Download Understanding Chemical Equilibrium: Constants, Solubility, Acids & Bases and more Study notes Chemistry in PDF only on Docsity! 1 Chemical Equilibrium & Activity Equilibrium Constant Most chemical systems are governed by equilibria such that if: aA + bB cC + dD, then ba dc ba dc BA DC K xx xxax aa aa K ba dc ][][ ][][ ,1 assume weif ,1 solutions, diluteIn tcoefficienactivity theis [X], where )()( )()( = =→ == γγ γγ Equilibrium constants may be written for dissociations, associations, reactions, or distributions. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Equilibrium Constant • If K is very large, that the equilibrium lies far to the right (or towards products). If K is small, the reaction lies towards reactants. • Knowledge of reaction stoichiometry and the equilibrium constant allows us to make some predictions about the system. ba dc BA DC K ][][ ][][ = aA + bB cC + dD Important items to regarding K expressions 1. All solute concentrations should be in mol/L (M). 2. All gas concentrations should be in atmospheres. 3. By convention, all K’s are calculated relative to 1 M solutions or 1 atm gas, so the resulting constants are dimensionless. 4. Concentrations of pure solids, pure liquids and solvents are omitted from the equilibrium constant expression. Equilibrium constant expressions are thermodynamic relations. Equilibrium and Thermodynamics • Gibbs free energy: • If K >1 Go<0 Spontaneous • If K<1 Go NOT Spontaneous RTG o o eK K -RTG / ln ∆−= = 2 Reaction Quotient (Q) The equilibrium constant expression with non- equilibrium concentrations plugged in. • If Q>K, the reaction must proceed to the left • If Q<K, the reaction must proceed to the right • If Q=K the reaction is at equilibrium Using equilibria to characterize systems Solubility: solubility product (Ksp) Complexation: formation constants (Kn), cumulative formation constants (n) Acids and Bases: acid dissociation constant (Ka), base hydrolysis constant (Kb) Solubility The solubility product (Ksp) describes the concentrations of species present when ions are in equilibrium with undissolved salt. Example: CaCO3 = Ca 2+ + CO3 2- K= [Ca2+][CO3 2-]/[CaCO3]= [Ca2+][CO3 2-] = Ksp = 6.0 x 10-9 What is the concentration of calcium in a saturated solution of calcium carbonate? Solubility - Separation by precipitation It is possible to quantitatively separate two or more species based on their solubility. Ability to do so is related to the magnitudes of the Ksp for each ion. Example: Is it possible to precipitate 99% of 0.010M Ce3+ by adding oxalate (C2O4 2-) without precipitate 0.010M Ca2+ CaC2O4 Ksp = 1.3 x 10-8 Ce2(C2O4)3 Ksp = 3.0 x 10-29 Acids and Bases • Definitions: Lewis – Electrons (acid: electron pair acceptor); BrØnsted-Lowry (acid: proton donor) • Conjugate Acid-Base Pairs: related by the gain or loss of one proton (ex. Acetic acid & acetate ion). • Neutralization Reactions: reactions of acid and base to form salts and water • Solvent Autoprotolysis or self-ionization: water is the most common, in which it acts both an acid and a base: H 2O OH - +H + 2 H 2O H 3O ++O H - 14 3 10]][[]][[ −++ === −− OHHOHOHKw Autoprotolysis constant (equilibrium constants) pH H 2O OH - +H + P-function: pX = - log10[X] pH = -log [H+] pH > 7 is basic, pH<7 is acidic pOH=-log[OH-] pH + pOH = pKw = 14.00 at 25oC Ex. Concentration of H+ and OH- in pure water at 25oC ANS: 1.0x10-7 M
