Chemistry
Reaction Quotient
The reaction quotient is a measure of the relative concentrations of products and reactants in a chemical reaction at any given time. It is calculated using the same formula as the equilibrium constant, but it is used to determine whether a reaction is at equilibrium or not. Comparing the reaction quotient to the equilibrium constant helps predict the direction a reaction will proceed.
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7 Key excerpts on "Reaction Quotient"
eBook - PDFChemistry
Structure and Dynamics
- James N. Spencer, George M. Bodner, Lyman H. Rickard(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
Because the reaction proceeds in both directions at the same rate, there is no apparent change in the concentrations of the reactants or the prod- ucts on the macroscopic scale (i.e., the level of objects visible to the naked eye). This model can also be used to predict the direction in which a reaction has to shift to reach equilibrium. If the concentrations of the reactants are too large for the reaction to be at equilibrium, the rate of the forward reaction will be faster than that of the reverse reaction, and some of the reactants will be converted to products until equilibrium is achieved. Conversely, if the concentrations of the reactants are too small, the rate of the reverse reaction will exceed that of the forward reaction, and the reaction will convert some of the excess products back into reactants until the system reaches equilibrium. We can determine the direction in which a reaction has to shift to reach equilibrium by comparing the Reaction Quotient (Q c ) for the reaction at some moment in time with the equilibrium constant (K c ) for the reaction. The Reaction Quotient expression is written in much the same way as the equilibrium constant expression. But the concentrations used to calculate Q c describe the system at any moment in time, whereas the concentrations used to calculate K c describe the sys- tem only when it is at equilibrium. To illustrate how the Reaction Quotient is used, consider the following gas- phase reaction. The equilibrium constant expression for the reaction is written as follows. By analogy, we can write the expression for the Reaction Quotient as follows. There are three important differences between the equilibrium constant expres- sion and the Reaction Quotient expression. First, we use brackets, such as [HI], in the equilibrium constant expression to indicate that the reaction is at equilibrium.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
446 Chemistry SUMMARY 9.1 Define chemical equilibrium. Chemical equilibrium occurs when the chemical composition of a system does not change with time. When the forward and reverse reactions in a chemical system occur at equal rates, a dynamic equilibrium exists and the concentrations of the reac- tants and products remain constant. 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. 9.2 Use the equilibrium constant, K, and the Reaction Quotient, Q. The equilibrium constant expression contains only gases and species in solution. When all components in an equilibrium equation are gases or species in solution, the equilibrium con- stant expression is a fraction. The concentrations of the products at equilibrium, each divided by c o and raised to a power equal to its stoichiometric coefficient in the balanced chemical equation, are multiplied together in the top line (numerator). The bottom line (denominator) is constructed in the same way from the concentrations of the reactants at equilibrium, each divided by c o and raised to a power equal to its stoichiometric coefficient. The numerical value of the equilibrium constant expression is the equilibrium constant, K c . If partial pressures of gases, divided by p o and raised to their stoichiometric coefficients, are used in the equilibrium constant expression, K p is obtained. The thermo- dynamic equilibrium constant is obtained when activities, rather than pressures or concentrations, are used in the equilibrium constant expression. The equilibrium constant is constant at constant temperature. The magnitude of the equilibrium constant reflects the extent to which the forward reaction has proceeded to completion when equilibrium is reached.
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- Juan J. de Pablo, Jay D. Schieber(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
For example, if we have one mole of hydrogen and one mole of oxygen, then we can make at most one mole of water, leaving half a mole of oxygen unreacted. If we make less water, we have more oxygen left over – the numbers of moles of oxygen and water are not independent. Therefore, we cast the criterion for chemical equilibrium using a new independent variable called extent of reaction . In general, we can write a chemical reaction in the form 0 r i ν i B i , (10.4) where a species is represented by B i , and its stoichiometric coefficient is ν i . If the stoichiomet-ric coefficient of a species is negative, it would normally show up on the left-hand side of the equation as a reactant. For example, the combustion of ethene can be written as C 2 H 4 + 3O 2 2CO 2 + 2H 2 O. (10.5) 10.2 Extent of reaction 331 Here, the stoichiometric coefficients are ν C 2 H 4 = − 1 and ν CO 2 = 2 for ethene and carbon dioxide, respectively. Note that there is a restriction on the stoichiometric coefficients, since the numbers of atoms must be the same on each side of the equation; for example, there are two carbon atoms, four hydrogen atoms, and six oxygen atoms on each side of the ethene reaction. However, in principle, all coefficients could be multiplied by an integer to yield a different set of equally valid values. Hence, it is important to be certain that all thermodynamic quantities considered below are associated with the same set of values for the stoichiometric coefficients. For a closed system, the only way in which the number of moles of a chemical species can change is by reaction. Hence, for a single chemical reaction, we can write dN 1 ν 1 = dN 2 ν 2 = . . . = dN m ν m = : d ε , single reaction, (10.6) which defines the extent of reaction ε , for a single reaction. The extent of reaction is typically (though arbitrarily) taken to be zero when the components are mixed.
