Rate Expression and Rate Constant
Introduces the mathematical representation of reaction rate in terms of reactant concentrations and defines the proportionality constant, known as the rate constant.
What is Rate Expression and Rate Constant?
A mathematical equation that expresses the rate of a reaction as a function of the concentrations of reactants.
Key formula / rule: Rate Law
Key points
- Understand the concept of reaction rate.
- Write the rate law for a given reaction.
- Define and explain the significance of the rate constant.
- Determine the units of the rate constant based on the reaction order.
Common exam trap
Assuming rate law exponents (x, y) are equal to stoichiometric coefficients (a, b).
Definitions
- Term
Rate Expression
- Meaning
A mathematical equation that expresses the rate of a reaction as a function of the concentrations of reactants.
- Term
Rate Constant (k)
- Meaning
The proportionality constant in the rate expression, which relates the rate of a reaction to the concentrations of reactants raised to their respective orders. It is temperature-dependent.
- Term
Reaction Order
- Meaning
The exponent to which the concentration of a reactant is raised in the rate law. It indicates how the rate of reaction changes with the concentration of that specific reactant.
- Term
Overall Reaction Order
- Meaning
The sum of the individual reaction orders with respect to all reactants in the rate law.
Learning objectives
Understand the concept of reaction rate.
Write the rate law for a given reaction.
Define and explain the significance of the rate constant.
Determine the units of the rate constant based on the reaction order.
Formulae
- Name
Rate Law
- Note
x and y are orders with respect to A and B; k is the rate constant.
- Expression
Rate = k[A]x[B]y
- Name
Overall Reaction Order
- Note
n is the overall order of the reaction.
- Expression
n = x + y
- Name
Units of Rate Constant (k)
- Note
Where n is the overall reaction order.
- Expression
Units of k = (mol L⁻¹)^(1-n) s⁻¹
Prerequisites
Chemical Kinetics: Introduction
Concentration of solutions
Units of measurement
Common mistakes
Assuming rate law exponents (x, y) are equal to stoichiometric coefficients (a, b).
Confusing rate with rate constant.
Incorrectly calculating units of the rate constant.
Keywords
Rate Law
Rate Constant
Reaction Order
Chemical Kinetics
Rate Expression
Elementary Reaction
Complex Reaction
Practice preview
Which of the following statements correctly defines the rate expression for a chemical reaction?…
easy
What does the rate constant (k) represent in a rate expression?…
easy
For a zero-order reaction, what are the units of the rate constant (k) if concentration is in mol L-1 and time is in seconds?…
easy