Introduction to Chemical Equilibrium

A major theme in chemistry is chemical equilibrium that point during the course of a chemical reaction where there is no further net change in the chemical composition of the system. One of the triumphs of thermodynamics is that it can be used to understand chemical equilibria. 

When you stop and think about it, very few chemical processes are actually at chemical equilibrium. Consider the chemical reactions going on in your body s cells. If they were at equilibrium, you wouldn t even be alive Many chemical reactions that occur on the industrial scale aren t at equilibrium, or else chemical producers wouldn t be making new chemicals for sale. 

Then why do we put so much stock in equilibria For one thing, a system at equilibrium is a system we can understand using thermodynamics. Also, though almost all chemical systems of interest aren t at equilibrium, the idea of equilibrium is used as a starting point. The concept of chemical equilibrium is the very basis for understanding systems that are not at equilibrium. An understanding of equilibrium is a central part of understanding chemistry. 

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L Meites An Introduction to Chemical Equilibrium and Kinetics, Pergamon Press, Oxford, 1981... 

These chapters provide the necessary background for a strong introduction to chemical equilibrium in Chapter 17. This is followed by three chapters on equilibria in aqueous solutions. A chapter on electrochemistry (Chapter 21) and nuclear chemistry (Chapter 22), completes the common core of the text. 

The N equations represented by Eq. (4-282) in conjunction with Eq. (4-284) may be used to solve for N unspecified phase-equilibrium variables. For a multicomponent system the calculation is formidable, but well suited to computer solution. The types of problems encountered for nonelectrolyte systems at low to moderate pressures (well below the critical pressure) are discussed by Smith, Van Ness, and Abbott (Introduction to Chemical Engineering Thermodynamics, 5th ed., McGraw-Hill, New York, 1996). 

FIGURE 7.5 Thermodynamic equilibrium constant for gas-phase reactions. (From Smith, J. M. and Van Ness, H. C., Introduction to Chemical Engineering Thermodynamics, 4th Ed., McGraw-Hill, New York, 1986.)... 

The application of the second law to chemical equilibrium is facilitated by the introduction of two more state functions. These are defined as (a) the Helmholtz free energy... 

See Chapter 9 in J. M. Smith, Introduction to Chemical Engineering Thermodynam-ias, 3rd ed., McGraw-Hill, New York, 1959,and Chapter 9 in S. 1. Sandler, Chemical and Engineering Thermodynamics, WUey, (1989) for a discussion of chemical equilibrium including nonideal effects. 

FIG. 4-9 Two PV isotherms at the same T for mixtures. The solid line is for a liquid-phase composition the dashed line is for a vapor-phase composition. Point B represents a bubble point with the liquid-phase composition point D represents a dew point with the vapor-phase composition. When these points lie at the same P (as shown), they represent phases in equilibrium. [Smith, Van Ness, and Abbott, Introduction to Chemical Engineering Thermodynamics, 7th ed., p. 560, McGraw-Hill, New York (2005). ]... 

Skill 2.2a-Predict the effect of temperature, pressure, and concentration on chemical equilibrium (LeChatelier s principle) and the reaction rate Introduction to dynamic equilibrium... 

As an introduction to the application of Eqs. 13.1-1 and 13.1-2 to chemical equilibrium problems, consider the prediction of the equilibrium state for the low-pressure, gas-phase reaction... 

Fig. 2-11. Energy bands of sodium as a function of internuclear distance. r0 represents the actual equilibrium distance. [Reproduced by permission from J. C. Slater, Introduction to Chemical Physics, McGraw-Hill Book Co., 1939.]...

Nowadays, a kinetic derivation of the Law of Mass Action is usually not included in chemistry textbooks. Instead, a qualitative introduction of chemical equilibrium, which uses kinetic ideas, has become quite common in many textbooks ( dynamic equilibrium ) for secondary education (age of students 14-16 years). The stabihty of chemical equilibrium and the ways these equilibria respond to external changes are usually addressed next. In this respect, much attention traditionally is paid to Le Chatelier sprinciple. This principle has been, and is still, used extensively to explain or predict the behaviour of a chemical system at equilibrium due to changes in pressure, volume, concentrations, or temperature. This is done in terms of changes in the concentrations of reactants and products followed by the establishment of a new state of equilibrium. This principle was applied to various examples... 

In introductory chemistry lessons, chemical reactions are usually associated with observable phenomena (e.g., change of colour, evolution and absorption of heat, precipitation of a solid, evolution of a gas) and chemical reactions are presented as proceeding to completion, taking place in one direction (Andersson, 1990). The introduction of chemical equilibrium at a later stage, however, demonstrates the reversibility of chemical reactions and the possibility that chemical reactions do not proceed to completion. Moreover, the dynamic nature of chemical equilibrium requires students to assume that two opposite chemical reactions are taking place, in spite of the fact that this cannot be deduced from observation. As a consequence, the introduction of chemical equilibrium requires students to revise their initial conception of chemical reactions. This is illustrated in Table 1. 

The introduction of chemical equilibrium in the latter part of a two semester ehemistry course exposes the student to the possibility of incompleteness and reversibility of chemical reactions. The students are confronted with the idea of two opposing chemical reactions occurring at the same time but for which no visible evidence is available. These concepts are at odds with well established conceptions that students have about chemical reactions. 

The first part (237 pp.) of Klotz s book, which is issued separately under the title Introduction to Chemical Thermodynamics , gives an account of the laws and basic equations of classical thermodynamics as applied to systems of constant composition. The second part is concerned with systems of variable composition. The book has a good treatment of the Third Law and its chemical applications but reaction equilibrium is covered only to a limited extent. 

Abstract In this chapter we present a brief introduction to chemical kinetics. Key concepts like reversibility of chemical reactions, reaction rate, reaction rate constant, and chemical equilibrium, are introduced and discussed. The most important of the results here derived is the so-called law of mass action which we discuss from the perspective of chemical kinetics. In this chapter we follow a heuristic rather than a formal approach. We start by analyzing a few simple chemical reactions to gain insight into the chemical kinetics basic concepts. After that, we heuristically derive and discuss the corresponding results for the most general case. The interested reader can consult any of the many available books on the subject. We particularly recommend the book by Houston (Chemical kinetics and reaction dynamics. McGraw-Hill, New York, 2001). 

This chapter is meant as a brief introduction to chemical kinetics. Some central concepts, like reaction rate and chemical equilibrium, have been introduced and their meaning has been reviewed. We have further seen how to employ those concepts to write a system of ordinary differential equations to model the time evolution of the concentrations of all the chemical species in the system. The resulting equations can then be numerically or analytically solved, or studied by means of the techniques of nonlinear dynamics. A particularly interesting result obtained in this chapter was the law of mass action, which dictates a condition to be satisfied for the equilibrium concentrations of all the chemical species involved in a reaction, regardless of their initial values. In the forthcoming chapters we shall use this result to connect different approaches like chemical kinetics, thermodynamics, etc. 

In spite of these limitations it is hoped that this chapter will provide an introduction to the unusual phenomena that chemically reacting systems exlribit when driven far from equilibrium and an indication of how these phenomena may be analysed. Although such systems were often regarded as curiosities in the past, it is now clear that they are the mle rather than the exception in nature and deserve our full attention. 

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