Le Chateliers Principle

Le Chatelier discovered that if a chemical system at equilibrium is stressed (disturbed) it will reestablish equilibrium by shifting the rates of the reactions involved. This means that the amounts of the reactants and products will change, but the ratio will remain the same. One can stress the equilibrium in a number of ways changes in concentration, pressure, and temperature. However, a catalyst will have no effect on the equilibrium amounts since it affects both the forward and reverse reactions equally. It will simply allow the reaction to reach equilibrium faster. 

If the stress to the equilibrium system is a change in concentration of one of the reactants or products, then the equilibrium will react to remove that stress. If, for example, we decrease the concentration of a chemical species, the equilibrium will shift to produce more of it. In doing so, the concentration of chemical species on the other side of the reaction arrows will be decreased. If the concentration of a chemical species is increased, the equilibrium will shift to consume it, increasing the concentration of chemical species on the other side of the reaction arrows. 

If one increases the concentration of hydrogen gas then the equilibrium shifts to the right in order to consume some of the added hydrogen. In doing so, the concentration of ammonia (NH3) will increase and the concentration of nitrogen gas will decrease. 

The concentrations may change, but the value of Kc or Kp would remain the same. 

Changes in pressure are only significant if there are gases involved. The pressure may be changed by changing the volume of the container or by changing the concentration of a gaseous species. If the container becomes smaller, the pressure increases because there are an increased number of collisions on the inside walls of the container. This stresses the equilibrium system and it will shift in order to reduce the pressure. A shift towards the side of the equation that has the least number of moles of gas will accomplish this. If the container 

To learn to predict the changes that occur when a system at equilibrium is disturbed. 

It is important to understand the factors that control the position of a chemical equilibrium. For example, when a chemical is manufactured, the chemists and chemical engineers in charge of production want to choose conditions that favor the desired product as much as possible. That is, they want the equilibrium to lie far to the right (toward products). When the process for the synthesis of ammonia was being developed, extensive studies were carried out to determine how the equilibrium concentration of ammonia depended on the conditions of temperature and pressure. 

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In this section we will explore how various changes in conditions affect the equilibrium position of a reaction system. We can predict the effects of changes in concentration, pressure, and temperature on a system at equilibrium by using Le Chatelier s principle, which states that when a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change. 

Let us consider the ammonia synthesis reaction. Suppose there is an equilibrium position described by these concentrations  

What will happen if 1.000 mol/L of N2 is suddenly injected into the system We can begin to answer this question by remembering that for the system at equilibrium, the rates of the forward and reverse reactions exactly balance, 

Henri Louis Le Chatelier (1850-1936), the French physical chemist and metallurgist, seen here while a student at the Ecole Polytechnique. 

However, this is not the whole story. Carrying out the process at low temperatures is not feasible because then the reaction is too slow. Even though the equilibrium tends to shift to the right as the temperature is lowered, the attainment of equilibrium would be much too slow at low temperatures to be practical. This emphasizes once again that we must study both the thermodynamics and the kinetics of a reaction before we really understand the factors that control it. 

To see how we can predict the effect of change in concentration on a system at equilibrium, we will consider the ammonia synthesis reaction. Suppose there is an equilibrium position described by these concentrations  

The Le Chatelier principle itself can also be derived from the Second Law of Thermodynamics (Frame 14) is of much wider applicability than just to chemical reactions. It applies to all systems which are in equilibrium. Other examples might be solid - liquid (see Frame 26 where conclusions regarding the effect of pressure on melting point are in agreement with the application of the Le Chatelier principle), solid - solid equilibria or equilibria in solution (see Section 49.7 below). 

3 Effect on an Equilibrium Reaction of Change in Temperature - Qualitative Interpretation 

To see how this works suppose we have an exothermic reaction, for which AH 0 (negative) then for the forward reaction we have  

In the case of endothermic reactions, the reverse is true, if the temperature is raised then we achieve greater product concentrations and K increases its value as T is raised. Here the reaction absorbs heat since AH 0, so if T is increased the reaction moves in the direction in which it can absorb the additional heat supplied from the rise in temperature, i.e. in the forward direction. 

Correspondingly inducing a fall in temperature produces the reverse effects to that caused by rise in temperature in each case. 

