Chemistry
Equilibrium Constants
Equilibrium constants are numerical values that express the extent of a chemical reaction at equilibrium. They are calculated from the concentrations of reactants and products at equilibrium and are used to predict the direction of a reaction and its equilibrium position. The value of the equilibrium constant provides information about the relative amounts of reactants and products at equilibrium.
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- Kevin Reel, Derrick C. Wood, Scott A. Best(Authors)
- 2014(Publication Date)
- Research & Education Association(Publisher)
12Equilibrium
Equilibrium Constants
• Equilibrium Constants are ratios. See the following reaction:The ratio of the product concentrations raised to their stoichiometric coefficients, divided by the reactant concentrations raised to their stoichiometric coefficients, is the equilibrium constant, K . This is also called the law of mass action .• Pure liquids or solids do not show up in the equilibrium expression because they do not change in concentration; only solutions measured as molar concentrations or pressures (as in the case of gases).• The equilibrium constant for a multi-step process is equal to the product of the Equilibrium Constants for each step. Example: For a set of three reactions that add to equal a total reaction:• The equilibrium constant for a reverse reaction is the inverse of the equilibrium constant for a forward reaction.• There are different Equilibrium Constants for different types of reactions. See Table 12.1 .Table 12.1. Types of Equilibrium ConstantsTEST TIPThe magnitude of the value of K indicates the strength of a particular species: Larger Ka values mean greater acid strength, larger Kb is a stronger base, and larger Ksp values mean that more solid is able to dissolve in a given amount of solvent.Le Chatelier’s Principle
• Le Chatelier’s principle states that when a system at equilibrium is disturbed by a change in pressure, temperature, or the amount (concentration) of product or reactant, the reaction will shift to minimize the change and establish a new equilibrium.
eBook - ePubChemistry
Concepts and Problems, A Self-Teaching Guide
- Richard Post, Chad Snyder, Clifford C. Houk(Authors)
- 2020(Publication Date)
- Jossey-Bass(Publisher)
equilibrium constant is the ratio of the concentration of the products divided by the concentration of the reactants at equilibrium and at a specified temperature. The equilibrium constant (product concentrations divided by reactant concentrations) is valid only at a specified temperature after the reaction has gone to (completion, equilibrium) __________Answer: equilibriumBelow is a reversible reaction and the expression for the equilibrium constant for this reversible reaction.The symbol Keqrepresents the equilibrium constant and the brackets [] represent the concentration (usually in moles per liter) of each product and reactant. Look at the placement of each reactant and product in the equilibrium constant expression. In the equilibrium constant expression for a reversible reaction, the (products, reactants) ____________ are located in the numerator or upper part of the fraction and the (products, reactants) ________________ are located in the denominator or lower part of the fraction.Answer: products; reactantsThe standard equation, then, for Keqis as follows.Write the equilibrium constant expression for the following reversible reaction.Keq= ___________________Answer:(Since there are two products, they should be placed in the upper part of the fraction. The one reactant belongs in the lower part of the fraction.)
eBook - ePub- Jeffrey Gaffney, Nancy Marley(Authors)
- 2017(Publication Date)
- Elsevier(Publisher)
7.2, when the reactants of a reversible reaction are first combined, the forward reaction occurs rapidly increasing the concentrations of the products and decreasing the concentrations of the reactants. As the reaction proceeds and the concentration of products increases, the forward reaction slows and the reverse reaction begins to occur. After some time, an equilibrium is reached where the concentrations of reactants and products do not change with time and the forward and reverse reaction rates are equal. This is known as a dynamic equilibrium because, although forward and reverse reactions still occur, the rate of the forward reaction equals the rate of the reverse reaction. So, the ratio of concentrations of the products to the concentrations of the reactants remains constant, although the reaction is constantly proceeding in both directions. Fig. 7.2 A reversible chemical reaction reaches dynamic equilibrium when the rate of the forward and reverse reactions are equal (left) and the concentrations of reactants and products do not change with time (right). 7.2 The Equilibrium Constant Since the concentrations of reactants and products of a reversible reaction at equilibrium are constant, an expression can be written for an equilibrium constant (K eq) that describes the concentrations of the products and reactants at equilibrium. For the generic reversible reaction (3) at equilibrium, the equilibrium constant expression is; K eq = C c D d A a B b (4) As with the acid ionization constants described in Section 5.2, the equilibrium constant for a reversible reaction is expressed as the ratio of the equilibrium concentrations of the products in the numerator to the equilibrium concentrations of the reactants in the denominator. The superscript letters are the stoichiometric coefficients of the reactants and products in the balanced chemical equation
