Le Chatelier’s Principle Chemical Equilibrium Reversible Reactions, Equilibrium Constant, and Le Chatelier’s Principle

Standards 9. Chemical equilibrium is a dynamic process at the molecular level. As a basis for understanding this concept: a. Students know how to use Le Chatelier’s principle to predict the effect of changes in concentration, temperature, and pressure. 9. b. Students know equilibrium is established when forward and reverse reaction rates are equal. Forward and reverse reactions at equilibrium are going on at the same time and at the same rate, causing overall concentrations of each reactant and product to remain constant over time. 9. c.* Students know how to write and calculate an equilibrium constant expression for a reaction. aA + bB ↔ c C + d D Keq = [C]^c x [D]^d _____________ [A]^a x [B]^b

Reaction Symbols Symbol Meaning (s), (l), (g) Substance is a solid, liquid, or gas (aq) Aqueous, substance is dissolved in H2O “Produces” or “makes” “Produces” through reversible reaction heat or Δ Heat is added to the reactants Pt A catalyst is used to speed up the reaction

Definitions completion reactions – reaction that only go forward reversible reactions – reaction that goes both forwards and backwards

Chemical Equilibrium starts out: Reactants Products after a while: Chemical equilibrium is reached when the rate of the forward reaction is equal to the rate of the reverse reaction. (Does not mean equal amounts of reactants and products).

+ H2 (g) + I2 (g) 2 HI (g) Example Reaction #2 hydrogen iodine hydrogen gas gas iodide +

H2 (g) + I2 (g) 2 HI (g) +0 +8

H2 (g) + I2 (g) 2 HI (g) – 2 +6

H2 (g) + I2 (g) 2 HI (g) – 4 +4

H2 (g) + I2 (g) 2 HI (g) – 4 +4

H2 (g) + I2 (g) 2 HI (g) – 4 +4

Equilibrium Constant a A + b B c C + d D [C]c[D]d [A]a[B]b Keq = coefficient [C]c[D]d [A]a[B]b concentration equilibrium constant Keq = Example: N2 + 3 H2 2 NH3 [NH3]2 [N2] [H2]3 Keq =

Equilibrium Constant Example: N2 + 3 H2 2 NH3 Keq = ??? [N2] = 3.0 [4.0]2 [3.0] [2.0]3 Keq = = 2 3

Equilibrium Constant a A + b B c C + d D [C]c[D]d [A]a[B]b Keq = coefficient [C]c[D]d [A]a[B]b concentration equilibrium constant Keq = Note: Only use concentrations for dissolved mixtures (e.g. gases, mixed liquids, aqueous solutes). Otherwise use a 1. Example: CaSO4 (s) Ca2+ (aq) + SO42– (aq) [Ca2+][SO42–] 1 Keq =

Equilibrium Constant a A + b B c C + d D [C]c[D]d [A]a[B]b Keq = coefficient [C]c[D]d [A]a[B]b concentration equilibrium constant Keq = Keq > 1: Reaction favors products Keq < 1: Reaction favors reactants

Exo/Endothermic Reactions exothermic reaction – the forward reaction releases heat. endothermic reaction – the forward reaction absorbs heat.

Le Châtelier’s Principle If a chemical system experiences a stress, the equilibrium will shift (away from that side) to relieve that stress. The stress can be a change in: Concentration Temperature (for endothermic or exothermic rxns) Pressure (for reactions involving gases) If a chemical system experiences a reduction in stress, the equilibrium will shift (toward that side) to add more of that stress.

+ + HCl(aq) + H2O(l) H3O+(aq) + Cl–(aq) hydro- water hydronium chloride chloric acid ion ion + +

N2 (g) + 3 H2 (g) 2 NH3 (g) + 92kJ nitrogen hydrogen ammonia heat gas gas gas + +

Example Reaction #1 CaCl2 (aq) + Na2SO4 (aq) CaSO4 (s)+ 2NaCl (aq) Ca2+ (aq) + SO42– (aq) CaSO4 (s)

Le Chatelier’s Principle

Le Chatelier’s Principle Add plunger

He H B C N O Ne F Li Be P Al Si S Cl Ar Na Mg Br Kr K Ca I Xe

He H B C N O Ne F Li Be P Al Si S Cl Ar Na Mg Br Kr K Ca I Xe

4 e– in valence shell