There is a general rule called Le Chatelier s principle that can often be applied to systems at equilibrium. Le Chatelier s principle states that when a system at equilibrium is stressed, the system will shift in a direction that will reduce that stress. 

There are three types of stress that usually obey Le Chatelier s principle 1) addition or removal of a product or reactant 2) changing the pressure of the system 3) heating or cooling the system. 

The Haber Process is an all gas reaction commonly used on the MCAT to test Le Chatelier s principle. The Haber Process is an exothermic reaction, so it creates heat. For Le Chltelier purposes, we can think of heat as a product in the Haber Process  

Imagine a rigid container with N2, H2, and NH gas at equilibrium, [f we add N2 gas to our system, the system attempts to compensate for the increased concentration of nitrogen by reducing the partial pressure of N2 with the forward reaction. The forward reaction uses up H2 as well, reducing its partial pressure. NH3 and heat are created by the forward reaction. 

If we raise the temperature by adding heat, the reaction is pushed to the left. The concentrations of N2 and H2 are increased, while the concentration of NH, is decreased. 

An important and interesting qualitative principle about equilibrium is the principle of Le Chatelier. This principle, which is named after the French chemist Henry Louis Le Chatelier (1850-1936), may be expressed in the following way if the conditions of a system, initially at ecjuilibrinni, are changed, the equilihrinni will shift in such a direction as to tend to restore the original conditions, if such a shift is possible. 

From the equilibrium equation 10-16 at constant temperature, we see that increasing the partial pressure of a reactant A or B causes the equi- 

Many of the products we use in everyday life are obtained from the chemical industry. Chemists and chemical engineers in industry spend a great deal of time and effort to maximize the yield of valuable products and minimize waste. For example, when Haber developed his process for making ammonia from N2 and H2, he examined how reaction conditions might be varied to increase yield. Using the values of the equilibrium constant at various temperatures, he calculated the equilibrium amounts of NH3 formed under a variety of conditions. Some of Haber s results are shown in FIGURE 15.9. 

At what combination of pressure and temperature shouid you run the reaction to maximize NH3 yieid  

FIGURE 15.9 Effect of temperataire and pressue on NH3 yield in the Haber 

Notice that the percent of NH3 present at equilibrium decreases with increasing temperature and increases with increasing pressure. 

We can understand these effects in terms of a principle first put forward by Henri-Louis Le Chatelier (1850-1936), a French industrial chemist If a system at equilibrium is disturbed by a change in temperature, pressure, or a component concentration, the system will shift its equilibrium position so as to counteract the effect of the disturbance. 

In 1888, the French chemist Henri Le Chatelier (1850-1936) set forth a simple, far-reaching generalization on the behavior of equilibrium systems. This generalization, Le Chatelier s principle known as Le Chatelier s principle, states  

Unfortunately, bacteria in plaque (resulting from not brushing) shift the equilibrium toward demineralization (shown in the figure), and a cavity can begin to form. Scientists realized that fluoride encourages remineralization in teeth by replacing hydroxyl ions in nature s calcium phosphate (hydroxyapatite). The substitution changes the hydroxyapatite to fluorapatite, which is more acid resistant. 

Several types of remineralizing chewing gums are currently available, including Trident Advantage and Trident 

If a stress is applied to a system in equilibrium, the system will respond in such a way as to relieve that stress and restore equilibrium under a new set of conditions. 

The application of Le Chatelier s principle helps us predict the effect of changing conditions in chemical reactions. We will examine the effect of changes in concentration, temperature, and volume. 

The direction of change in the position of equilibrium due to change in the external variables such as T and P may generally be found by the application of Le Chatelier s Principle which states Perturbation of a system at equilibrium will cause the equilibrium position to change in such a way as to tend to remove the perturbation. f 

For instance, if heat is evolved in a reaction (AH 0), lowering the temperature will promote more reaction because moving the equilibrium toward the product side tends to raise the temperature of the system. If a reaction proceeds with a positive volume change, then application of pressure shifts the equilibrium in the direction of the reactants, These conclusions are given quantitative expression in the equations 

Le Chatelier s Principle provides a good guide to the effects of pressure and temperature changes. To make it universally true however it would have to be stated in a more rigorous form, so it is wise to regard it as a useful guide or aide memoire rather than as a fundamental thermodynamic principle. 

