Chemistry
Application of Le Chatelier's Principle
Le Chatelier's Principle is a concept in chemistry that predicts how a system at equilibrium will respond to changes in concentration, pressure, or temperature. It states that if a system at equilibrium is disturbed, the system will adjust to counteract the disturbance and restore equilibrium. This principle is widely used to understand and predict the behavior of chemical reactions in various applications.
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6 Key excerpts on "Application of Le Chatelier's Principle"
eBook - PDFPhysical Chemistry
Thermodynamics
- Horia Metiu(Author)
- 2006(Publication Date)
- Taylor & Francis(Publisher)
For example, for the reaction N 2 + 3H 2 → 2NH 3 I have V 0 = 2 v 0 (NH 3 ) − v 0 (N 2 ) − 3 v 0 (H 2 ). This is the volume of the products minus the volume of the reactants. Le Chatelier’s Principle §4. The Formulation of the Principle. Imagine that you have performed a reac-tion, at a fixed temperature and pressure, and that it reached equilibrium. The equilibrium composition depends on T and p . If I change T or p , the composition will change. In 1884, Le Chatelier formulated a principle governing such changes. 412 Dependence of Equilibrium Constant on T and p He said that if you have a system in equilibrium and you make a change in the con-ditions (i.e., temperature or pressure), the composition will shift in a direction that diminishes the change you are making. This is a most sophisticated formulation of Murphy’s Law: no matter what you want to do, the system will try to make you fail. This may seem a bit vague, but consider how Le Chatelier formulated it, at the time of its discovery: Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its con-densation (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 a change of temperature or of condensation of opposite sign to that resulting from the external cause. You can see that there is very little correlation between high intelligence and clear writing. I will clarify Le Chatelier’s principle by giving a few examples. Consider a reaction A B at equilibrium. The reaction is such that if the composition changes to make more B, then heat is produced (A → B is exothermic); if it makes more A, then heat is absorbed (B → A is endothermic). Now try to raise the temperature, by giving heat to the system. How can the system frustrate me? If the reaction consumes B to make A, it will absorb heat.
- Morris Hein, Scott Pattison, Susan Arena, Leo R. Best(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
In 1888, the French chemist Henri Le Châtelier (1850–1936) set forth a simple, far-reaching generalization on the behavior of equilibrium systems. This generalization, known as Le Châtelier’s principle, states: KEY TERMS Le Châtelier’s principle catalyst activation energy E X A M P L E 1 6 . 2 Is a reversible chemical reaction at equilibrium a static or a dynamic system? Explain. SOLUTION A reversible chemical reaction is a dynamic system in which two opposing reactions are taking place at the same time and at the same rate of reaction. LEARNING OBJECTIVE 16.3 • Le Châtelier’s Principle 367 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 Châtelier’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. New Ways in Fighting Cavities and Avoiding the Drill Dentists have understood for more than 20 years what causes cavities, but, until now, there have been only a limited number of over-the-counter products to help us avoid our dates with the drill. Bacteria in the mouth break down sugars remaining in the mouth after eating. Acids produced during this process slip through tooth enamel, dissolving minerals below the surface in a process called demineralization. Saliva works to rebuild teeth by adding calcium and phosphate back in a process called reminer- alization. Under ideal conditions (assuming that you brush after eating), these two processes form an equilibrium. Unfortunately, bacteria in plaque (resulting from not brush- ing) 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).
eBook - PDF- William R. Robinson, Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
If, however, we put a stress on the system by cooling the mixture (withdrawing energy), the equilibrium shifts to the left to supply some of the energy lost by cooling. The concentration of colorless N 2 O 4 increases, and the concentration of brown NO 2 decreases, causing the brown color to fade. This interactive animation (http://openstaxcollege.org/l/16chatelier) allows you to apply Le Châtelier's principle to predict the effects of changes in concentration, pressure, and temperature on reactant and product concentrations. 13.4 Equilibrium Calculations By the end of this section, you will be able to: • Write equations representing changes in concentration and pressure for chemical species in equilibrium systems • Use algebra to perform various types of equilibrium calculations • Explain how temperature affects the spontaneity of some proceses • Relate standard free energy changes to equilibrium constants We know that at equilibrium, the value of the reaction quotient of any reaction is equal to its equilibrium constant. Thus, we can use the mathematical expression for Q to determine a number of quantities associated with a reaction at equilibrium or approaching equilibrium. While we have learned to identify in which direction a reaction will shift Link to Learning Chapter 13 | Fundamental Equilibrium Concepts 695 to reach equilibrium, we want to extend that understanding to quantitative calculations. We do so by evaluating the ways that the concentrations of products and reactants change as a reaction approaches equilibrium, keeping in mind the stoichiometric ratios of the reaction. This algebraic approach to equilibrium calculations will be explored in this section. Changes in concentrations or pressures of reactants and products occur as a reaction system approaches equilibrium. In this section we will see that we can relate these changes to each other using the coefficients in the balanced chemical equation describing the system.
