: Reaction will proceed to form more products (right shift).
• Q > K: Reaction will form more reactants (left shift).

Examples and Applications

• Biochemical Relevance: Oxygen transport in red blood cells involves equilibrium between hemoglobin and gases like ( CO_2 ) and ( O_2 ).
• Medical Context: Carbon monoxide poisoning treatment involves shifting equilibrium by introducing excess ( O_2 ) to displace ( CO ).

Important Equations

• For a general reaction: ( aA + bB \rightleftharpoons cC + dD )
• Equilibrium Constant: ( K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b} )
• Reaction Quotient: Used similarly to evaluate reaction direction relative to ( K )._

Contributors and Attributions

• Allison Soult, Ph.D., Department of Chemistry, University of Kentucky.