Chapter 13 Part 1 Chemistry Student Notes PDF

Chapter 13: Part 1 CHEM 0120 2  Arrow Conventions Chemists commonly use two kinds of arrows in reactions to indicate the degree of completion of the reactions. A single arrow indicates that all the reactant molecules are converted to product molecules at the...

Chapter 13: Part 1 CHEM 0120 2  Arrow Conventions Chemists commonly use two kinds of arrows in reactions to indicate the degree of completion of the reactions. A single arrow indicates that all the reactant molecules are converted to product molecules at the end. A double arrow indicates that the reaction reaches equilibrium with reactant and products present. 3  Reaction Dynamics When a reaction starts, the reactants are consumed and products are made. The reactant concentrations decrease and the product concentrations increase. Eventually, the products can react to re-form some of the reactants, assuming that the products are not allowed to escape. Processes that proceed in both the forward and reverse directions are said to be reversible. Reactants ⇌ Products 4  Example Catalytic methanation: CO(g) + 3H2(g) → CH4(g) + H2O(g) Steam reforming: CH4(g) + H2O(g) → CO(g) + 3H2(g) Consists of a forward and reverse reaction; therefore we can write it as: CO g + 3H2 g ⇌ CH4 g + H2 O(g) The favorability of products or reactants depends upon the conditions of the reaction. 5  Chemical Equilibrium Chemical equilibrium: the state reached by a reaction mixture when the rates of forward and reverse reactions have become equal 6  Equilibrium notes At equilibrium, the rates of the forward and reverse reactions are equal. This does not mean the concentrations of reactants and products are equal. Some reactions reach equilibrium after almost all of the reactant molecules are consumed The position of equilibrium favors the products. Other reactions reach equilibrium when only a small amount of the reactant molecules are consumed The position of equilibrium favors the reactants. 7  Equilibrium Constant, Kc 𝑎A + 𝑏B ⇌ 𝑐C + 𝑑D [C]𝑐 [D]𝑑 𝐾𝑐 = [A]𝑎 [B]𝑏 Kc is the value obtained for the equilibrium-constant expression when equilibrium concentrations are substituted Law of mass action: a relation that states that the values of the equilibrium-constant expression Kc are constant for a particular reaction at a given temperature, whatever equilibrium concentrations are substituted 8  Writing Equilibrium Constant Expressions So, for the reaction 2 N2O5(g) ⇌ 4 NO2(g) + O2(g) Write the expression for the equilibrium constant, Kc 9  Kinetics of Equilibrium Dynamic equilibrium: consists of a forward reaction and a reverse reaction occurring at the same speed. Once at equilibrium, the forward and reverse reactions continue. 𝑎A ⇌ 𝑏B At equilibrium, kf[A]a = kr[B]b (rate of forward reaction = rate of reverse reaction) Kc can be identified as the ratio of rate constants for the forward and reverse reactions 𝑘𝑓 𝐾𝑐 = 𝑘𝑟 This ratio is just a comparison of the rate constants. The rate is dependent on the concentrations, coefficients and rate constants. 10  K for Reactions involving Gases The concentration of a gas in a mixture is proportional to its partial pressure. Kp: an equilibrium constant for a gaseous reaction in terms of partial pressures For 𝑎A(𝑔) + 𝑏B(𝑔) ⇌ 𝑐C(𝑔) + 𝑑D(𝑔), 𝑐 𝑑 𝑃C 𝑃D 𝐾𝑝 = 𝑎 𝑏 𝑃A 𝑃B 11  Relationship between Kc and Kp The relationship between Kc and Kp is: Kp = Kc(RT)Δn Δn is the sum of the coefficients of gaseous products minus the sum of the coefficients of gaseous reactants in the chemical equation 12  Example Nitrogen monoxide, a pollutant in automobile exhaust, is oxidized to nitrogen dioxide in the atmosphere. 2NO g + O2 g ⇌ 2NO2 (g) Kp = 2.2 x 1012 at 298 K Solve Kc for this reaction. 13  Equilibrium Constant for the Sum of Reactions Useful rule: If a given chemical equation can be obtained by taking the sum of other equations, the equilibrium constant for the given equation equals the product of the equilibrium constants of the added equations. For the reactions (1) 𝒂𝐀 ⇌ 𝒃𝐁 and (2) 𝒃𝐁 ⇌ 𝒄𝐂, the equilibrium constant expressions are as follows: [B]𝑏 [C]𝑐 𝐾1 = 𝐾2 = [A]𝑎 [B]𝑏 For the reaction 𝒂𝐀 ⇌ 𝒄𝐂, the equilibrium constant expression is as follows: [𝐶]𝑐 𝐾1 𝐾2 = 𝐾3 = [𝐴]𝑎 14  Heterogeneous Equilibria Heterogeneous equilibrium: an equilibrium involving reactants and products in more than one phase. The concentrations of pure solids and pure liquids do not change during the course of a reaction. Because their concentration doesn’t change, solids and liquids are not included in the equilibrium constant expression. 15  Example for Heterogeneous Equilibria Write the expression for Kc for the following reactions: CaCO3(s) ⇌ CaO(s) + CO2(g) H2O(l) ⇌ H2O(g) 16  Using the Equilibrium Constant Ways to use the equilibrium constant: Qualitatively interpret the equilibrium constant. Does the equilibrium favor reactants or products? To predict the direction of the reaction. To calculate the equilibrium concentrations. 17  Qualitatively Interpreting Kc 𝑎A + 𝑏B ⇌ 𝑐C + 𝑑D [C]𝑐 [D]𝑑 𝐾𝑐 = [A]𝑎 [B]𝑏 When Kc >> 1, the equilibrium mixture is mostly products. When Kc Kc, the reaction will go to the left. Indicates that the reaction mixture has more products. If Qc < Kc, the reaction will go to the right. Indicates that the reaction mixture has more reactants. If Qc = Kc, the reaction mixture is at equilibrium. 20  Reaction Quotient, Qc If Qc > Kc, the reaction will go to the left. If Qc < Kc, the reaction will go to the right. If Qc = Kc, the reaction mixture is at equilibrium. 21