[H2 Chemistry] 2021 Kinetics 3

This paragraph introduces section 5, focusing on experimental methods for analyzing reaction rates. It discusses two types of graphs: concentration-time and rate against concentration. The initial rate method is used for concentration-time graphs, and the speaker references section 4.2 for details on determining reaction order. The sampling method and clock method are highlighted as key experimental techniques. The sampling method involves taking regular time interval samples from the reaction mixture using a pipette and analyzing changes in concentration using various methods like colorimetry or titration. The clock method, briefly mentioned, is used to determine the average rate of reaction through color changes. The speaker emphasizes the importance of practical application for understanding these concepts and mentions a forthcoming video on the clock method.

The speaker delves deeper into the sampling method, explaining the process of taking samples at regular intervals using pipettes and analyzing them for changes in reactant or product concentration over time. The importance of accurate timing when withdrawing samples is emphasized, as is the need to quench the reaction at precise moments to prevent further changes. Quenching can be done with various agents, such as infinite dilution with cold water or specific reagents that stop the reaction. The paragraph also discusses the analysis of iodine remaining in a reaction mixture by titration with thiosulfate, a common technique in sampling experiments. The speaker advises students to understand the importance of accurate titration and to strategize their approach during practical experiments to manage time effectively.

This paragraph discusses factors that can affect the accuracy of sampling and titration methods in kinetic experiments. It stresses the importance of correct timing for quenching reactions and maintaining a constant temperature, unless the experiment specifically aims to study the effect of temperature on reaction rates. The paragraph also mentions that other reactants should be used in large excess so their concentration changes do not significantly affect the reaction rate. This assumption allows for simpler rate calculations based on the concentration of one reactant, as demonstrated in the iodine clock reaction example. The speaker also provides a generic procedure for writing about sampling reactions by titration, advising students to become familiar with these procedures after completing a few practicals.

The speaker presents an exercise involving the decomposition of a diluted H2O2 solution, which produces oxygen gas. The reaction is monitored by sampling at various times and titrating with KMnO4. The time interval for sampling is every five minutes, and the volume of KMnO4 required for complete reaction decreases over time. Students are instructed to plot a graph of KMnO4 volume against time and use it to determine the order of the reaction with respect to H2O2. The speaker explains that a constant half-life indicates first-order kinetics and provides the steps to calculate the rate constant, emphasizing the importance of plotting accurate graphs and understanding the relationship between the volume of titrant and the concentration of the reactant.

This paragraph explores various methods for measuring the rates of different reactions by monitoring physical property changes. It suggests using gas collection techniques for soluble gases, calorimetry to measure color intensity changes, and conductivity measurements for reactions involving ions. The speaker also discusses changes in gas pressure as a method to study reaction kinetics. Each method is tailored to the specific reaction, such as monitoring the disappearance of color in a displacement reaction or the formation of water in an esterification reaction. The paragraph aims to highlight the diverse approaches available for studying reaction rates through physical property changes.

The speaker introduces clock reactions, which are designed to measure reaction rates by observing the time taken for a certain amount of product to form, leading to a visible change. The paragraph describes a video demonstration of a clock reaction using hydrogen peroxide, starch, sulfuric acid, sodium thiosulfate, and potassium iodide. The reaction involves the production of elemental iodine, which is consumed by thiosulfate, and the sudden appearance of a blue-black color indicates the end of the reaction. The speaker explains the importance of using a fixed amount of thiosulfate to ensure a fair comparison of reaction rates in different experiments and clarifies that the rate measured in clock reactions is an average rate, not the initial rate.

This paragraph clarifies the relationship between time and initial rates, emphasizing that initial rates are inversely proportional to time, not equal to one over time. The speaker discusses the importance of recognizing this relationship when conducting experiments and interpreting results. They also provide a summary of the two methods for studying reaction rates: the sampling method, which allows for the plotting of concentration against time to obtain initial rates, and the clock method, which uses one over time as a proxy for initial rates. The clock method is noted for being faster and requiring less time for data collection compared to the sampling method.

The speaker discusses collision theory, explaining that for a reaction to occur, particles must collide with sufficient energy and the correct orientation. Activation energy is identified as the minimum energy required for a reaction to proceed past the activation barrier. The paragraph introduces the concept of the Maxwell-Boltzmann distribution curve, which shows the distribution of kinetic energy among particles. It explains that increasing temperature increases the number of particles with energy above the activation barrier, leading to more effective collisions and a faster reaction rate. The speaker also touches on reactions with low activation energy, such as acid-base neutralization, and those with high activation energy, like the production of ammonia.

This paragraph delves into the concepts of transition states and intermediates in the context of reaction mechanisms. It emphasizes that transition states are transient, hypothetical states that cannot be isolated, existing only briefly during a reaction. In contrast, intermediates are unstable species that can sometimes be isolated and studied. The speaker clarifies that transition states are not the same as intermediates and that activation energy is associated with the highest energy transition state along the reaction pathway, not necessarily the highest individual activation barrier. The paragraph aims to provide a clearer understanding of these complex concepts, which are crucial for studying reaction mechanisms.