Chemistry
Reaction Quotient and Le Chatelier's Principle
The reaction quotient is a measure of the relative concentrations of products and reactants at any point during a chemical reaction. Le Chatelier's Principle states that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift its position to counteract the disturbance and restore a new equilibrium.
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12 Key excerpts on "Reaction Quotient and Le Chatelier's Principle"
eBook - ePub- 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
eBook - PDF- H. Donald Brooke Jenkins(Author)
- 2008(Publication Date)
- Wiley-Blackwell(Publisher)
In a reaction proceeding in the forward direction, reactant concentrations will be falling at the same time that the product concentrations are rising, hence Q , the reaction quotient, will be increasing until it reaches a value = K when the reaction will be at equilibrium. Conversely, in a reaction proceeding in the backward direction, reactant concentrations will be rising at the same time that the product concentrations are falling, and hence Q , the reaction quotient, will be decreasing until it again reaches the appropriate value of K , when the reaction will be at equilibrium and will then cease, under the current conditions of temperature and pressure. 49.2 Le Chatelier’s Principle Using the equilibrium constant and v’ant Hoff equations (Frames 46 and 47) it is possible to determine quantitatively the effect of pressure and temperature on the position of equilibrium taken up by a reversible reaction (see Section 49.4, this frame). Similar conclusions can be reached qualitatively using the principle of “ mobile equilibrium ” originally developed independently by Le Chatelier and by Braun, but now referred to simply as Le Chatelier’s principle. Le Chatelier proposed a principle that is useful in predicting qualitatively the likely effect that a ‘ constraint ’ placed on a reaction, at equilibrium, will have on the concentrations of the products and reactants. The constraint could be brought about by a change in the overall temperature, T , at which the reaction is being carried out or by alteration of the external pressure, P , for the process. According to Le Chatelier’s Principle : If a constraint is applied to a system at equilibrium then the system will tend to adjust the position of equilibrium so as to oppose (or tend to nullify) the effects of this constraint. 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.
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
13.3 • Shifting Equilibria: Le Châtelier’s Principle 669 Reaction rates are affected primarily by concentrations, as described by the reaction’s rate law, and temperature, as described by the Arrhenius equation. Consequently, changes in concentration and temperature are the two stresses that can shift an equilibrium. Effect of a Change in Concentration If an equilibrium system is subjected to a change in the concentration of a reactant or product species, the rate of either the forward or the reverse reaction will change. As an example, consider the equilibrium reaction The rate laws for the forward and reverse reactions are When this system is at equilibrium, the forward and reverse reaction rates are equal. If the system is stressed by adding reactant, either H 2 or I 2 , the resulting increase in concentration causes the rate of the forward reaction to increase, exceeding that of the reverse reaction: The system will experience a temporary net reaction in the forward direction to re-establish equilibrium (the equilibrium will shift right). This same shift will result if some product HI is removed from the system, which decreases the rate of the reverse reaction, again resulting in the same imbalance in rates. The same logic can be used to explain the left shift that results from either removing reactant or adding product to an equilibrium system. These stresses both result in an increased rate for the reverse reaction and a temporary net reaction in the reverse direction to re-establish equilibrium. As an alternative to this kinetic interpretation, the effect of changes in concentration on equilibria can be rationalized in terms of reaction quotients. When the system is at equilibrium, If reactant is added (increasing the denominator of the reaction quotient) or product is removed (decreasing the numerator), then Q c < K c and the equilibrium will shift right.
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.
eBook - ePub- Patrick E. McMahon, Rosemary McMahon, Bohdan Khomtchouk(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
In this case, the concentrations of all reactants and products must change in order to re-establish the required equilibrium ratios. The outside changes may be an addition or removal of some of the moles of a reactant or product; this will then change the concentration of the compound added or removed and, thus, temporarily will change the [product] to [reactant] ratio. The outside change may be an addition (temperature increase) or removal (temperature decrease) of some heat. Heat can be considered a product or reactant based on the sign of the enthalpy (ΔH). A change in temperature results in the change of the heat product or reactant and will also produce a temporary change in the actual concentrations described by the [product] to [reactant] ratio. Le Chatelier’s principle states that if an additional outside change is induced on an established equilibrium reaction, the reaction will respond by attempting to counteract the outside change. (1) If reactants are added to or products are removed from an equilibrium, the forward reaction increases its rate to convert some of the reactant molecules to product molecules; the result is to remove some of the extra reactant molecules or replace some of the depleted product molecules. This response decreases the concentration of the reactants, increases the concentration of product molecules, and, thus, reestablishes the original required equilibrium ratio. If the rate of the forward reaction must increase, the reaction is said to shift to the right or to shift toward products. (2) If reactants are removed from or products are added to an equilibrium, the reverse reaction increases its rate to convert some of the product molecules to reactant molecules; the result is to remove some of the extra product molecules or replace some of the depleted reactant molecules
eBook - PDF- Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
13.3 Shifting Equilibria: Le Châtelier’s Principle LEARNING OBJECTIVES By the end of this section, you will be able to: • Describe the ways in which an equilibrium system can be stressed • Predict the response of a stressed equilibrium using Le Châtelier’s principle A system at equilibrium is in a state of dynamic balance, with forward and reverse reactions taking place at equal rates. If an equilibrium system is subjected to a change in conditions that affects these reaction rates differently (a stress), then the rates are no longer equal and the system is not at equilibrium. The system will subsequently experience a net reaction in the direction of greater rate (a shift) that will re-establish the equilibrium. This phenomenon is summarized by Le Châtelier’s principle: if an equilibrium system is stressed, the system will experience a shift in response to the stress that re-establishes equilibrium. Reaction rates are affected primarily by concentrations, as described by the reaction’s rate law, and temperature, as described by the Arrhenius equation. Consequently, changes in concentration and temperature are the two stresses that can shift an equilibrium. Effect of a Change in Concentration If an equilibrium system is subjected to a change in the concentration of a reactant or product species, the rate of either the forward or the reverse reaction will change. As an example, consider the equilibrium reaction The rate laws for the forward and reverse reactions are When this system is at equilibrium, the forward and reverse reaction rates are equal. If the system is stressed by adding reactant, either H 2 or I 2 , the resulting increase in concentration causes the rate of the forward reaction to increase, exceeding that of the reverse reaction: The system will experience a temporary net reaction in the forward direction to re-establish equilibrium (the equilibrium will shift right).
