Study Guide 7 Chemical Equilibria PDF

University of the Philippines Visayas TACLOBAN COLLEGE CHEM 18 University Chemistry STUDY GUIDE 7 Chemical Equilibria...

University of the Philippines Visayas TACLOBAN COLLEGE CHEM 18 University Chemistry STUDY GUIDE 7 Chemical Equilibria All discussions and figures in the study guide are taken from the book of Chang, R & Goldsby, K. Chemistry. 12th Ed. McGraw-Hill Education, 2 Penn Plaza, New York, NY, USA, pp. 1083. I. Introduction to Chemical Equilibrium A chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, and the concentrations of the reactants and products remain constant over time. Chemical equilibrium is a dynamic process that even at equilibrium, both forward and reverse reactions continue to occur, but with no net change in concentrations of the reactants and products. aA + bB ⇌ cC + dD The equilibrium constant (Keq) expression is based on the concentrations of the products and reactants at equilibrium. It is expressed as: [𝐶]# [𝐷]$ 𝐾!" = [𝐴]% [𝐵]& where [A], [B], [C], and [D] are the molar concentrations of the chemical species; and a, b, c, and d are the stoichiometric coefficients for the reacting species A, B, C, and D. Above equation is the mathematical expression of their law of mass action, which holds that for a reversible reaction at equilibrium and a constant temperature, a certain ratio of reactant and product concentrations has a constant value, Keq (the equilibrium constant). The validity of the equation and the law of mass action has been established by studying many reversible reactions. The magnitude of the Keq tells us whether an equilibrium reaction favors the products or reactants. If Keq is much greater than 1, the equilibrium will lie to the right and favors the products. Conversely, if Keq is much smaller than 1, the equilibrium will lie to the left and favor the reactants. For reactions that have not reached equilibrium, we obtain the reaction quotient (Qc by substituting the initial concentrations into the equilibrium constant expression. To determine the direction in which the net reaction will proceed to achieve equilibrium, we compare the values of Qc and Kc. The three possible cases are as follows: 𝑄# < 𝐾# To reach equilibrium, reactants must be converted to products 𝑄# = 𝐾# The initial concentrations are equilibrium concentrations. The system is at equilibrium 𝑄# > 𝐾# To reach equilibrium, products must be converted to reactants Types of Equilibrium Constants 1. Kc: In terms of concentrations (mol/L) 2. Kp: In terms of partial pressures (for gases) Relationship Between Kp and Kc: 𝐾' = 𝐾# (𝑅𝑇)∆) where: R is the gas constant T is the temperature in Kelvin Δn is the difference in moles of gas (products – reactants). Read: Chapter 14.1 Chang, R & Goldsby, K. Chemistry. 12th Ed. McGraw-Hill Education, 2 Penn Plaza, New York, NY, USA Question: 1. Consider the equilibrium X ⇌ Y, where the forward reaction rate constant is greater than the reverse reaction rate constant. Which of the following is true about the equilibrium constant? (a) Kc > 1, (b) Kc < 1, (c) Kc = 1. II. Types of Equilibria 1. Homogenous Equilibria applies to reactions in which all reacting species are in the same phase. N2O4(g) ⇌ 2NO2(g) ! [+,! ]!.#$ 𝐾# = [+! ," ] 𝐾' = !. #! $" Kp expression applies only to gaseous reactions and the concentration of solvent does not appear in the equilibrium constant expression.  Sample Problem 7.1: Write expressions for Kc, and Kp if applicable, for the following reversible reactions at equilibrium: HF(aq) + H2O(l) ⇌ H3O+(aq) + F-(aq) Solution: Because there are no gases present, Kp does not apply, and we have only Kc. [#! $ " ][& # ] Answer: 𝐾! = [#&] Sample Problem 7.2: The following equilibrium process has been studied at 230°C: 2NO(g) + O2(g) ⇌ 2NO2(g) In one experiment, the concentrations of the reacting species at equilibrium are found to be [NO] = 0.0542 M, [O2] = 0.127 M, and [NO2] = 15.5 M. Calculate the equilibrium constant (Kc) of the reaction at this temperature. [+,! ]! Solution: 𝐾# = [+,]! [,! ] Note that Kc is given without units. Also, the (12.2)! large magnitude of Kc is consistent with the high 𝐾# = (5.5267)! (5.178) product (NO2) concentration relative to the 𝐾# = 6.44 x105 concentrations of the reactants (NO and O2). 2. Heterogeneous Equilibria results from a reversible reaction involving reactants and products that are in different phases Sample Problem 7.3: The reaction of calcium carbonate that is heated in a closed vessel. CaCO3(s) ⇌ CaO(s) + CO2(g) Write its equilibrium constant expression in terms of Kp and Kc. Solution: The “concentration” of a solid, like its density, is an intensive property and does not depend on how much of the substance is present. For this reason, the terms [CaCO3] and [CaO] are themselves constants and can be combined with the equilibrium constant. Answer: 𝐾! = [𝐶𝑂( ] 𝐾) = 𝑃*$$ 3. Multiple Equilibria the product molecules in one equilibrium system are involved in a second equilibrium process: A+B ⇌ C+D 𝐾!+ C+D ⇌ E+F 𝐾!++ Overall Reaction: A + B ⇌ E + F 𝐾! If a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions. [:] 𝐾# = 𝐾%& 𝐾%&& 𝐾# = [;][