eBook - PDF- Ageetha Vanamudan(Author)
- 2023(Publication Date)
- Delve Publishing(Publisher)
QUANTITIES OF REACTANTS AND PRODUCTS CHAPTER4 CONTENTS 4.1 Categorization of Chemical Reactions and Reactive Substances ........ 57 4.2 Chemical Reaction Signs ................................................................... 63 4.3 Mechanical Equilibrium .................................................................... 67 Introduction to Molecular Science 56 A chemical reaction is one in which the bonds between the reactant molecules and their products are broken and new bonds between the products are formed. A chemical reaction occurs when two or more chemicals combine to generate new ones. Chemical interactions are essential to many industries, societies, and even our daily lives. A chemical reaction must involve a change in chemical characteristics, such as precipitation, heat production, color alterations, and other comparable physical changes. When two or more elements from a new connection that does not entail the destruction or production of any of its elements, numerous types of reactions occur (Hasanuzzaman et al., 2013). The rate of a chemical reaction is affected by pressure, temperature, and reactant concentration. Chemical compound formulas: Because of the huge number of chemical reactions in our environment, a nomenclature was developed to make it easier to represent a chemical reaction as a chemical equation. A chemical equation is a mathematical formula that describes a chemical reaction and the formation of a product from reactants. On the left side of the figure, an arrow with one or two heads connects reactants to products, while products are on the right. Consider this an example of a possible reaction. A+B C+D; C and D are the by-products of the reaction between the reactants A and B. Chemical equations make use of reactant formulas. To preserve mass conservation, the number of atoms on both sides of a chemical equation must be the same. As a result, the equation is balanced. Figure 4.1: Chemical reactants in reaction bottles.
eBook - PDFChemistry
Principles and Reactions
- William Masterton, Cecile Hurley(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
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. 11-2 Reaction Rate and Concentration 281 In this equation m is referred to as “the order of the reaction with respect to A.” Similarly, n is “the order of the reaction with respect to B.” The overall order of the reaction is the sum of the exponents, m 1 n. If m 5 1, n 5 2, then the reaction is first-order in A, second-order in B, and third-order overall. When more than one reactant is involved, the order can be determined by holding the initial concentration of one reactant constant while varying that of the other reactant. From rates measured under these conditions, it is possible to deduce the order of the reaction with respect to the reactant whose initial concen-tration is varied. To see how to do this, consider the reaction between A and B referred to above. Suppose we run two different experiments in which the initial concentra-tions of A differ ([A] 1 , [A] 2 ) but that of B is held constant at [B]. Then rate 1 5 k [A] 1 m 3 [B] n rate 2 5 k [A] 2 m 3 [B] n Dividing the second equation by the first ▲ rate 2 rate 1 5 k [A] 2 m 3 [B] n k [A] 1 m 3 [B] n 5 [A] 2 m [A] 1 m 5 1 [A] 2 [A] 1 2 m Knowing the two rates and the ratio of the two concentrations, we can readily find the value of m. This way, the concentration terms cancel for that reactant. EXAMPLE Consider the reaction between t-butylbromide and a base at 55°C: (CH 3 ) 3 CBr( aq ) 1 OH 2 ( aq ) 9: (CH 3 ) 3 COH( aq ) 1 Br 2 ( aq ) A series of experiments is carried out with the following results: Expt.
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
Equilibrium effects are most obvious in reactions with K values between 10 3 and 10 2 3 . In such cases, the reaction goes part of the way and significant concentrations of all substances are present at equilibrium. An example is the reaction between carbon monoxide and water discussed in Section 7.5, for which K is equal to 10 at 600 8 C. HOW TO Interpret the Value of the Equilibrium Constant, K Position of Equilibrium The first question about the value of an equilibrium constant is whether the number is larger than one or smaller than one. If the number is larger than one, it means the ratio of product concentrations to reactant concentrations favors products. In other words, the equilibrium lies to the right. If the number is smaller than one, it means the ratio of prod-uct concentrations to reactant concentrations favors reactants. In other words, the equilibrium lies to the left. 216 | Chapter 7 Reaction Rates and Chemical Equilibrium Copyright 2020 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. The equilibrium constant for a given reaction remains the same no mat-ter what happens to the concentrations, but the same is not true for changes in temperature. As pointed out earlier in this section, the equilibrium expression is valid only after equilibrium has been reached. Before that point, there is no Numerical Value of K The next question focuses on the numerical value of the equilibrium constant. As we saw in Section 1.3, we frequently write numbers with exponents, with positive exponents for very large numbers and negative exponents for very small numbers.
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