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)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. N ow that you have completed the chapter, let’s review the concepts of equilibrium condition, reaction quotient, and equilibrium constant in the context of understanding Le Châtelier’s principle. Consider the reaction + + where and represent two different types of atoms. Suppose we mix the two types of molecules together as seen in the left box, and the equilibrium mixture, as seen in the right box, results. Since there is a 1;1;1;1 mole ratio in the balanced equation, we can calculate the equilibrium constant by finding the ratio of products to reactants. Doing so, we get (N (1)(1) )(N ) (N (3)(3) ) (N ) = = 0.111 Suppose we then add 16 more molecules of to the mixture at equilibrium. Initially, the mixture will look like this: We can see this reaction is not at equilibrium because the reaction quotient is now: (N (1)(1) )(N ) (N (19)(3) ) (N ) = = 0.0175 Note that the value of the reaction quotient is 0.0175, which is less than the equilibrium constant. This means that the reac- tion should proceed to the right to reach equilibrium. This matches our prediction using Le Châtelier’s principle since we have disturbed the equilibrium system by increasing the con- centration of one of the reactants. That is, the system will respond by shifting equilibrium “to the right” (product side). We can see the new equilibrium system here: In this case, the ratio of products to reactants is (N (2)(2) )(N ) (N (18)(2) ) (N ) = = 0.111 And thus, the system is at equilibrium. The equilibrium condi- tion is different from the first case, but the equilibrium con- stant is the same. CONNECTING TO ATOMS 12.1 The Difference Between Equilibrium Constant and Equilibrium Condition 521 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.
eBook - PDF- 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+ .
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.
- 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 - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
For a given overall chemical composition, the amounts of reactants and products that are present at equilibrium are the same regardless of whether the equilibrium is approached from the direction of pure “reactants,” pure “products,” or any mixture of them. Explain the basics of equilibrium laws The mass action expression is a fraction. The concentrations of the products, raised to powers equal to their coefficients in the chemical equation for a homogeneous equilibrium, are multi- plied together in the numerator. The denominator is con- structed in the same way from the concentrations of the reactants raised to powers equal to their coefficients. The numerical value of the mass action expression is the reaction quotient, Q. At equilibrium, the reaction quotient is equal to the equilibrium constant, K c . If partial pressures of gases are used in the mass action expression, is represented as K P . The magnitude of the equilibrium constant is roughly proportional to the extent to which the reaction proceeds to completion when equilibrium is reached. Equilibrium equations can be manipulated by multiplying the coefficients by a common fac- tor, changing the direction of the reaction, and by adding two or more equilibria. The rules given in the description of the Tools for Problem Solving below apply. Write and convert between equilibrium laws based on molar concentration and gas pressures The values of K P and K c are only equal if the same number of moles of gas are represented on both sides of the chemical equa- tion. When the numbers of moles of gas are different, K P is related to K c by the equation K P = K c (RT) n g . Remember to use R = 0.0821 L atm mol -1 K -1 and T = absolute temperature. Also, be careful to calculate ∆n g as the difference between the number of moles of gaseous products and the number of moles of gaseous reactants in the balanced equation.
eBook - PDF- Gary D. Christian, Purnendu K. Dasgupta, Kevin A. Schug(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Chapter 5 GENERAL CONCEPTS OF CHEMICAL EQUILIBRIUM “The worst form of inequality is to try to make unequal things equal.” —Aristotle KEY THINGS TO LEARN FROM THIS CHAPTER The equilibrium constant (key Equations: 5.12, 5.15) Calculation of equilibrium concentrations Using Excel Goal Seek to solve one-variable equations The systematic approach to equilibrium calculations: mass balance and charge balance equations Activity and activity coefficients (key Equation: 5.19) Thermodynamic equilibrium constants (key Equation: 5.23) Even though in a chemical reaction the reactants may almost quantitatively react to form the products, reactions never go in only one direction. In fact, reactions reach an equilibrium in which the rates of reactions in both directions are equal. In this chapter we review the equilibrium concept and the equilibrium constant and describe gen- eral approaches for calculations using equilibrium constants. We discuss the activity of ionic species along with the calculation of activity coefficients. These values are required for calculations using thermodynamic equilibrium constants, that is, for the diverse ion effect, described at the end of the chapter. They are also used in potentio- metric calculations (Chapter 20). 5.1 Chemical Reactions: The Rate Concept In 1863 Guldberg and Waage described what we now call the law of mass action, which states that the rate of a chemical reaction is proportional to the “active masses” of the reacting substances present at any time. The active masses may be concentrations or pressures. Guldberg and Waage derived an equilibrium constant by defining equilib- rium as the condition when the rates of the forward and reverse reactions are equal.
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