Pre-Equilibrium Approximation - AP Chemistry

Equilibrium condition. Forward and reverse rates equal in fast step.

Identify the fast equilibrium step. Required before deriving rate law from mechanism.

Relates intermediate and reactant concentrations. Links intermediate concentration to initial reactants.

$k' = k \times K$. Product of rate constant and equilibrium constant.

It remains nearly constant. Equilibrium condition maintains stable intermediate concentration.

Rapidly forms and decomposes. Fast equilibration between formation and decomposition reactions.

Intermediate is in equilibrium with reactants. Fast step equilibrium before slow step determines rate.

Concentration remains constant over time. Equilibrium established faster than consumption in slow step.

Correct: 'assumes equilibrium, not steady state.'. Pre-equilibrium involves equilibrium, not steady-state conditions.

Rate = $k'[\text{reactants}]^{\text{coefficients}}$. Combined rate and equilibrium constants for overall reaction.

$k' = k \times K$. Effective rate constant combines kinetic and equilibrium factors.

Correct: 'applies to reversible reactions.'. Pre-equilibrium requires reversible steps to establish equilibrium.

Simplifies calculation of rate laws. Reduces complex mechanisms to simple rate expressions.

Correct: 'applies to equilibrium conditions.'. Pre-equilibrium requires equilibrium conditions, not steady-state.

Simplifies the rate law expression. Eliminates need for complex intermediate concentration terms.

The fast initial step. The reversible step that reaches equilibrium quickly.

They quickly reach equilibrium with reactants. Fast equilibrium maintained throughout reaction course.

Fast step reaches equilibrium before slow step occurs. Fast equilibrium must precede slow rate-determining step.

It is rapid and reversible. Fast forward and reverse rates establish equilibrium.

Assumes early equilibrium in a reaction mechanism. Fast step reaches equilibrium before proceeding to slow step.

Initial step is fast and reversible. Fast reversible step required for equilibrium assumption.

Simplifies complex mechanisms into rate laws. Makes complex multi-step reactions mathematically tractable.

Applying to irreversible reactions. Requires reversible steps to establish equilibrium conditions.

Relates initial concentrations and intermediate. Connects initial species concentrations to intermediate levels.

Equilibrium vs. constant concentration assumptions. Different underlying principles for each approximation method.

$K = \frac{[\text{products}]}{[\text{reactants}]}$. Ratio of product to reactant concentrations at equilibrium.

Derived from the slow step using equilibrium expressions. Rate law comes from slow step with equilibrium substitution.

Allows simplification of complex mechanisms. Converts complex mechanisms into manageable rate expressions.

Steady-state assumes constant intermediate concentration. Pre-equilibrium assumes equilibrium, not steady state.