K vs Q
Related Examples and Practice Problems
Additional Worked Out Examples/ Practice
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Separation techniques: Selected and explaining limitation of appropriate separation
Relating Properties to Composition: Predicting classification based on descriptive properties
Topic Summary & Highlights
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Core Concept
K is the equilibrium constant and describes the ratio of product concentrations to reactant concentrations at equilibrium, with each raised to the power of their stoichiometric coefficients.
Q is the reaction quotient and it is a snapshot of the ratio of product concentrations to reactant concentrations at any point during the reaction, not necessarily at equilibrium.
Practice Tips
Using the Wrong Expression: Ensure the Q and K expressions match the balanced equation.
Ignoring States of Matter: Exclude solids and liquids from the Q and K expressions.
Confusing Kc and Kp: Use concentrations for Kc and partial pressures for Kp.
K, Equilibrium Constant
K describes the ratio of product concentrations to reactant concentrations at equilibrium, with each raised to the power of their stoichiometric coefficients.
Expression: $K = \frac{[\text{products}]^{\text{coefficients}}}{[\text{reactants}]^{\text{coefficients}}}$
Key Points:
K is calculated using equilibrium concentrations or pressures.
K is constant at a given temperature.
Q, the Reaction Quotient
Definition: Q is a snapshot of the ratio of product concentrations to reactant concentrations at any point during the reaction, not necessarily at equilibrium.
Expression: $Q = \frac{[\text{products}]^{\text{coefficients}}}{[\text{reactants}]^{\text{coefficients}}}$
Key Points:
Q is calculated using the current (non-equilibrium) concentrations or pressures.
Q changes as the reaction progresses toward equilibrium.
Comparing K and Q
The relationship between Q and K determines the direction in which the reaction will proceed to reach equilibrium:
| Relationship | Interpretation | Direction of Reaction |
|---|---|---|
| Q < K | Too few products; too many reactants | Reaction shifts right (toward products) |
| Q = K | Reaction is at equilibrium | No shift; reaction is balanced |
| Q > K | Too many products; too few reactants | Reaction shifts left (toward reactants) |