ChemicalEquilibrium_3[2]

ChemicalEquilibrium_3[2] - Introductory University...

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Unformatted text preview: Introductory University Chemistry 2 Principles of Chemical Equilibrium Unit 2 Introduction • All chemical reactions can be regarded as reversible. A + B C + D • If a chemical reaction is carried out in a closed vessel (neither the reactant nor the product can escape), a state of equilibrium will eventually reach. Some examples of equilibrium systems Water H 2 O ( l ) H 2 O (g) Saturated salt vsolution NaCl NaCl (s) Na + (aq) + Cl- (aq) Vapor pressure of the liquid Solubility of the solid (solubility equilibrium) Some examples of equilibrium systems I 2 in water Distribution coefficient of the solute I 2 in dichloromethane I 2 (aq) I 2 (CH 2 Cl 2 ) Distribution coefficient [I 2 ] CH2Cl2 = [I 2 ] H2O Dynamic Equilibrium N 2 O 4 (g) 2NO 2 (g) colorless brown Equilibrium is reached Reaction starts from NO 2 Reaction starts from N 2 O 4 Characteristics of an equilibrium state: 1. The equilibrium is dynamic 2. Concentrations of the reactants and products are constant The reactions are still occurring although no net change can be observed Rate forward = Rate reverse 3. Rate of forward reaction = rate of reverse reaction. The Equilibrium Constant Expression For a chemical reaction: a A + b B c C + d D Rate (forward) = k (forward) [A] a x [B] b Rate (reverse) = k (reverse) [C] c x [D] d At equilibrium, k (forward) [A] a x [B] b = k (reversed) [C] c x [D] d [C] c x [D] d k (forward) = = K c [A] a x [B] b k (reverse) K c is called the equilibrium constant and is dependent only on temperature. K c = Equilibrium constant expressed in terms of concentrations. Equilibrium constant K does not have units Because in thermodynamics, concentration is expressed as ‘activity’ which is without unit . [C] c [D] d K c = [A] a [B] b For example: This ratio is called the equilibrium constant expression N 2 O 4 (g) 2NO 2 (g) K c = [NO 2 ] 2 [N 2 O 4 ] For example: Magnitude of K : • Small K • Large K An indication of how far a reaction proceeds toward product at a given temperature. N 2 (g) + O 2 (g) 2NO (g) K = 1 x 10-30 Very little product is formed (actually no reaction) • Intermediate K 2CO (g) + O 2 (g) 2CO 2 (g) K = 2.2 x 10 22 So much product formed. (reaction goes to completion) 2BrCl (g) Br 2 (g) + Cl 2 (g) K = 5 The reaction mixture contains a considerate amount of both the reactant and the product For the chemical reaction: N 2 (g) + 3H 2 (g) 2NH 3 (g) at 127 o C Initial concentrations Equilibrium concentrations (from experiments) K c 1 [NH 3 ] = 0 mol / L [N 2 ] = 1.000 mol / L = 1.000 mol / L [NH 3 ] = 0.157 mol / L [N 2 ] = 0.921 mol / L = 0.763 mol / L Reaction starts om the left [H 2 ] = 1.000 mol / L [H 2 ] = 0.763 mol / L 2 [NH 3 ] = 1.000 mol / L [N 2 ] = 0 mol / L [H 2 ] = 0 mol / L [NH 3 ] = 0.203 mol / L [N 2 ] = 0.399 mol / L [H 2 ] = 1.197 mol / L 3 [NH 3 ] = 3.00 mol / L [N 2 ] = 2.00 mol / L [H 2 ] = 1.00 mol / L [NH 3 ] = 1.82 mol / L [N 2 ] = 2.59 mol / L [H 2 ] = 2.77 mol / L from the left Reaction starts from the right Reaction starts from both sides N...
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ChemicalEquilibrium_3[2] - Introductory University...

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