33. Kinetics and Temperature

The script begins with an introduction to MIT OpenCourseWare, highlighting its commitment to providing free, high-quality educational resources, with a call to support its mission. It then transitions into a classroom discussion led by Catherine Drennan, focusing on the impact of temperature on reaction rates. The concept of the Arrhenius equation is introduced, which describes the exponential relationship between reaction rates and temperature. The lecture aims to quantitatively explain how temperature changes can affect reaction rates, using the Arrhenius plot to illustrate the linear relationship between the natural logarithm of the rate constant and the inverse of temperature.

This paragraph delves deeper into the Arrhenius equation, explaining the significance of the Arrhenius factor (A) and activation energy (Ea). It clarifies that factor A is the rate constant at an infinitely high temperature, representing the theoretical maximum reaction speed. The activation energy is established as a constant for a given reaction, independent of temperature, but varying between different reactions. The paragraph also discusses different ways to express the Arrhenius equation and emphasizes the importance of understanding these concepts for solving kinetics problems.

The script presents a practical example of using the Arrhenius equation to calculate the rate constant of sucrose hydrolysis at different temperatures. Given the activation energy and the rate constant at normal body temperature, the students are guided through the process of determining the rate constant at a lower temperature of 35 degrees Celsius. This example serves to illustrate the quantitative application of the Arrhenius equation in predicting reaction rates under varying thermal conditions.

This section discusses the relationship between temperature and reaction rates, emphasizing that lower temperatures result in slower reaction rates. The script uses the body's temperature as an example, explaining that the human body operates optimally at a specific temperature, and deviations from this can slow down essential processes like digestion. The narrative also touches on the use of liquid nitrogen in research to 'freeze' reactions for structural analysis, highlighting the dramatic effects of extreme cold on reaction rates.

The script describes a classroom demonstration using glow sticks to visually represent the effect of temperature on chemical reactions. The glow sticks, which emit light through a chemical reaction, are cooled with liquid nitrogen to observe the reaction's cessation as the temperature decreases. This experiment serves to reinforce the concept that chemical reactions, including enzymatic and non-enzymatic ones, are significantly influenced by temperature.

This paragraph introduces the concept of activation energy and the reaction coordinate, explaining the process of reactants forming products. It describes the necessity of molecules having sufficient energy to overcome the activation energy barrier, which is critical for a reaction to proceed. The script uses an analogy of a relationship requiring effort to illustrate the need for this 'critical energy' and introduces the terms 'activated complex' and 'transition state' to describe the high-energy state during a reaction.

The script explores the relationship between kinetic energy, temperature, and reaction rates, using a plot to illustrate how a higher temperature results in a greater fraction of molecules possessing the necessary activation energy to react. It explains how temperature influences the kinetic energy distribution of molecules, thereby affecting the likelihood of successful collisions and reactions. The paragraph emphasizes the importance of temperature in increasing the rate of reactions by providing the kinetic energy needed to overcome activation energy barriers.

This section of the script discusses the construction and interpretation of reaction coordinate diagrams, which depict the potential energy changes during a reaction. It explains the concepts of forward and reverse activation energies and how they relate to the overall energy change (delta E) of a reaction. The script also touches on the relationship between delta E and thermodynamics, specifically the connection to enthalpy changes (delta H) in reactions.

The script examines the impact of temperature on overall reactions, noting that it is more complex than for elementary reactions due to the potential presence of multiple steps in a reaction mechanism. It explains that while increasing temperature always increases the rate of an elementary reaction, the effect on the equilibrium constant and overall reaction rate can vary depending on whether the reaction is exothermic or endothermic. The importance of understanding reaction mechanisms to predict temperature effects is highlighted.

This paragraph discusses how temperature affects both the rate constants (k) and equilibrium constants (K) of reactions. It emphasizes that while rate constants always increase with temperature due to the presence of activation energy barriers, the effect on equilibrium constants depends on the reaction's thermodynamic nature (exothermic or endothermic). The script also introduces the van 't Hoff equation, which relates temperature changes to equilibrium constants, and illustrates how the interplay between rate constants and equilibrium constants determines the overall effect of temperature on reaction rates.

The script concludes with a summary of the key points discussed in the lecture, including the consistent increase of rate constants with temperature and the variable effect on equilibrium constants. It previews the continuation of the topic in the next class, hinting at a deeper exploration of temperature effects and introducing the concept of Le Chatelier's principle in the context of temperature changes. The lecture ends with a reminder of an upcoming exam and an invitation for students to discuss any difficulties they may be facing with the material.

The final paragraph of the script focuses on the concept of activation energy barriers and their temperature sensitivity. It explains that large activation energy barriers make rate constants very sensitive to temperature changes, while small barriers have less impact on reaction rates. The script uses the analogy of overcoming challenges to illustrate the concept and emphasizes that increasing temperature allows more molecules to overcome activation energy barriers, leading to a shift in reaction direction towards the side with the larger barrier.