Naphthalene melts at 353 K at 1 atm pressure with an enthalpy change on fusion of 19kjmol h The volume increase on fusion is 19 x 10- im3. What change in the melting temperature will be observed if the pressure is raised by 100 atm (1 atm - 105Nm 2.) 

Calculate the enthalpy change on vaporization and the entropy of vaporization. Calculate the standard Gibbs free-energy change accompanying vaporization at 

Chemical equilibria being of a dynamic type, equilibrium states are altered by changes in the variables controlling them. The effect of such changes can be interpreted qualitatively on the basis of a principle which was enunciated independently by Le Chatelier in 1885 and by Braun one year later. It states that when a system in a state of dynamic equilibrium is subjected to a stress imposed by variation in anyone of the variables controlling the equilibrium state, the system will tend to adjust itself in such a way as to minimize the effect of the stress. The variables of interest in this connection are temperature of the system, pressure on the system, and concentrations for the reactants and products taken individually. 

In order to illustrate this principle, let the effect of temperature on the equilibrium constant of an exothermic reaction, involving the oxidation of a metal to its oxides, be considered. Upon increasing the temperature of this reaction some of the metal oxides will dissociate into the metal and oxygen and thereby reduce the amount of heat released. This qualitative conclusion based on Le Chatelier s principle can be substantiated quantitatively from the Varft Hoff isochore. 

It may be added here that Le Chatelier s principle is quite general in nature, and that its applicability is not restricted only to chemical equilibria. It can also be applied to physical equilibria, as for example, to explain qualitatively the effects of temperature and pressure on solubility or the effect of pressure on the melting of a solid. 

The content provided to the present topic on chemical equilibrium may be concluded with the following summarized presentation  

In the last section you saw how concentrations and reaction rates jockey with each other until the rates become equal and equilibrium is reached. In this section we start with a system already at equilibrium and see what happens to it when the equilibrium is upset. To upset an equilibrium you must somehow make the forward and reverse reaction rates unequal, at least temporarily. One way to do this is to change the temperature. Another is to change the concentration of at least one substance in the system. A gaseous equilibrium can often be upset simply by changing the volume of the container. 

How an equilibrium responds to a distmbance can be predicted from the concentration and temperatme effects already considered. The predictions may be summarized in Le Chatelier s Principle, which says that if an equilibrium system is subjected to change, processes occur that tend to partially counteract the initial change, thereby bringing the system to a new position of equilibrium. 

We will now see how Le Chatelier s Principle explains three different equilibrium changes. 

8 Given the equation for a chemical equilibrium, predict the direction in which the equilibrium will shift because of a change in the concentration of one species. 

The reaction of hydrogen and iodine to produce hydrogen iodide comes to equilibrium with hydrogen iodide as the favored species, that is, the species having the higher concentration. The equal forward and reverse reaction rates are shown by the equal-length arrows in the equation 

The shift of Keq with P change can therefore be predicted from the sign of AV if reaction volume / creases (Angas 0), then a P increase shifts Keq toward the reactant side, whereas if reaction volume decreases (Angas 0), a P increase will promote product formation. These inferences are consistent with the expectations of Le Chatelier s principle. 

In 1884, French chemist Henri Louis Le Chatelier enunciated a qualitative principle of great insight and generality pertaining to the responses of chemical equilibrium when subjected 

It is fortunate that there is a general qualitative principle, called Le Chatelier s principle, that relates to all the applications of the principles of chemical equilibrium. .. Some years after you have finished your college work, you may (unless you become a chemist or work in some closely related field) have forgotten all the mathematical equations relating to chemical equilibrium. I hope, however, that you will not have forgotten Le Chatelier s principle. 

Le Chatelier s principle has been stated in many forms, some excessively vague or anthropomorphic and subj ect to misuse. For our present purposes, we adopt the following statement  

Le Chatelier s principle When a variable affecting the position of equilibrium is changed, the system re-equilibrates in the direction tending to counteract the change. 

As mentioned for the Haber process, temperature and pressure can help control the position of an equilibrium, the amount of product and reactant present at equilibrium, and thus the equilibrium constant. Temperature and pressure are parameters that, when varied, create a stress on the equilibrium. Stresses on equilibrium systems cause changes in the system. The nature of these changes are described by Le Chatelier s principle. 

