Chemical equilibria changing temperature

I See the Saunders Interactive General Chemistry CD-ROM, Screen 16.12, Disturbing a Chemical Equilibrium (2) Temperature Changes. 

Although the resulting formulae are correct enough in so far as they are applied to the calculation of changes of entropy or free energy, for example, in consideration of shifts of chemical equilibrium with temperature, they still prove to be wrong in absolute magnitude. 

Factors That Affect Chemical Equilibrium Changes in concentration can affect the position of an equilibrium state—that is, the relative amounts of reactants and products. Changes in pressure and volume may have the same effect for gaseous systems at equilibrium. Only a change in temperature can alter the value of equilibrium constant. A catalyst can establish the equilibrium state faster by speeding the forward and reverse reactions, but it can change neither the equilibrium position nor the equilibrium constant. 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

Chemical relaxation techniques were conceived and implemented by M. Eigen, who received the 1967 Nobel Prize in Chemistry for his work. In a relaxation measurement, one perturbs a previously established chemical equilibrium by a sudden change in a physical variable, such as temperature, pressure, or electric field strength. The experiment is carried out so that the time for the change to be applied is much shorter than that for the chemical reaction to shift to its new equilibrium position. That is to say, the alteration in the physical variable changes the equilibrium constant of the reaction. The concentrations then adjust to their values under the new condition of temperature, pressure, or electric field strength. 

Why Do We Need to Know This Material The second law of thermodynamics is the key to understanding why one chemical reaction has a natural tendency to occur bur another one does not. We apply the second law by using the very important concepts of entropy and Gibbs free energy. The third law of thermodynamics is the basis of the numerical values of these two quantities. The second and third laws jointly provide a way to predict the effects of changes in temperature and pressure on physical and chemical processes. They also lay the thermodynamic foundations for discussing chemical equilibrium, which the following chapters explore in detail. 

Why Do We Need to Know This Material The dynamic equilibrium toward which every chemical reaction tends is such an important aspect of the study of chemistry that four chapters of this book deal with it. We need to know the composition of a reaction mixture at equilibrium because it tells us how much product we can expect. To control the yield of a reaction, we need to understand the thermodynamic basis of equilibrium and how the position of equilibrium is affected by conditions such as temperature and pressure. The response of equilibria to changes in conditions has considerable economic and biological significance the regulation of chemical equilibrium affects the yields of products in industrial processes, and living cells struggle to avoid sinking into equilibrium. 

Concentration (1), pressure (1) and temperature (1) may, if changed, alter the position of a chemical equilibrium. These factors often, but not always, have an effect on the position of equilibrium. 

Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its condensation (pressure, concentration, number of molecules in unit volume), either as a whole or in some of its parts, can only undergo such internal modifications as would, if produced alone, bring about a change of temperature or of condensation of opposite sign to that resulting from the external cause. 

A chemical equilibrium results when two exactly opposite reactions occur at the same place, at the same time, and with the same rate. An equilibrium constant expression represents the equilibrium system. Le Chatelier s principle describes the shifting of the equilibrium system due to changes in concentration, pressure, and temperature. 

With the availability of perturbation techniques for measuring the rates of rapid reactions (Sec. 3.4), the subject of relaxation kinetics — rates of reaction near to chemical equilibrium — has become important in the study of chemical reactions.Briefly, a chemical system at equilibrium is perturbed, for example, by a change in the temperature of the solution. The rate at which the new equilibrium position is attained is a measure of the values of the rate constants linking the equilibrium (or equilibria in a multistep process) and is controlled by these values. 

A valuable guide is available to assist you in estimating how chemical equilibrium will shift in response to changes in the conditions of the reaction, such as a modification of temperature or pressure. The French chemist Henri Le Chatelier realized in 1884 that if a chemical system at equilibrium is disturbed, the system would adjust itself to minimize the effect of the disturbance. This qualitative reasoning tool is cited as Le Chatelier s principle. 

This simple result for the equilibrium melting temperature offers a guidance in the chemical design of elastomers (low 7 ) and engineering plastics (high T ). For example, if the chain backbone is more flexible, then the change in... 

If a chemical system at equilibrium is disturbed by a change of concentration, pressure, or temperature, the system tends to counteract this change in order to reestablish a new equilibrium. In a chemical equilibrium, this principle is called Le Chatelier s principle. 

As described in Section 1,1, both the procedures of changing Po. at fixed temperature and changing temperature at fixed Po. control the nonstoichiometry in Nii O, Let us consider the relation between a and 3 in Nil O, in which the non-stoichiometry S is believed to originate from metal vacancies. By use of the notation based on the effective charge, described in Section 1,3,7, the chemical equilibrium between the oxygen gas in the atmosphere and the oxygen in the solid may be expressed as... 

CHEMICAL EQUILIBRIUM. The fundamental law of chemical equilibrium was enunciated by Le Chalclier (I884i. and may be stated as follows If any stress or force is brought to bear upon a system in equilibrium, the equilibrium is displaced in a direction which lends to diminish the intensity ol the stress or force. This is equivalent to the principle of least aclion. Its great value to the chemist is that it enahles him to predict the effect upon systems in equilibrium ol changes in temperature, pressure, and concentration. 

The effect ol change ol temperature nn a system in chemical equilibrium is thin the equilibrium point is shifted ll) toward the side itiii/v from that which evolves heat when the temperature is wised, and (2) toward the side which evolves heal when the temperature is lowered. It is tis if the amount of heal were a muterinl reactant and its concentration (temperature or intensity of heal) increased, in respect to the tUrttiitm of the sltilt of tile equilibrium point. The amount of die shill at constant pressure can he calculated in eases where one possesses the proper data. 

Chemical examples showing this type of behaviour include processes associated with sudden changes in concentration, phase, crystal structure, temperature, etc. For example, Figure 2.9 shows how the equilibrium concentration of a chemical species changes suddenly when a temperature jump is applied at time t. Although there are no discontinuities in this function, its derivative is undefined at time t0. 

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