Chemical Kinetics - Units of Reaction Rates
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Reaction rate
- The change in concentration of reactants or products per unit time
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Units of reaction rate:
- Molarity per second (M/s)
- Moles per liter per second (mol/L·s)
- Grams per liter per second (g/L·s)
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Example: Consider the reaction: 2A + B → C
- The rate of this reaction can be defined as the rate of disappearance of A or B or the rate of appearance of C
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The rate of reaction can be determined experimentally by measuring the change in concentration of any reactant or product with time.
Chemical Kinetics - Units of Reaction Rates
Slide 11:
- Integrated Rate Laws
- Describe how the concentration of a reactant or product changes with time
- Different reactions have different rate laws based on their stoichiometry and mechanism
Slide 12:
- First-Order Reactions
- Reaction rate is directly proportional to the concentration of a single reactant
- Example: A → products
- Rate = k[A]
- The integrated rate law for a first-order reaction is given by: ln[A]t = -kt + ln[A]0
Slide 13:
- Second-Order Reactions
- A reaction rate is directly proportional to the square of the concentration of a reactant or to the product of the concentrations of two reactants
- Example: 2A → products
- Rate = k[A]² or Rate = k[A][B]
- The integrated rate law for a second-order reaction is given by: 1/[A]t = kt + 1/[A]0
Slide 14:
- Zero-Order Reactions
- Reaction rate is independent of the concentration of a reactant
- Example: A → products
- Rate = k
- The integrated rate law for a zero-order reaction is given by: [A]t = -kt + [A]0
Slide 15:
- Half-Life of a Reaction
- The time required for the concentration of a reactant or product to decrease to half of its initial value
- For first-order reactions, the half-life is constant and independent of initial concentration
- For second-order reactions, the half-life depends on the initial concentration
Slide 16:
- Activation Energy
- The minimum energy required for a reaction to occur
- Denoted as Ea
- Determines the rate of a reaction
- Higher activation energy leads to slower reaction rates
Slide 17:
- Arrhenius Equation
- Describes the temperature dependence of reaction rates
- k = Ae^(-Ea/RT)
- k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin
Slide 18:
- Factors Affecting Reaction Rates
- Concentration: Increasing concentration of reactants leads to faster reaction rates
- Temperature: Higher temperature increases the rate of reaction due to increased kinetic energy
- Catalysts: Substances that increase the rate of a reaction by providing an alternate reaction pathway with lower activation energy
Slide 19:
- Reaction Mechanisms
- Complex reactions often occur through a series of smaller steps called elementary reactions
- Reaction mechanism explains the individual steps and intermediates involved in a reaction
- Rate-determining step is the slowest step that determines the overall rate of the reaction
Slide 20:
- Collision Theory
- Particles must collide with proper orientation and sufficient energy to react
- Collision frequency and energy play crucial roles in determining reaction rates
- Activation energy determines the fraction of particles with sufficient energy to react
Slide 21:
- Reaction Rate Law
- Describes the relationship between the rate of a reaction and the concentrations of its reactants
- The general form of a rate law is: Rate = k[A]^m[B]^n
- The exponents m and n are determined experimentally and may not necessarily match the coefficients in the balanced chemical equation
Slide 22:
- Rate Determination by Initial Rates Method
- In the initial rates method, the rate of the reaction is measured at different initial concentrations of reactants
- By comparing the rates, the exponents (m and n) in the rate law can be determined
- Example: 2A + B → C
- Initial concentrations: [A] = 0.1 M, [B] = 0.2 M, [C] = 0 M
- Rate 1 = k[A]^m[B]^n
- Rate 2 = k’[A’]^m[B’]^n
- Compare rates to determine m and n
Slide 23:
- Rate Determination by Integrated Rate Laws Method
- In the integrated rate laws method, the concentrations of reactants/products are measured at different times
- By using the appropriate integrated rate law, the rate constant (k) can be determined
- Example: A → products
- Initial concentration: [A] = [A]0
- Concentration at time t: [A]t
- Use the integrated rate law to determine k
Slide 24:
- Reaction Order
- The sum of the exponents in the rate law equation is the reaction order
- Reaction order can be zero, first, second, or even fractional/negative
- For example, a reaction with the rate law Rate = k[A]²[B]³ has a reaction order of 2 + 3 = 5
Slide 25:
- Rate Constant
- The rate constant (k) is a proportionality constant in the rate law equation
- The value of k depends on the specific reaction and the temperature at which the reaction occurs
- The units of k depend on the overall reaction order
Slide 26:
- Factors Affecting Rate Constant
- Temperature: Increasing temperature usually increases the value of k due to increased collision frequency and energy
- Presence of catalysts: Catalysts can increase the rate constant by providing an alternative reaction pathway with lower activation energy
- Physical state: The rate constant may vary depending on whether the reactants are in the gas, liquid, or solid state
Slide 27:
- Reaction Mechanism and Elementary Steps
- A reaction mechanism describes the sequence of individual steps by which a reaction occurs
- Elementary steps are the individual chemical reactions that make up the overall reaction
- The rate law for the overall reaction can be determined by the slowest step, known as the rate-determining step
Slide 28:
- Reaction Intermediate
- An intermediate is a species that is formed during the course of a reaction but does not appear in the balanced chemical equation
- Intermediates are consumed in subsequent steps and are not present in the final products
Slide 29:
- Reaction Intermediates and Catalysts
- Catalysts are substances that increase the rate of a reaction without being consumed in the process
- Catalysts provide an alternative reaction pathway with a lower activation energy, increasing the rate of the reaction
- Catalysts can react with reactants to form intermediates, which are then regenerated in subsequent steps
Slide 30:
- Summary of Key Points
- Reaction rate is the change in concentration of reactants or products per unit time
- Reaction rates can be defined by the disappearance of reactants or the appearance of products
- Rate laws describe the relationship between the rate of a reaction and the concentrations of its reactants
- Rate constants (k) depend on temperature, presence of catalysts, and physical state of reactants
- Reaction mechanisms describe the sequence of elementary steps and the rate-determining step.
Chemical Kinetics - Units of Reaction Rates Reaction rate The change in concentration of reactants or products per unit time Units of reaction rate: Molarity per second (M/s) Moles per liter per second (mol/L·s) Grams per liter per second (g/L·s) Example: Consider the reaction: 2A + B → C The rate of this reaction can be defined as the rate of disappearance of A or B or the rate of appearance of C The rate of reaction can be determined experimentally by measuring the change in concentration of any reactant or product with time.