Le Chatelier's Principle

9 Key excerpts on "Le Chatelier's Principle"

• 

  eBook - ePub

  General Chemistry for Engineers

  • Jeffrey Gaffney, Nancy Marley(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  This is known as Le Chatelier's Principle, after the nineteenth century chemist Henry-Louis Le Chatelier, who studied the effects of changes in reaction conditions on a chemical equilibrium. The exact statement of Le Chatelier's Principle is; • When a chemical reaction at equilibrium is subjected to a change in reaction conditions, the position of the equilibrium will shift to counteract the effect of the change until a new equilibrium is established. The ways that a chemical reaction at equilibrium will respond when disturbed by a change in the original conditions can be predicted. These predicted responses and their effects on the equilibrium constant are summarized in Table 7.1 for each of the changing conditions. If the concentration of a reactant or product in a reversible chemical reaction at equilibrium is changed, by the addition or the removal of either a reactant or a product, the equilibrium will be reestablished by adjusting the concentrations of reactants and products so that their ratio and the value of K eq will remain the same. For example, if the concentration of reactant A in the generic Eq. (3) of Section 7.1 is increased by the addition of A to the reaction system, the reaction will shift to consume the added reactant A and form more of the products C and D. This increases the value of the numerator of K eq and decreases the value of the denominator compensating for the increase in the value of the denominator caused by the added reactant. This response can be useful in an industrial process if it is necessary to convert the maximum possible amount of reactant B into the products C and D when reactant B is an expensive or rare material and reactant A is inexpensive and abundant. In this case, the addition of excess reactant A becomes a means of conserving the more expensive reactant B while producing as much product as possible

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Physical 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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry

  • John A. Olmsted, Gregory M. Williams, Robert C. Burk(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  Le Châtelier’s principle is a com- pact summary of how different factors influence equilibrium. Introducing a reagent causes a reaction to proceed in the direction that consumes the reagent. Reducing the temperature removes heat from the system and causes the reaction to produce heat by proceeding in the exothermic direction. Key Concept When a change is imposed on a system at equilibrium, the system will react in the direction that reduces the amount of change (Le Châtelier’s principle). Changes in Amounts of Reagents A change in the amount of any substance that appears in the reaction quotient displaces the system from its equilibrium position. As an example, consider an industrial reactor containing a mixture of methane, hydrogen, steam, and carbon monoxide at equilibrium: CH 4 ( g) + H 2 O( g) ⟶ ⟵ CO( g) + 3 H 2 ( g)  = ( p CO )( p H 2 ) 3 ___________ ( p CH 4 )( p H 2 O ) How does this system respond if more steam is injected into the reactor? Adding one of the starting materials decreases the value of , making  < K eq . Because the system responds in a way that restores equilibrium, adding steam causes the reaction to proceed to the right, consuming CH 4 and H 2 O and producing CO and H 2 until equilibrium is restored. Figure 14.6 provides a graphical illustration of how concentrations change when steam is added. Removing a product from a system at equilibrium also makes  < K eq and leads to the formation of additional products. This behaviour is commonly used to advantage in chemical synthesis. For example, calcium oxide (lime), an important material in the con- struction industry, is made by heating calcium carbonate in a furnace to about 1100 K: CaCO 3 (s) ⟶ ⟵ CaO(s) + CO 2 ( g) K eq = ( p CO 2 ) eq = 1.0 at 1100 K If the pressure of CO 2 in the furnace were to reach 1.0 bar, the system would attain equilibrium, and no additional products would form.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to General, Organic, and Biochemistry

  • 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).

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry: Atoms First

  • 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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry

  • Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
  • 2015(Publication Date)
  • Openstax

    (Publisher)

  Since this stress affects the concentrations of the reactants and the products, the value of Q will no longer equal the value of K. To re-establish equilibrium, the system will either shift toward the products (if Q < K) or the reactants (if Q > K) until Q returns to the same value as K. 730 Chapter 13 | Fundamental Equilibrium Concepts This OpenStax book is available for free at http://cnx.org/content/col11760/1.9 This process is described by Le Châtelier's principle: When a chemical system at equilibrium is disturbed, it returns to equilibrium by counteracting the disturbance. As described in the previous paragraph, the disturbance causes a change in Q; the reaction will shift to re-establish Q = K. Predicting the Direction of a Reversible Reaction Le Châtelier's principle can be used to predict changes in equilibrium concentrations when a system that is at equilibrium is subjected to a stress. However, if we have a mixture of reactants and products that have not yet reached equilibrium, the changes necessary to reach equilibrium may not be so obvious. In such a case, we can compare the values of Q and K for the system to predict the changes. Effect of Change in Concentration on Equilibrium A chemical system at equilibrium can be temporarily shifted out of equilibrium by adding or removing one or more of the reactants or products. The concentrations of both reactants and products then undergo additional changes to return the system to equilibrium. The stress on the system in Figure 13.8 is the reduction of the equilibrium concentration of SCN − (lowering the concentration of one of the reactants would cause Q to be larger than K). As a consequence, Le Châtelier's principle leads us to predict that the concentration of Fe(SCN) 2+ should decrease, increasing the concentration of SCN − part way back to its original concentration, and increasing the concentration of Fe 3+ above its initial equilibrium concentration. Figure 13.8 (a) The test tube contains 0.1 M Fe 3+ .

