[H2 Chemistry] 2021 Kinetics 2

The lecturer begins by continuing from a previous lecture on chemical kinetics, focusing on section 4.2, which is about determining the order of a reaction using the initial rates method. This method involves studying how the initial rate of reaction changes with varying initial concentrations of reactants. The lecturer plans to discuss this concept extensively using exercise 4.4 as an example and emphasizes the importance of understanding the concepts being presented. The initial rate can be obtained through two methods: from a concentration-time graph by drawing a tangent at time equals zero, which represents the instantaneous rate, or through the 'clock method' which measures an average rate over a certain period. The lecturer points out the time-consuming nature of the first method and the inaccuracies of the second, setting the stage for a deeper dive into these concepts.

The lecturer delves deeper into the methods of obtaining the initial rate, starting with the concentration-time graph method. This involves plotting the concentration of a reactant over time and drawing a tangent at the origin to find the initial rate. The process is illustrated with a hypothetical experiment where the concentration of reactant 'a1' is measured, and the tangent at time zero is drawn to determine the initial rate 'g1'. The lecturer then discusses the second method, the clock method, which involves stopping the experiment at a certain point, usually indicated by a color change in the solution, to determine the average rate. The lecturer also mentions a drawback of this method, which is that it doesn't measure the instantaneous rate but rather an average over time. The importance of understanding these methods is stressed, as they are fundamental to the study of reaction kinetics.

The lecturer introduces two methods for determining the order of reactions: inspection and the analytical method. The inspection method involves observing changes in initial rates in relation to changes in reactant concentrations. For example, if the concentration of reactant 'a' triples and the initial rate also triples, this indicates first-order kinetics with respect to 'a'. The analytical method, on the other hand, involves mathematical calculations to determine the order of the reaction. The lecturer demonstrates this by comparing two experiments, showing how the rate law can be used to solve for the order of the reaction with respect to different reactants. The analytical method is particularly useful when the concentrations of multiple reactants are not kept constant, making it difficult to determine the reaction order through inspection alone.

The lecturer moves on to discuss how to write rate equations for reactions, providing an example where the rate equation is written in terms of the rate constant 'k' and the concentrations of reactants 'a', 'b', and 'c' raised to their respective orders. The lecturer then guides the students through calculating the rate constant using the values from a given experiment, emphasizing the importance of units in these calculations. The units for the rate constant 'k' are derived from the units of the initial rate and the concentrations of the reactants. The lecturer also calculates the initial rate for another experiment, demonstrating how to substitute the values of 'k' and the reactant concentrations into the rate equation to find the initial rate.

The lecturer explains the concept of pseudo-order reactions, particularly in the context of experiments where the concentrations of some reactants are kept in excess. This is done to ensure that the rate of reaction is determined primarily by the reactant whose concentration is being varied, rather than by changes in the concentrations of other reactants. The lecturer uses the example of a reaction involving reactants 'a', 'b', and 'c', where 'b' and 'c' are in much higher concentrations than 'a'. This allows the reaction to be treated as a pseudo-first-order reaction with respect to 'a', simplifying the analysis. The lecturer emphasizes the importance of this concept in experimental design and kinetics studies.

The lecturer addresses a common question about why changes in the concentrations of excess reactants can affect the initial rate of a reaction. It is explained that increasing the excess of a reactant increases the probability of effective collisions, thus affecting the rate. The lecturer also discusses the intelligent design of experiments that can simultaneously determine the effects of multiple reactants on the reaction rate. This understanding is highlighted as crucial for solving tutorial questions and for practical applications in kinetics.

The lecturer guides the students through analyzing experimental data to determine the orders of reactions with respect to different reactants. Using the method of inspection and mathematical calculations, the lecturer demonstrates how to compare different sets of experimental data to deduce the reaction orders. The importance of keeping two reactants constant while varying the third is emphasized for accurate determination. The lecturer also discusses how to calculate the rate constant using the initial rates and concentrations from the experiments, correcting a common mistake in the calculation process.

In the final part of the lecture, the lecturer summarizes the key points covered, particularly emphasizing the importance of understanding pseudo-order reactions, which are frequently encountered in practical sessions. The students are encouraged to ask questions, especially about complex topics like pseudo-order reactions, and to take their time in understanding the concepts. The lecturer also advises against memorizing graphs and instead suggests deriving them as part of the learning process, which is beneficial for practical exams and papers.