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Cover Making the Transition to University Chemistry


This chapter discusses rates of reactions in kinetics. The rate of reaction is based on the rate of change of concentration per unit time. The activation energy for a reaction is the minimum energy necessary for a collision to lead to a successful reaction. The rate of reaction, then, depends on the concentration of the reactants, temperature, presence of a catalyst, and state of subdivision. The Maxwell–Boltzmann distribution pins the number of molecules in a gas with given energy against energy. On the other hand, the Arrhenius equation measures the activation energy on how the rate constant depends on the temperature.


Cover Atkins’ Physical Chemistry

Heterogeneous catalysis  

This chapter explains how the chemical industry relies on the use of efficient catalysts to facilitate a wide variety of transformations, noting that the majority of these catalysts involve reactions at surfaces. It describes how certain concepts relating to adsorption and desorption can be extended to provide a way to model surface reactions. It also points out that the chemical industry relies on heterogeneous catalysis for many of its most important large-scale processes. The chapter highlights heterogeneous catalysis, which commonly involves chemisorption of one or more reactants and a consequent lowering of the activation energy. It builds on the discussion of reaction mechanisms and uses the Arrhenius equation and adsorption isotherms.


Cover Chemistry for the Biosciences

Kinetics: what affects the speed of a reaction?  

This chapter discusses chemical kinetics, looking at the factors that control the rate of a chemical reaction. The rate of a chemical reaction is the speed of change in concentration of reactants or products per unit time as the reaction proceeds. One can determine the rate of a reaction by measuring the concentrations of reactants or products at different times during the course of a reaction, and plotting on a graph the change of concentration with time. Every reaction has an energy threshold called the activation energy that must be reached before the reaction can occur. The chapter then considers catalysis and enzymes, and their impact upon the activation energy and reaction kinetics, before explaining enzyme kinetics and enzyme inhibition and the ways in which these can be described.


Cover Physical Chemistry for the Life Sciences

Electron transfer  

This chapter demonstrates how electron transfer rates can be expressed in terms of Gibbs activation energies. Many biological processes depend on the transfer of electrons between electron donors and acceptors. In some instances, the electron donors and acceptors are free to diffuse in the aqueous solutions of the cell or in the fluid bilayers that make up the membranes, and electron transfer takes place when the donor and acceptor meet. However, in many other instances electron donors and acceptors exist as redox centres within protein molecules and electron transfer from donor to acceptor occurs without a diffusional encounter being necessary. Thus, this chapter introduces Marcus theory, an adaptation of transition-state theory, which is widely used to account for the rates of electron transfer. It also discusses protein electron transfer processes and examines electron transfer in photosystem I as a case study.


Cover Reaction Dynamics

Thermal rate coefficients  

This chapter assesses thermal rate coefficients. It begins by looking at canonical transition state theory (CTST), focusing on the derivation and application of transition state theory to bimolecular reactions. The discrepancy between CTST and the experimental rate coefficients can be ascribed to the neglect of tunnelling in the CTST calculation. However, classical mechanics works rather well, suggesting that the absence of a tunnelling contribution to the classical rate coefficient is offset by the absence of zero point vibrational energy in the barrier region, which lowers the effective barrier height compared with the quantum mechanical one. The chapter then examines thermally activated unimolecular reactions. In a thermally activated unimolecular reaction, the molecule acquires the energy necessary to react by energy-transferring collisions, and like the elementary reaction step, the rate coefficients of these activating and deactivating collisions also depend on the energy of the energized molecule.


Cover Structure and Reactivity in Organic Chemistry
Structure and Reactivity in Organic Chemistry starts with a consideration of molecular vibrations and intermolecular interactions, and introduces the use of potential energy profiles and reaction maps to describe organic chemical transformations. The relationship between kinetics and organic reaction mechanisms is then explored with special emphasis on the interpretation of activation parameters. The relationship between molecular structure and chemical reactivity, i.e. correlation analysis, is then covered, followed by a chapter on catalysis of organic chemical reactions in solution by small molecules. The treatment of catalysis explores how the molecular structure of compounds determines their reactivity either as substrates or as catalysts. The final chapter is devoted to isotope effects in mechanistic organic chemistry, concentrating on deuterium kinetic isotope effects.


Cover Mechanisms of Organic Reactions

Description and investigation of organic reaction mechanisms  

This chapter discusses the terminology necessary for describing mechanisms of reactions of organic compounds. Each individual step of a complex reaction that is a proper chemical reaction in its own right is called an elementary reaction. As the concerted bonding of the nucleophile and unbonding of the nucleofuge, the molecular potential energy of the ion–molecule system increases until, at a certain stage, it reaches a maximum with both halogen atoms partially bonded to the carbon which is penta-coordinate. The chemical species corresponding to the molecular potential energy maximum is called the activated complex. The chapter then looks at molecularity, reaction coordinate, molecular potential energy reaction profile, and reaction maps. It also introduces the main techniques deployed in the investigation of organic reaction mechanisms, which can be divided into two groups: kinetics methods and non-kinetics methods.