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2015(Publication Date)
- Openstax(Publisher)
In accordance with Le Châtelier's principle, a shift in the equilibrium that reduces the total number of molecules per unit of volume will be favored because this relieves the stress. The reverse reaction would be favored by a decrease in pressure. Consider what happens when we increase the pressure on a system in which NO, O 2 , and NO 2 are at equilibrium: 2NO(g) + O 2 (g) ⇌ 2NO 2 (g) The formation of additional amounts of NO 2 decreases the total number of molecules in the system because each time two molecules of NO 2 form, a total of three molecules of NO and O 2 are consumed. This reduces the total pressure exerted by the system and reduces, but does not completely relieve, the stress of the increased pressure. On the other hand, a decrease in the pressure on the system favors decomposition of NO 2 into NO and O 2 , which tends to restore the pressure. Now consider this reaction: N 2 (g) + O 2 (g) ⇌ 2NO(g) Because there is no change in the total number of molecules in the system during reaction, a change in pressure does not favor either formation or decomposition of gaseous nitrogen monoxide. Effect of Change in Temperature on Equilibrium Changing concentration or pressure perturbs an equilibrium because the reaction quotient is shifted away from the equilibrium value. Changing the temperature of a system at equilibrium has a different effect: A change in temperature actually changes the value of the equilibrium constant. However, we can qualitatively predict the effect of the temperature change by treating it as a stress on the system and applying Le Châtelier's principle. Link to Learning 732 Chapter 13 | Fundamental Equilibrium Concepts This OpenStax book is available for free at http://cnx.org/content/col11760/1.9 When hydrogen reacts with gaseous iodine, heat is evolved. H 2 (g) + I 2 (g) ⇌ 2HI(g) ΔH = −9.4 kJ (exothermic) Because this reaction is exothermic, we can write it with heat as a product.
eBook - PDFChemistry
An Atoms First Approach
- Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste, , Steven Zumdahl, Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Thus the amount of NH 3 present at equilibrium is favored by conditions of low temperature and high pressure. 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. We can qualitatively predict the effects of changes in concentration, pressure, and temperature on a system at equilibrium by using Le Châtelier’s principle, which states that if a change is imposed on a system at equilibrium, the posi- tion of the equilibrium will shift in a direction that tends to reduce that change. Although this rule sometimes oversimplifies the situation, it works re- markably well. The Effect of a Change in Concentration To see how we can predict the effect of change in concentration on a system at equi- librium, we will consider the ammonia synthesis reaction. Suppose there is an equilib- rium position described by these concentrations: fN 2 g 5 0.399 M fH 2 g 5 1.197 M fNH 3 g 5 0.202 M TABLE 12.2 The Percent by Mass of NH 3 at Equilibrium in a Mixture of N 2 , H 2 , and NH 3 as a Function of Temperature and Total Pressure* Total Pressure Temperature (8C) 300 atm 400 atm 500 atm 400 48% NH 3 55% NH 3 61% NH 3 500 26% NH 3 32% NH 3 38% NH 3 600 13% NH 3 17% NH 3 21% NH 3 *Each experiment was begun with a 3;1 mixture of H 2 and N 2 . ▲ Henri Louis Le Châtelier (1850–1936), the French physical chemist and metallurgist, seen here while a student at the École Polytechnique. SPL/Science Source 514 CHAPTER 12 Chemical Equilibrium Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
eBook - ePub- Bruce H. Mahan(Author)
- 2013(Publication Date)
- Dover Publications(Publisher)
IVApplications of the Thermodynamic Principles
IN THIS CHAPTER we shall demonstrate the application of thermodynamics to some simple practical problems of interest to chemists. Our purpose is to increase understanding of the thermodynamic principles and to show how thermodynamics helps us to understand chemical phenomena.4-1 PHASE EQUILIBRIA
Vapor Pressure
As our first application, we shall consider the temperature dependence of the vapor pressure of a condensed material, such as liquid water.H 2 0(l ) = H 2 0(g ) ΔH °298 = 10,510 cal/moleGenerally, we refer to this, and any similar process, as a physical change, but it is equally valid to think of it as the simplest possible of chemical reactions. With this in mind we can write an equilibrium constant for the reaction:IIIIf we agree for the present to work with pure liquid water, then the concentration of liquid water is a constant and we can define a new equilibrium constant by the equationK = K ′ [H 2 0(l )] = [H 2 0(g )]This will be true as long as the water contains no dissolved solutes. Since the concentration of material in the gas phase is conveniently expressed in pressure units, we could better write the equilibrium constant asK = P vapThat is, the equilibrium constant for the reaction is the vapor pressure of water. A similar expression holds for the equilibrium between any liquid or solid and its vapor.The relation between the vapor pressure of water and the standard free energy change ΔG ° of evaporation isΔG °.= −RT ln K = −RT ln P vapWe could evaluate the standard free energy change for the reaction from the standard entropies of liquid and gaseous water and the enthalpy of vaporization. However, in this case we shall reverse this procedure and obtain ΔG ° from a direct measurement of the vapor pressure. At 298°K it is found thatThis value of ΔG
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