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry 2e

  • Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
  • 2019(Publication Date)
  • Openstax

    (Publisher)

  Chemical equilibria are dynamic processes characterized by constant amounts of reactant and product species. 13.2 Equilibrium Constants The composition of a reaction mixture may be represented by a mathematical function known as the reaction quotient, Q. For a reaction at equilibrium, the composition is constant, and Q is called the equilibrium constant, K. A homogeneous equilibrium is an equilibrium in which all components are in the same phase. A heterogeneous equilibrium is an equilibrium in which components are in two or more phases. 13.3 Shifting Equilibria: Le Châtelier’s Principle Systems at equilibrium can be disturbed by changes to temperature, concentration, and, in some cases, volume and pressure. The system’s response to these disturbances is described by Le Châtelier’s principle: An equilibrium system subjected to a disturbance will shift in a way that counters the disturbance and re-establishes equilibrium. A catalyst will increase the rate of both the forward and reverse reactions of a reversible process, increasing the rate at which equilibrium is reached but not altering the equilibrium mixture’s composition (K does not change). 13.4 Equilibrium Calculations Calculating values for equilibrium constants and/or equilibrium concentrations is of practical benefit to many applications. A mathematical strategy that uses initial concentrations, changes in concentrations, and equilibrium concentrations (and goes by the acronym ICE) is useful for several types of equilibrium calculations. 13 • Key Terms 683 Exercises 13.1 Chemical Equilibria 1. What does it mean to describe a reaction as “reversible”? 2. When writing an equation, how is a reversible reaction distinguished from a nonreversible reaction? 3. If a reaction is reversible, when can it be said to have reached equilibrium? 4. Is a system at equilibrium if the rate constants of the forward and reverse reactions are equal? 5.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry

  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)

  This reduces the effect of the addition. Thus Le Châtelier’s principle correctly predicts that adding nitrogen will cause the equilibrium to shift to the right (Fig. 12.9). If ammonia had been added instead of nitrogen, the system would have shifted to the left to consume ammonia. So another way of stating Le Châtelier’s principle is to say that if a component (reactant or product) is added to a reaction system at equi- librium (at constant T and P or constant T and V ), the equilibrium position will shift in the direction that lowers the concentration of that component. If a compo- nent is removed, the opposite effect occurs. The system shifts in the direction that compensates for the imposed change. 515 12.7 Le Châtelier’s Principle Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Using Le Châtelier’s Principle I Arsenic can be extracted from its ores by first reacting the ore with oxygen (called roasting) to form solid As 4 O 6 , which is then reduced using carbon: As 4 O 6 ssd 1 6Cssd m As 4 sgd 1 6COsgd Predict the direction of the shift of the equilibrium position in response to each of the following changes in conditions. a. Addition of carbon monoxide b. Addition or removal of carbon or tetraarsenic hexoxide (As 4 O 6 ) c. Removal of gaseous arsenic (As 4 ) SOLUTION a. Le Châtelier’s principle predicts that the shift will be away from the substance whose concentration is increased. The equilibrium position will shift to the left when carbon monoxide is added.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Basic Physical Chemistry

  The Route to Understanding

  • E Brian Smith(Author)
  • 2012(Publication Date)
  • ICP

    (Publisher)

  This formulation is particularly helpful when we move on from our study of mechanical systems to gain an understanding of physical chemical systems. 7.2 Why we need a second principle Understanding equilibrium in mechanical systems is quite straightforward and appears to involve little more than common sense. Unfortunately, this is not the case in the world of physicochemical phenomena. When the block in the dish illustrated in Fig. 7.1 moves to its equilibrium position, where its potential energy is at a minimum, the excess potential energy may be given out as heat (unless the block is harnessed to lift a weight and do work). If chemical processes behaved in a similar manner, we would expect them to liberate heat as they approached equilibrium, which would cause the system to warm up (again assuming they do no work). If we add some solid sodium hydroxide to water, this is indeed what occurs. The equilibrium position, a solution of sodium hydroxide in water, has lower energy, and energy in the form of heat is liberated (Fig. 7.3). Such observations led scientists in the 19th century to suggest, erroneously, that all spontaneous chemical reactions should be accompanied by the evolution of heat (that is, are exothermic ). However, this is not always the case. If we add solid sodium nitrate to water, we find that heat is absorbed and the system cools down (the process is endothermic ). This system ‘climbs uphill’ on the energy scale to reach its position of equilibrium (Fig. 7.3). This tells us that, in this system, the tendency to minimise energy cannot be the sole factor determining the position of equilibrium. Let us consider two systems whose energy is constant and where energy can play no part in determining the position of equilibrium. (i) Expansion of a gas . Consider a system in which a gas is confined to one of two bulbs connected by a stopcock, the other bulb being evacuated (Fig. 7.4).

  Sign up to read

  Learn more about book