Chemical Kinetics
- Assumption of temperature independence of A and Ea
- Rate equation: rate = k[A]
- Arrhenius equation: k = Ae^(-Ea/RT)
- Determining rate constants
- Experimental methods
- Integrated rate laws
- Reaction order and molecularity
- Zero order reactions
- First order reactions
- Second order reactions
- Rate-determining step
- Slowest step in a reaction
- Determines overall reaction rate
- Catalysts
- Increase rate of reaction
- Provide an alternative reaction pathway
Reaction Mechanisms
- Elementary steps
- Simplest individual steps in a reaction
- Reaction intermediates
- Formed and consumed during reaction
- Not present in the overall balanced reaction equation
- Rate-determining step revisited
- Identifying the slowest step in a mechanism
- Unimolecular and bimolecular reactions
- Unimolecular: one molecule involved in the rate-determining step
- Bimolecular: two molecules collide to form a product
- Multistep reactions
- Overall reaction composed of multiple elementary steps
- Rate law for multistep reactions
- Determining the rate expression from the mechanism
Collision Theory
- Effect of concentration and temperature on reaction rate
- Activation energy
- Energy required for reactants to form the transition state
- Orientation factor
- Role of molecular orientation in successful collisions
- Reaction rate and collision frequency
- Relationship between number of collisions and reaction rate
- Factors affecting collision frequency
- Concentration, temperature, and pressure
- Effect of catalysts on collision theory
- Providing an alternative reaction pathway
- Lowering activation energy
- Effective collision
- Successful collision leading to product formation
Effect of Temperature on Reaction Rate
- Temperature dependence of reaction rate
- Higher temperature, faster reaction rate
- Activation energy and reaction rate
- Higher activation energy, slower reaction rate
- Arrhenius equation revisited
- Boltzmann distribution
- Distribution of kinetic energies among molecules
- Transition state theory
- Formation of an activated complex during reaction
- Effect of temperature on rate constant
- Exponential increase with temperature
Reaction Rate and Concentration
- Relationship between reactant concentration and rate
- Rate law and rate constant
- Determining the reaction order
- Method of initial rates
- Determining the rate constant
- Rate law for elementary steps in a mechanism
- Using data to determine the rate law
- Integrated rate laws
- Half-life of a reaction
- Zero order reactions and their characteristics
Integrated Rate Laws
- Zero order reactions
- Rate = k
- Integrated rate law: [A] = [A]_0 - kt
- First order reactions
- Rate = k[A]
- Integrated rate law: ln[A] = -kt + ln[A]_0
- Second order reactions
- Rate = k[A]^2
- Integrated rate law: 1/[A] = kt + 1/[A]_0
- Half-life of a reaction
- Graphical representation of integrated rate laws
Factors Affecting Reaction Rate
- Nature of reactants
- Different substances react at different rates
- Surface area
- Smaller particle size increases reaction rate
- Temperature
- Higher temperature, faster reaction rate
- Concentration of reactants
- Higher concentration, faster reaction rate
- Catalysts
- Increase reaction rate without being consumed
- Pressure (for gaseous reactions)
- Higher pressure, faster reaction rate
Rate Laws and Reaction Mechanisms
- Determining the rate law from the overall reaction
- Identifying the intermediates and rate-determining step
- Experimental methods to determine rate laws and mechanisms
- Techniques: initial rates, method of isolation, titration
- Collision theory and reaction mechanisms
- Relationship between rate constants and molecularity
- Elementary steps and overall reaction order
- Rate-determining step and the slowest step
- Catalysis and the effect on reaction mechanisms
Collision Theory
- Effect of concentration and temperature on reaction rate
- Higher concentration increases the likelihood of collisions
- Higher temperature increases the kinetic energy of molecules, leading to more collisions
- Activation energy
- The minimum energy required for a collision to result in a reaction
- Orientation factor
- Certain orientations of molecules are more favorable for a successful collision
- Reaction rate and collision frequency
- A higher collision frequency leads to a higher reaction rate
- Factors affecting collision frequency
- Concentration of reactants
- Temperature
- Presence of a catalyst
- Effective collision
- A collision that results in a reaction
- Influenced by kinetic energy, orientation, and activation energy
Reaction Mechanisms
- Elementary steps
- Individual steps that make up a complex reaction
- Reaction intermediates
- Species formed and consumed in the reaction mechanism
- Not present in the overall balanced equation
- Rate-determining step
- The slowest step in the reaction mechanism
- Determines the overall rate of the reaction
- Unimolecular and bimolecular reactions
- Unimolecular: A single molecule is involved in the rate-determining step
Example: Decomposition of a substance
- Bimolecular: Two molecules collide to form a product
Example: A + B -> AB
- Multistep reactions
- Reactions that occur in multiple elementary steps
- Rate law for multistep reactions
- Determined by the rate-determining step
- Only the slowest step affects the rate law
Effect of Temperature on Reaction Rate
- Temperature dependence of reaction rate
- Increasing temperature usually increases the rate of a reaction
- Higher temperature provides more kinetic energy to reactant molecules
- Activation energy and reaction rate
- Higher activation energy leads to a slower reaction rate
- Lower activation energy allows for more collisions to overcome the energy barrier
- Arrhenius equation revisited
- k = Ae^(-Ea/RT)
- A: pre-exponential factor (related to collision frequency)
- Ea: activation energy
- R: gas constant
- T: temperature in Kelvin
- Boltzmann distribution
- Describes the distribution of kinetic energies among molecules
- Transition state theory
- Involves the formation of an activated complex during the reaction
- Effect of temperature on rate constant
- Rate constant increases with increasing temperature exponentially
Reaction Rate and Concentration
- Relationship between reactant concentration and rate
- As reactant concentration increases, the reaction rate generally increases
- Directly proportional relationship for many reactions
- Rate law and rate constant
- Rate law: A mathematical expression relating rate to reactant concentrations
- Rate = k[A]^m[B]^n
- Rate constant (k) is specific to a particular reaction at a specific temperature
- Determining the reaction order
- Reaction order (m and n) determined from the rate law
- Method of initial rates
- Comparing the initial rates at different concentrations to determine the rate order
- Determining the rate constant
- Using the rate equation and experimental data
- Rate law for elementary steps in a mechanism
- Based on stoichiometry of the elementary steps
- Determining the rate law from data
- Plotting concentration vs. time and determining the slope
Integrated Rate Laws
- Zero order reactions
- Rate = k
- Integrated rate law: [A] = [A]₀ - kt
- Example: Decomposition of a substance
- First order reactions
- Rate = k[A]
- Integrated rate law: ln[A] = -kt + ln[A]₀
- Example: Radioactive decay
- Second order reactions
- Rate = k[A]²
- Integrated rate law: 1/[A] = kt + 1/[A]₀
- Example: A + B -> C
- Half-life of a reaction
- The time taken for the concentration of a reactant to decrease by half
- Graphical representation of integrated rate laws
- Plotting concentration vs. time or ln(concentration) vs. time
- Relationship between rate constant and slope of the graph