Chemical equilibria changing concentration

Equilibrium is when two opposing reactions occur at the same rate. In chemical equilibrium, the concentrations of the reactants and products remain constant, and no change is observed in the system. Any chemical process will achieve equilibrium over time. 

When a reaction system has reached chemical equilibrium, the concentrations of fhe reacfanfs and products no longer change with time. Why does the amount of producf no longer increase, even fhough large concenfrafions of the reactants may still be present ... 

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. 

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. 

One difficulty Haber faced is that the reactions used to produce compounds from nitrogen do not go to completion, but appear to stop after only some of the reactants have been used up. At this point the mixture of reactants and products has reached chemical equilibrium, the stage in a chemical reaction when there is no further tendency for the composition of the reaction mixture—the concentrations or partial pressures of the reactants and products—to change. To achieve the greatest conversion of nitrogen into its compounds, Haber had to understand how a reaction approaches and eventually reaches equilibrium and then use that... 

A knowledge of the concentrations of all reactants and products is necessary for a description of the equilibrium state. However, calculation of the concentrations can be a complex task because many compounds may be Imked by chemical reactions. Changes in a variable such as pH or oxidation potential or light intensity can cause large shifts in the concentrations of these linked species. Aggregate variables may provide a means of simplifying the description of these complex systems. Here we look at two cases that involve acid-base reactions. 

As an indispensable source of fertilizer, the Haber process is one of the most important reactions in industrial chemistry. Nevertheless, even under optimal conditions the yield of the ammonia synthesis in industrial reactors is only about 13%. This Is because the Haber process does not go to completion the net rate of producing ammonia reaches zero when substantial amounts of N2 and H2 are still present. At balance, the concentrations no longer change even though some of each starting material is still present. This balance point represents dynamic chemical 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. 

In 1864, two Norwegian chemists, Cato Guldberg and Peter Waage, summarized their experiments on chemical equilibrium in the law of chemical equilibrium At equilibrium, there is a constant ratio between the concentrations of the products and reactants in any change. 

What happens to a system at equilibrium if the concentration of one of the reacting chemicals is changed This question has practical importance, because many manufacturing processes are continual. Products are removed and more reactants are added without stopping the process. For example, consider the Haber process that was mentioned in the previous Sample Problem. 

In this section, you compared strong and weak acids and bases using your understanding of chemical equilibrium, and you solved problems involving their concentrations and pH. Then you considered the effect on pH of buffer solutions solutions that contain a mixture of acid ions and base ions. In the next section, you will compare pH changes that occur when solutions of acids and bases with different strengths react together. 

A condition of balance in a chemical system, at which no further change in reactant and product concentrations occurs. 2. If actions occurring within the reaction result in no net change in the reactant and product concentrations, the system is said to be in dynamic equilibrium. A chemical equilibrium is a dynamic equilibrium when the reaction rate in the forward direction is balanced by the rate in the reverse direction. The potential energy... 

Sometimes, when a chemical reaction takes place, it proceeds for a period of time and then seems to stop before all the reactants are consumed. But the reaction does not actually stop. Instead, the reaction reaches a point of chemical equilibrium in which the reverse reaction is converting products into reactants as fast as products are formed in the forward reaction. At equilibrium, with both the forward and reverse reactions taking place equally fast, the concentration of every species no longer changes. 

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. 

The Ca(II) concentration in blood is closely controlled normal values He between 2.1 and 2.6 mmol/L (8.5—10.4 mg/dL) of semm (21). The free calcium ion concentration is near 1.2 mmol/L the rest is chelated with blood proteins or, to a lesser extent, with citrate. It is the free Ca(II) in the serum that determines the calcium balance with the tissues. The mineral phase of bone is essentially in chemical equilibrium with calcium and phosphate ions present in blood semm, and bone cells can easily promote either the deposition or dissolution of the mineral phase by localized changes in pH or chelating... 

If the absorbing molecule participates in a concentration-dependent chemical equilibrium, the absorptivity changes with concentration. For example, in concentrated solution, a weak acid, HA, may be mostly undissociated. As the solution is diluted, dissociation increases. If the absorptivity of A" is not the same as that of HA, the solution will appear not to obey Beer s law as it is diluted. 

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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