TLDR;
This lecture on chemical kinetics covers the law of mass action, rate laws, order of reactions, and units of rate constants. It explains the differences between the law of mass action and rate laws, defines rate constants, and discusses the characteristics of rate constants. The lecture also covers how to determine the order of a reaction and provides examples of zero, first, second, and third-order reactions. Finally, it explains how to calculate the units of rate constants for different orders of reactions.
- Law of Mass Action vs. Rate Law
- Rate Constant Definition and Properties
- Determining Reaction Order
- Calculating Units of Rate Constants
Quiz 1: Law of Mass Action [1:30]
The lecture starts with a quick quiz based on the previous class. The first question asks who proposed the law of mass action, with the correct answer being Goldberg and Waage. The second question asks what the rate of an elementary reaction is directly proportional to, according to the law of mass action, with the correct answer being molarity.
Law of Mass Action and Rate Law [4:04]
The lecture discusses the law of mass action, which states that the rate of a reaction is directly proportional to the concentration of the reactants raised to the power of their stoichiometric coefficients. However, this law is not always valid experimentally. The rate law, on the other hand, is an experimental determination of how the rate of a reaction depends on the concentration of reactants. The powers in the rate law may or may not be equal to the stoichiometric coefficients. The key difference is that the law of mass action is a theoretical concept, while the rate law is based on experimental data.
Rate Constant (k) [7:10]
The rate law is expressed as rate = k[A]^α[B]^β, where k is the rate constant, and α and β are the orders of the reaction with respect to reactants A and B. The rate constant (k) is the rate of the reaction when the concentration of each reactant is one molar. It is also known as the specific reaction rate. The units of the rate constant vary depending on the order of the reaction, while the units of the rate are always mol L⁻¹ s⁻¹. The rate constant is a measure of the rate of reaction, with a higher rate constant indicating a faster reaction. The rate constant is fixed for a particular reaction at a specific temperature and is independent of the concentration of the reactants.
Characteristics of Rate Constant [12:44]
The rate constant is a measure of the rate of reaction, meaning a higher rate constant indicates a faster reaction. Each reaction has a definite value of the rate constant at a particular temperature. The rate constant is independent of the concentration of the reactants. The units of the rate constant depend on the order of the reaction. If a reaction is given and you need to write the rate law, use the stoichiometric coefficients as the powers, unless the rate law is explicitly given.
Quiz 2: Factors Affecting Rate of Reaction [17:20]
Another quiz is conducted. The first question involves applying the law of mass action to a given reaction (2A + B), and the second question asks which factors affect the rate of a reaction. The correct answer includes surface area, temperature, and concentration/pressure of reactants.
Order of Reaction [20:05]
The order of a reaction is the sum of the powers to which the concentration terms are raised in the rate law expression (α + β). If the sum is zero, it's a zero-order reaction; if it's one, it's a first-order reaction, and so on. The syllabus mainly focuses on zero-order and first-order reactions. Examples of different order reactions are provided:
- Zero Order: Decomposition of ammonia (NH₃) on a platinum catalyst.
- First Order: Decomposition of N₂O₅.
- Second Order: Decomposition of NO₂.
- Third Order: Reaction of NO with O₂.
True or False: Rate Dependence on Reactant Concentration [30:58]
A question is posed: "Does the rate of reaction always depend upon the concentration of the reactant?" The answer is false because, in zero-order reactions, the rate is independent of the reactant concentration.
Calculating Overall Order of Reaction [34:14]
Examples are given to calculate the overall order of a reaction from the rate expression. The order can be zero, one, two, or even a fraction.
Quiz 3: Order of Chemical Reaction [35:48]
A quiz is conducted to test understanding of the order of reactions. The questions focus on the definition of the order of a chemical reaction and calculating the order from a given reaction.
Units of Rate Constant [38:16]
The lecture explains how to determine the units of the rate constant (k) for different orders of reactions. The units of the rate are always mol L⁻¹ s⁻¹, but the units of k vary. The units of k are derived for zero, first, and second-order reactions. A general formula is provided to calculate the units of k for any order:
Units of k = (mol L⁻¹)^(1-n) s⁻¹
where n is the order of the reaction.
Identifying Order from Units [45:44]
A question is asked to identify the order of a reaction based on the units of the rate constant. Homework is assigned to derive the units of third and fourth-order reactions using both the formula and the conceptual method.