Le Chatelier s Principle can be expressed as follows when a stress is placed on a system at equilibrium, the equilibrium shifts to partially relieve the stress. 

Here we have an equilibrium with a position lying a little to the left. 

Imagine applying a stress that favors the products. 

A momentary surge to the right occurs as a result of the stress. SURGE 

Approximations can simplify complicated math, but their validity should be carefully checked. 

Making this approximation allows us to simplify the equilibrium expression  

A good way to assess whether a 4% error is acceptable here is to examine the precision of the data given. For example, note that the value of K is 1.6 x 10 5, which can be interpreted as (1.6 0.1) x 10-5. Thus the uncertainty in K is at least 1 part in 16, or about 6%. Therefore, a 4% error in [NOCl] is acceptable. 

The difference between 0.50 and 0.48 is 0.02, or 4% of the initial concentration of NOCl, a relatively small discrepancy that will have little effect on tjie outcome. That is, since lx is very small compared with 0.50, the value of x obtained in the approximate solution should be very close to the exact value. We use this approximate value of x to calculate the equilibrium concentrations  

Since the given value of K is 1.6 X 10 5, these calculated concentrations are correct. 

If a substance is added to a system at equilibrium, the system reacts to consume some of the substance. If a substance is removed from a system, the system reacts to produce more of substance. 

In this chapter the values of G and K appear without a source being shown. The basis for computing these values is shown in Chapter 12 and Appendix F. 

The fundamental criterion for all kinds of physical and chemical equihbrium is that at constant T and P, dGsys = 0 for any infinitesimal change. The constant T and P restriction does not prevent this from being almost universally applicable. 

For finite changes away from the equihbrium state, in any direction, dG ys 0. Thus, the equilibrium state is one of minimum Gibbs energy, subject to the external constraints. 

For two or more phases in equilibrium there is a separate equilibrium relationship for each of the chemical species present, and that relationship is that the partial molar Gibbs energy, called the chemical potential, for species i is the same in all of the phases (1, 2,. ..). The same statement applies to species j, k, and so on. 

A system is at chemical equilibrium when it has adjusted its chemical compositions so that the overall Gibbs energy of the system is the minimum possible, subject to the external constraints (T, P, initial chemical composition, etc.). The shorthand way of showing this is the law of mass action or chemical equilibrium constant. The relation between the two is explored in Chapter 12. 

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Le Chateliers principle (Section 6 10) A reaction at equilib rium responds to any stress imposed on it by shifting the equihbnum in the direction that minimizes the stress Lewis acid See acid Lewis base See base... 

Le Chateliers principle can be used to predict the effect of a change in temperature on the position of an equilibrium. In general, an increase in temperature causes Ihe endothermic reaction to occur. This absorbs heat and so tends to reduce the temperature of the system, partially compensating for the original temperature increase. 

We saw in Chapter 13 that under these conditions, it is usually a good approximation to take [HB] = [HB]0. The approximation is even more accurate when a considerable amount of B-is added, as is the case with a buffer. By Le Chateliers principle, the reverse reaction occurs, the ionization of HB is repressed, and [HB] = [HB]0. 

Le Chatelier principle / principio de Le Chatelier establece que si un sistema en equihbrio es perturbado, el sistema cambia en el senhdo que le permita eliminar la perturbacidn. (pag. 569)... 

Le Chatelier principle concerns the conservation of energy or matter. There are corresponding laws in several other areas of science. 

Begin by considering how an equilibrium system adjusts to a change in the concentration of any substance. At equilibrium, the concentrations of all substances are fixed, and their ratio yields the equilibrium constant. Le Chateliers principle tells you that changing the concentration of a substance causes the system to adjust to minimize the change in that substance. The decomposition of carbonyl bromide provides an illustration ... 

Please realize that the effect of temperature on the equilibrium constant depends on which of the two opposing reactions is exothermic and on which is endothermic. You must have information on the heat of a reaction before you can apply Le Chateliers principle to judge how temperature alters the equilibrium. 

We may, thus, formulate a qualitative rule (which looks like a modification of the Le-Chatelier principle) ... 

The stability constant of a complex is temperature dependent—increased temperature generally leads to increased dissociation of the complex. Qualitatively, this can be explained by the Le Chatelier principle, which states that if there is a change in a reaction parameter, the reaction will proceed in a direction that opposes that change. Thus an increase in temperature will cause the reaction to go in the direction in which heat is absorbed, which is dissociation of the complex. More quantitatively, the relation between equilibrium constant and temperature is given approximately by... 

