AP Chemistry Unit 5: Kinetics — The Mathematics of Moving Molecules

Kinetics

The study of the rate of a reaction and the molecular pathway it takes.

Reaction Rate

The change in concentration of a reactant or product per unit of time, typically expressed in Molarity per second (M/s).

Stoichiometric Coefficient

The number that appears before a reactant or product in a balanced chemical equation; used to define relative rates of reaction.

Average Rate

The rate calculated over a specific time interval; represented by the slope of the secant line.

Instantaneous Rate

The rate of a reaction at a specific moment in time; represented by the slope of the tangent line at that point.

Differential Rate Law

An equation that relates the rate of a reaction to the concentration of the reactants.

Rate Constant (k)

A proportionality constant in the rate law that varies with temperature.

Zero Order Reaction

A reaction whose rate does not depend on the concentration of the reactants; doubling concentration does not change the rate.

First Order Reaction

A reaction where the rate is directly proportional to the concentration of one reactant; doubling concentration doubles the rate.

Second Order Reaction

A reaction where the rate is proportional to the square of the concentration of one reactant; doubling concentration quadruples the rate.

Method of Initial Rates

A technique to determine reaction orders by comparing rates and concentrations from initial measurements.

Integrated Rate Law

An expression that relates reactant concentration to time for a particular order of reaction.

Overall Order

The sum of the individual orders in a rate law, indicating how the rate is affected by the concentration of all reactants.

Half-Life (t_{1/2})

The time required for the concentration of a reactant to decrease to half of its initial value.

Common Mistake 1

Confusing stoichiometric coefficients with reaction orders in the rate law.

Common Mistake 2

Incorrectly determining the units of the rate constant (k) based on the overall order of the reaction.

Graph for Zero Order

A graph of concentration [A] vs. time that shows a straight line.

Graph for First Order

A graph of ln[A] vs. time that shows a straight line.

Graph for Second Order

A graph of 1/[A] vs. time that shows a straight line.

Rate = k[A]^m[B]^n

The general form of the rate law, relating rate to concentrations of reactants raised to their respective reaction orders.

Doubling concentration effect

Specific to zero, first, or second order; affects the rate differently based on the order.

Consumed Reactants

Reactants whose concentrations decrease over the course of the reaction.

Formed Products

Products whose concentrations increase over the course of the reaction.

Rate Proportionality

The concept that the rate of a reaction is proportional to the concentrations of reactants raised to specific powers.

Positive Rate Convention

The rule that reaction rates are reported as positive values, regardless of whether reactants are consumed or products formed.

Experimental Determination of Orders

Reaction orders must be determined through experimentation, not deduced from the equation coefficients.