AP Chemistry: Chemical Kinetics (Unit 5)

Chemical Kinetics

The study of the speed (rate) at which chemical reactions occur and the mechanisms by which they happen.

Rate of a reaction

The change in concentration of reactants or products per unit of time.

Reactants

Substances that disappear (negative change in concentration) during a chemical reaction.

Products

Substances that appear (positive change in concentration) during a chemical reaction.

Rate expression

Rate = -1/a * Δ[A]/Δt = -1/b * Δ[B]/Δt = 1/c * Δ[C]/Δt = 1/d * Δ[D]/Δt.

Factors affecting reaction rate

Concentration, pressure, surface area, temperature, and catalysts.

Effect of Concentration on reaction rate

Higher concentration leads to more frequent collisions, resulting in a faster rate.

Effect of Pressure on reaction rate

Increasing pressure increases concentration, leading to a faster reaction rate.

Effect of Surface Area on reaction rate

Greater surface area results in more collision sites and a faster reaction rate.

Effect of Temperature on reaction rate

Higher temperature increases particle speed and energy, resulting in more effective collisions.

Catalysts

Substances that speed up a reaction by providing an alternative pathway with a lower activation energy.

Differential Rate Law

Describes the relationship between the rate of the reaction and the concentration of reactants.

Rate Constant (k)

The proportionality constant in the rate equation; temperature-dependent and does not change with concentration.

Reaction Order

The exponents (m and n) in the rate law that indicate the sensitivity of the rate to changes in concentration.

Zero Order Reaction

Rate is independent of the concentration of the reactant. Doubling concentration has no effect on the rate.

First Order Reaction

Rate is directly proportional to the concentration of one reactant. Doubling concentration doubles the rate.

Second Order Reaction

Rate is proportional to the square of the concentration of a reactant. Doubling concentration quadruples the rate.

Method of Initial Rates

A method to determine the rate law by observing how the initial rate changes with varying reactant concentrations.

Rate-Determining Step (RDS)

The slowest step in a reaction mechanism that determines the overall reaction rate.

Integrated Rate Laws

Relate concentration of reactants/products to time, allowing calculations of remaining concentrations after a period.

Half-Life (t_{1/2})

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

Zero Order Half-Life

t{1/2} = [A]0 / 2k, depends on initial concentration.

First Order Half-Life

t_{1/2} = 0.693 / k, constant and independent of concentration.

Collision Theory

For a reaction to occur, particles must collide with correct orientation and sufficient energy (greater than activation energy).

Activation Energy (E_a)

The minimum energy required for a reaction to occur.

Maxwell-Boltzmann Distribution

Describes the distribution of speeds of molecules in a gas, illustrating how temperature affects reaction rates.

Arrhenius Equation

k = Ae^{-E_a/RT}, relates the rate constant to temperature and activation energy.

Elementary Step

A single molecular event in a multi-step reaction mechanism.

Molecularity

The number of reactant molecules involved in an elementary step (unimolecular, bimolecular, termolecular).

Intermediate

A substance produced in one step of a mechanism and consumed in a later step.

Catalyst

A substance that increases the rate of a reaction without being consumed, by lowering the activation energy.

Exothermic Reaction

A reaction where the energy of products is lower than that of reactants, releasing heat.

Endothermic Reaction

A reaction where the energy of products is higher than that of reactants, absorbing heat.

Catalysis Types

Includes acid-base catalysis, surface catalysis, and enzyme catalysis.

Correct Orientation

The specific angle at which reacting particles must collide to result in a reaction.

Common Mistake: Confusing Rate with Rate Constant

The rate can change with concentration, but rate constant k changes only with temperature or catalysts.

Slopes of Integrated Rate Laws

For zero and first order the slope is negative, while for second order the slope is positive.

Overall Order of Reaction

The sum of the powers of the concentration terms in the rate equation.

Rate Law from Elementary Steps

The rate law of an overall reaction is equal to the rate law of the rate-determining step.

Common Pitfall: Stoichiometric Coefficients as Orders

Assuming reaction orders are equal to stoichiometric coefficients from the balanced equation, rather than determined experimentally.

Units of the Rate Constant (k)

Change depending on the overall order of the reaction to ensure units balance as M/s.

Graphical Method for Reaction Order

Identifying the order of reaction graphically by plotting [A], ln[A], or 1/[A] against time.

Overall Reaction Rate

The speed at which reactants are converted into products across all elementary steps.