eBook - ePub- Kevin R. Reel(Author)
- 2013(Publication Date)
- Research & Education Association(Publisher)
law of mass action .• Pure substances, such as water or solids, do not show up in the equilibrium expression; only molar solutions or, as with Kp , gaseous pressures.• The equilibrium constant for a multi-step process is equal to the product of the Equilibrium Constants for each step.Example: For a set of three reactions that add to equal a total reaction,Ktotal=K1×K2×K3• The equilibrium constant for a reverse reaction is the inverse of the equilibrium constant for a forward reaction.Example: • There are different Equilibrium Constants for different types of reactions.REACTION QUOTIENT, Q
• The reaction quotient, Q, can be used to calculate the direction and degree to which a reaction will shift when new products or reactants are added (Le Chatelier’s principle). • The reaction quotient for a reaction is found using the same ratio as the equilibrium constant, but at non-equilibrium conditions.For the reaction, aA + bB ↔ cC + dD• If the reaction quotient is greater than the equilibrium expression, there are more products than there would be at equilibrium. By Le Chatelier’s principle, the reaction will shift toward equilibrium by using products and producing more reactants.When Q > K, reaction proceeds to the left toward reactants. When Q < K, reaction proceeds to the right toward products. When Q = K, reaction is at equilibrium. Example: In the following reaction,N2 (g ) + 3 H2 (g ) ↔ 2 NH3 (g )The Kc is 5.9 × 10−2 . The molar concentrations of each reactant and product are: [N2 ] = 0.40 M, [H2 ] = 0.80 M, and [NH3
eBook - ePub- Adrian Dingle, Derrick C. Wood(Authors)
- 2014(Publication Date)
- Research & Education Association(Publisher)
concentrations of reactants and products. In fact, they are very rarely the same numerical value.
5. Graphically, the concentrations of reactants and products can be represented as: i. For a product-favored reaction: ii. For a reactant-favored reaction: C. Quantitative TreatmentAll concentrations being constant (not changing) is not the same thing as concentrations being equal (all having the same value). This is a common misconception and one that you must not have.1. Equilibrium Constants for gaseous reactions: Kp , Kci. Equilibrium constant relates the concentrations of reactants and products at equilibrium at a given temperature.For the general reaction: aA + bB cC + dDThe equilibrium constantProduct molar concentrations are in the numerator.Reactant molar concentrations are in the denominator.Each concentration is raised to the power of its stoichiometric coefficient in the balanced chemical equation.Pure solids and pure liquids (e.g., water) are not placed into the expression.The expression Kc indicates that concentrations are used (moles per liter).The expression Kp indicates that partial pressures are used (pressure units, often atm) and only gases are included.Examples1. N2 (g) + 3H2 (g) 2NH3
eBook - ePub- Mark E. Davis, Robert J. Davis(Authors)
- 2013(Publication Date)
- Dover Publications(Publisher)
APPENDIX AReview of ChemicalEquilibria
A.1 | Basic Criteria for Chemical Equilibriumof Reacting Systems
The basic criterion for equilibrium with a single reaction is:where ΔG is the Gibbs function, NCOMP is the number of components in the system,viis the stoichiometric coefficient of species i , and i is the chemical potential of species i . The chemical potential is:where R g is the universal gas constant, is the standard chemical potential of species i in a reference state such that a i = 1, and a i is the activity of species i . The reference states are: (1) for gases (i.e., 0 = 1) (ideal gas, P = 1 atm) where is the fugacity, (2) for liquids, the pure liquid at T and one atmosphere, and (3) for solids, the pure solid at T and one atmosphere. If multiple reactions are occurring in a network, then Equation (A.1.1) can be extended to give:where NRXN is the number of independent reactions in the network.In general it is not true that the change in the standard Gibbs function, ΔG 0 , is zero. Thus,Therefore, or by using Equation (A.1.2): Now consider the general reaction:Application of Equation (A.1.6) to Equation (A.1.7) and recalling that ΔG = 0 at equilibrium gives:Thus, the equilibrium constant K a is defined as:Differentiation of Equation (A.1.8) with respect to T yields:Note that ΔG 0 = ΔH 0 − T ΔS 0 , where ΔH 0 and ΔS0 are the standard enthalpy and entropy, respectively, and differentiation of this expression with respect to T gives:Equating Equations (A. 1.10) and (A.1.l1) provides the functional form for the temperature dependence of the equilibrium constant:or after integration (assume ΔH 0 is independent of T ):Notice that when the reaction is exothermic (ΔH 0 is negative), K a increases with decreasing T
