Chemical Kinetics

Factors Affecting Reaction Rates

Chemical kinetics studies the speed at which chemical reactions occur and the factors that influence these rates.

• Temperature: Increasing temperature generally increases reaction rate by providing more energy for collisions.

• Concentration: Higher concentration of reactants leads to more frequent collisions, increasing the rate.

• Surface Area: For reactions involving solids, increasing surface area (e.g., by grinding) exposes more particles to react, increasing rate.

• Catalyst: Catalysts lower the activation energy, increasing the rate without being consumed.

Example: A reaction proceeds faster at 50°C than at 25°C due to increased molecular motion.

Expressing and Calculating Reaction Rates

The rate of a reaction is the change in concentration of a reactant or product per unit time.

• Average Rate: , where is concentration and is time.

• Instantaneous Rate: The rate at a specific moment, found by the slope of the concentration vs. time curve.

Example: If decreases from 0.10 M to 0.08 M in 20 seconds, M/s.

Rate Laws and Determining Reaction Order

A rate law expresses the relationship between the rate and the concentration of reactants.

• General Form:

• Order of Reaction: The exponents and indicate the order with respect to each reactant; the sum is the overall order.

• Determining Order: Use experimental data to see how rate changes with concentration.

Example: If doubling doubles the rate, the reaction is first order in .

Integrated Rate Laws and Calculations

Integrated rate laws relate concentrations of reactants to time for different reaction orders.

• Zero Order:

• First Order:

• Second Order:

Half-life ():

• First Order:

• Second Order:

• Zero Order:

Example: For a first-order reaction with s, s.

Reaction Mechanisms and Rate Laws

A reaction mechanism is a sequence of elementary steps that describes how a reaction occurs at the molecular level.

• Elementary Step: A single step in a mechanism, with its own rate law.

• Rate-Determining Step: The slowest step, which controls the overall rate.

• Writing Rate Laws from Mechanisms: The rate law is based on the rate-determining step, using only reactants involved in that step.

Example: If the slow step is , then .

Chemical Equilibrium

Equilibrium Expressions

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, and concentrations remain constant.

• Equilibrium Constant (): for the reaction .

• Only gases and aqueous species appear in ; solids and pure liquids are omitted.

Example: For , .

Calculations Involving Equilibrium (ICE Tables)

ICE (Initial-Change-Equilibrium) tables help organize data for equilibrium calculations.

• Initial: Starting concentrations

• Change: Amounts that react or form (use variables like or )

• Equilibrium: Final concentrations, used in the equilibrium expression

Example: If M, M, and mol/L of reacts, then at equilibrium , .

Reaction Quotient () and Direction of Reaction

The reaction quotient is calculated like but with current (not necessarily equilibrium) concentrations.

• If : Reaction proceeds forward (toward products).

• If : Reaction proceeds in reverse (toward reactants).

• If : System is at equilibrium.

Example: If and , the reaction will shift right to reach equilibrium.

Le Chatelier’s Principle

Le Chatelier’s Principle states that if a system at equilibrium is disturbed, it will shift to counteract the disturbance.

• Concentration: Adding reactant shifts equilibrium toward products; removing reactant shifts toward reactants.

• Pressure/Volume (for gases): Increasing pressure (decreasing volume) shifts equilibrium toward the side with fewer moles of gas.

• Temperature: Increasing temperature favors the endothermic direction; decreasing favors exothermic.

Example: For an exothermic reaction, increasing temperature shifts equilibrium toward reactants.

Common Equations Provided