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Chapter 15

Lecture and Textbook Collaborated Notes - Chapter 15 - CHEM 1A03

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McMaster University
Aadil Merali Juma

Chem 1A03 Chapter 15: Principles of Chemical Equilibrium 15.1 Dynamic Equilibrium  Vapor Pressure – When a liquid vaporizes within a closed container, after a time, the vapor molecules begin to condense to the liquid state at the same rate at which liquid molecules vapor o Vapor pressure of a liquid is a property resulting from an equilibrium condition  Solubility – When a solute is added to a solvent, the system may reach a point at which the rate o dissolution is just matched by the rate at which dissolved solute crystallizes – the solution is saturated o The solubility of a solute is a property resulting from an equilibrium condition  Distribution Coefficient – when an aqueous solution of iodine,2I , is shaken with pure tetrachloride, 4(l),e I2 molecules move into the CCL l4yer; as the concentration of I2builds up in the CCL4, the rate of return o2 I to the water layer becomes significant; when I2molecules pass between the two liquids at equal rates, the concentration of I2in each layer remains constant (at this point,2[I ] in the4CCL is 85x 2 [I ] in H2O) o The distribution coefficient, which represents the partitioning of a solute between two immiscible solvents, is a property resulting from an equilibrium condition  Chemical Equilibrium – when gaseous phosphorous pentachloride is heated, it decomposes to phosphorous trichloride and chlorine gases: PCL5(g) PCL 3(g) Cl2(g) consider PCL5(g)xerting a pressure of 1.0 atm, pressure continues to rise and then reaches an unchanging 1.7 atm – 2 moles of gas are produced for each mole of PCL 5(g)if the reaction went to completion, the final pressure would have been 2.0 atm o The decomposition of PCL is a reversible reaction that reaches an equilibrium condition 5 15.2 The Equilibrium Constant Expression Equilibrium Constant (K)  K – a thermodynamic parameter characterizing the tendency for a chemical reaction or process to occur o Determines the extent of reversible chemical processes (amount of product formed – yield) o Is a ratio of product to reactant activities, a (effective concentration or pressure) at equilibrium o K = [products ]/[reactants ] (aq) (aq) o The form of K depends on the balanced chemical equation  Activity o The activity (a) of a solute, S, is given by a = γ[S] / [So a = [S] (no units) [S]o= 1 mol L -1  Reference State γ = 1  Activity Coefficient γ ≈ 1 under “ideal conditions (dilute solutions of < 0.001 M) o The activity (a) of a gas, G is given by A = γ PG/P o a =P Gno units) Po = 1 atm  Reference State Assume γ = 1 o That activity (a) of a pure solid or liquid is 1  Ignore pure solids or liquids when setting up the equilibrium constant Relationship Between the Equilibrium Constant and Rate Constants Chem 1A03 k1  aA + bB +…  gG + hH+… k-1 a b  Rate of forward reaction: k [1] [B]  Rate of backward reaction: k- [1] [H] h  At equilibrium: k [A] [B] = k- [G] [H] h 1 1 k1 = [G] [H] h k-1 [A] [B] b k1 = K k-1  Multi-step mechanism: k1+ k 2 …k n = K k-1 k-2 …k- n 15.3 Relationships Involving Equilibrium Constants Relationship of K to the Balanced Chemical Equation  Reverse an equation – Invert the value of K  Multiply the coefficients in a balanced equation by a common actor – Raise K to the corresponding power  Divide the coefficients in a balanced equation by a common factor – Take the corresponding root of K Combining Equilibrium Constant Expressions  When individual equations are combined (added), their K values are multiplied to obtain the equilibrium constant for the overall reaction K vs. Kcand K p  Subscript on K – denotes the type of reaction o Eg/ K spdissolution reactions  K expression may include concentrations (aqueous species) and/or pressures (gages)  From the equation:aA (aq) bB (aq) gG (aq) hH (aq) K = (aG) (aH) h (aA) (aB)b  K C when activities are concentrations  K - when activities are partial pressures P Equilibria Involving Gases  Can be measured by K orcK p  K p K cRT) Δngas Equilibria Involving Pure Liquids and Solids  Homogenous Reactions – gas phase reactions and reactions in aqueous solutions; occur within a single phase  Heterogeneous Reactions – reactions involving one or more condensed phases, solids and liquids, in contact with a gas or solution phase  Equilibrium constant expressions do not contain concentration terms for solid or liquid phases of a single component (pure solids and liquids) o An equilibrium constant expression include terms only for reactants and products whose concentrations and/or partial pressures can change during a chemical reaction; the concentration of the single component within a pure solid or liquid phase cannot change o Activities of pure liquids and solids are set equal to 1 – the effect on the numerical value of the equilibrium constant is the same as not including terms for pure solids and liquids at all Chem 1A03 15.4 The Magnitude of an Equilibrium Constant  A very large numerical value of K signifies that the forward reaction, as written, goes to completion or very nearly so o Equilibrium constant may not be used 83 o Eg/ At 298K, 2H 2(g) O2(g) 2H O2 (l) K = 1.4 x 10 Term in the denominator approaches zero and makes the value of K very large  H 2(g)nd O 2(g)re stable at room temperature, despite their high K value, because H2O (l)much more thermodynamically stable than a mixture of H an2 O (it2lies at a lower energy state)  The rate of a chemical reaction is strongly governed by the activation energy, a  E as very high for the synthesis or H2O(l)om H a2d O , 2he rate of reaction is
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