Equilibrium
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Additional Worked Out Examples/ Practice
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Relating Properties to Composition: Predicting classification based on descriptive properties
Topic Summary & Highlights
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Core Concept
Equilibrium is a state in a reversible chemical reaction where the forward and reverse reaction rates are equal, resulting in constant concentrations of reactants and products.
The reaction is dynamic—reactants and products continue to interconvert, but their concentrations remain unchanged.
Equilibrium can be achieved from either direction (starting with reactants or products).
Equilibrium expressions quantitatively describe the balance between reactants and products in a reversible chemical reaction at equilibrium, using the ratio of their concentrations or partial pressures raised to the power of their stoichiometric coefficients.
Practice Tips
The equilibrium expression relates the concentrations or partial pressures of reactants and products at equilibrium using the reaction's stoichiometry.
For a reaction aA + bB ↔ cC + dD, the equilibrium constant (K) is written as $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$.
Only gases and aqueous species are included in the equilibrium expression; solids and liquids are excluded.
The value of K indicates the reaction's favorability; K>1 favors products, while K<1 favors reactants.
The equilibrium constant KcK_cKc uses molar concentrations, and $K_p$ uses partial pressures for gaseous reactions.
$K_p$ and $K_c$ are related by $K_p = K_c(RT)^{\Delta n}, where Δn is the change in moles of gas.
Reversible Reactions
Definition: Reactions that can proceed in both forward and reverse directions.
Notation: A+B⇌C+D\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}A+B⇌C+D
Forward reaction: A+B→C+D\text{A} + \text{B} \to \text{C} + \text{D}A+B→C+D.
Reverse reaction: C+D→A+B\text{C} + \text{D} \to \text{A} + \text{B}C+D→A+B.
The Equilibrium Constant (K)
Expression: $K = \frac{[\text{C}]^c[\text{D}]^d}{[\text{A}]^a[\text{B}]^b}$
Square brackets [ \] represent molar concentrations.
a,b,c,da, b, c, da,b,c,d are the stoichiometric coefficients from the balanced equation.
Types:
KcK_cKc: Based on concentrations.
KpK_pKp: Based on partial pressures (used for gases).