Chapter 14:Chemical Equilibrium Dynamic Equilibrium and the Equilibrium Constant

14.2-Dynamic Equilibrium ● When a chemical reaction reaches dynamic equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction. ○ the products of a reaction are being consumed or depleted at the same rate that they are being produced. ○ because the rates of both the forward and reverse reactions are the same, the concentration(s) of the reactant(s) and the concentration(s) of the product(s) are constant. ■ DO NOT CONFUSE “CONSTANT” WITH “EQUAL”. ■ the concentrations of the reactants and products become fixed when a reaction reaches dynamic equilibrium. This does not necessarily mean that there are equal amounts of reactant and product on either side of the reaction. ■ constant concentration simply means that something is being consumed at the same rate is is being replenished! ○ a reaction does not cease when it reaches dynamic equilibrium. Hence the term dynamic

14.2-Dynamic Equilibrium Consider the following reaction between H 2 and I 2 …. H 2 (g) + I 2 (g) 2HI (g) ● the double headed arrow classifies it as a reversible reaction ○ most chemical reactions are, in theory, reversible reactions. ● In the forward reaction, diatomic hydrogen and diatomic iodine react to form hydrogen iodide. ○ H 2 (g) + I 2 (g) 2HI (g) ● In the reverse reaction, newly formed hydrogen iodide may react and break apart to reform diatomic hydrogen and diatomic iodine ○ 2HI (g) H 2 (g) + I 2 (g) ● when this reaction reaches dynamic equilibrium, the rate of the forward reaction will equal the rate of the reverse reaction.

14.2-Dynamic Equilibrium ● as the concentration of reactant decreases, the concentration of the product increases. ● the rate of the forward reaction slows as the reactant is being used up, and the rate of the reverse reaction increases with the formation of new product.

14.3-The Equilibrium Constant K ● The equilibrium constant is used to quantify the concentrations of reactants and products. ● It is, by definition, the ratio of the concentrations of the products (raised to their stoichiometric coefficients) divided by the concentration of the reactants (raised to their stoichiometric coefficients). aA + bB cC + dD K = ○ the equilibrium constant is denoted by an italicized K. Not to be confused with K(kelvin) ○ this relationship is called The Law of Mass Action and may only be used if the reaction if in dynamic equilibrium. [C] c [D] d [A] a [B] b

14.3-Expressing Equilibrium Constants Make sure the given chemical equation in balanced! 2 N 2 O 5 (g) 4 NO 2 (g) + 1 O 2 (g) Plug values into Law of Mass Action K=K= the coefficients become the exponents! [C] c [D] d [A] a [B] b [NO 2 ] 4 [O 2 ] 1 [N 2 O 5 ] 2

4.3-Practice! Find and Fix each mistake in the equilibrium constant expressions…. A. 2H 2 S (g) 2H 2 (g) + S 2 (g) K = B. CO (g) + Cl 2 (g) COCl 2 (g) K = [H 2 ] [S 2 ] [H 2 S] [CO] [Cl 2 ] [COCl 2 ] K = [H 2 ] 2 [S 2, ] [H 2 S] 2 [COCl 2 ] [CO] [Cl 2, ]

14.3-The Significance of the Equilibrium Constant K << 1: A small equilibrium constant means that the reverse reaction is favored. when a reaction reaches dynamic equilibrium, there will be a higher concentration of reactants than products. (More of what exists on the left side of the equation) K = 1: neither direction is favored K >>1: A large equilibrium constant means that the forward reaction is favored. when a reaction with a constant greater that 1 reaches dynamic equilibrium, there will be a higher concentration of products than reactants. (more of what exists on the right side of the equation) the equilibrium constant has nothing to do with how fast a reaction is! It only signifies how far along the reaction is when it hits equilibrium.

14.3-Relationships Between Equilibrium Constant and Chemical Equation If a chemical equation is changed in any way, the equilibrium constant must also change Here are three very common equation modifications… I. If the chemical equation is reversed, the equilibrium constant should invert. II. If all coefficients are multiplied by a common factor, raise the entire equilibrium constant to the same factor III. If you add two or more individual chemical equations together, multiply the equilibrium constants together.

14.3-Relationships Between Equilibrium Constant and Chemical Equation I. A + 2B 3C when reversed…. 3C A + 2B K forward = I. A + 2B 3C when multiplied by n…. nA + 2nB 3nC K = I. A 2B then….. 2B 3C cancel out intermediates (2B) A 3C K = [C] 3 [A] [B] 2 K reverse = [A] [B] 2 [C] 3 [A] [B] 2 K=K= ( [C] 3 [A] [B] 2 )n)n [B] 2 [A] K=K= [C] 3 [B] 2 cancel out intermediates (2B) K=K= [C] 3 [A]

14.3- Modification Practice Express the original equilibrium constant of the following chemical equation… CO (g) + 2H 2 (g) CH 3 OH (g) How will the equilibrium constant change when the chemical equation becomes…. CH 3 OH (g) CO (g) + 2H 2 (g).5CO (g) + H 2.5CH 3 OH (g) 2CH 3 OH (g) 2CO (g) + 4H 2