Since the reaction is reversible, yields are greater in ether than in alcohol because alcohol is a product (Le Chatelier principle). 

Le Chatelier principle If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce the effect of that change. 

From available, though approximate, estimates, about 1023 g of carbon-containing gases are concentrated in the rocks of the Earth s crust and mantle (lithosphere) (Korstenshtein, 1984 Sokolov, 1971). This mass of carbon exceeds by approximately 104 times the amount present today in the biosphere (over the Earth surface). Between the biosphere and lithosphere there is a constant, very intensive exchange of carbon that is self-regulatory. From the data of Barenbaum (2000, 2002), due to the Le Chatelier principle (Krapivin et al., 1982), the content of mobile carbon in the system tries to attain a stable relationship ... 

The second term is always positive because as the system is heated, the acid dissociations shift in such a way as to absorb heat, as predicted by the Le Chatelier principle. 

As would be expected from the Le Chatelier principle, the use of reduced pressure gave improved yields. n-Octyl alcohol at 125 to 135 mm. pressure yielded 73.9% di-n-heptyl ketone compared with 56.0% conversion obtained at atmospheric pressure. 

The Law of Chemical Equilibrium is based on the constancy of the equilibrium constant. This means that if one disturbs the equilibrium, for example by adding more reactant molecules, there will be an increase in the number of product molecules in order to maintain the producl/reactant ratio unchanged and thus preserving the numerical value of the equilibrium constant. The Le Chatelier Principle expresses this as follows If an external stress is applied to a system in equilibrium, the system reacts in such a way as to partially relieve the stress. In our present experiment, we demonstrate the Le Chatelier Principle in two manners (a) disturbing the equilibrium by changing the concentration of a product or reactant (b) changing the temperature. 

If we add a few drops of aqueous HC1 to the solution, we will have added a common ion, H30+, that already was present in the equilibrium mixture. We expect, on the basis of the Le Chatelier Principle, that the equilibrium will shift to the left. Thus the solution will not become acidic. 

Since it is highly endothermic (DH = + 234 kJ/mol) so the equilibrium can be shifted in favour of products at higher temperature (according to be Le Chateliers principle). In a blast furnace the reaction above 1000° C, heat being provided by combustion of carbon in air, which is blown through the reaction mixture. 

The reaction is reversible, exothermic and proceeds with a decrease in volume. According to Le Chatelier principle, favourable conditions for maximum yield of ammonia are ... 

A second and important consideration in selecting the derivative is to choose the one with the highest chemical potential or affinity of reaction for the particular system. The Le Chatelier principle postulates that a system under the influence of unbalanced forces will shift its equilibrium in such a direction as to minimize the unbalanced forces. This is the case with a spray solution on a plant surface. If the solution... 

This inequality is identical to the Le Chatelier principle for nonequilibrium stationary states, since the disturbance 8Xk has the same sign as the flow Jk, indicating a decrease in the disturbance. For example, an increase in the gradient of the chemical potential will cause the mass flow and diminish the gradient. Hence, the stationary state will return to its original status. 

Using the Le Chatelier principle, one may calculate the lower (L) and upper (U) concentration limits of the ignition of polymer decomposition products in any oxidative medium... 

There is no violation of the Le Chatelier principle, which applies to equilibria, but not to rates. Equilibrium conversion is indeed higher at higher pressure, but is close to 100% anyway even at lower pressure. 

The rate of the forward reaction in (i.e. the number of C molecules formed per unit of time) is favored by a small volume since the probability of reactants A and B meeting is higher. However, the backward reaction is determined by the rate of dissociation of C, and is independent of volume. According to the Van t-Hoff-Le Chatelier principle, it is thus possible to shift the equilibrium concentrations of A, B, and C by changing the container volume. The formation of C is favored by a small volume, and vice versa. If the volume increase or decrease is made at a constant rate, dV/dt — fey, the concentration of A, B, and C change over time as ... 

Quantum Chemical le Chatelier Principle for Molecular Shapes (QCLCP-MS) the shape change due to relaxation tends to reduce the effect of the initial vertical shape change. 

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