ACID BASE EQUILIBRIA ACIDBASE THEORIES 4 theories ARRHENIUS

ACID BASE EQUILIBRIA ACID-BASE THEORIES (4 theories)

ARRHENIUS THEORY-H+ AND OHˉ • An acid is any substance that ionizes (partially or completely) in water to give hydrogen ions • Means associate with solvent to give hydronium ions • HA +H 2 O ↔ H 3 O+ + Aˉ

• A base ionizes in water to give hydroxyl ions. • Weak (partially ionizes) bases ionizes as follows: • B + H 2 O ⇋ BH+ + OHˉ For strong bases e. g. : (Na. OH) M(OH)n →M^n+ + n. OHˉ • this theory is obviously restricted to water as a solvent only

Franklin’s theory • Recognizes the ionization of a solvent to give a cation and an anion • E. g: 2 H 2 O ⇋ H 3 O+ + OHˉ • 2 NH 3 ⇋ NH 4 + NH 2¯ • Acid = a solute that yields the cation of the solvent • Base = a solute that yields the anion of the solvent • Eg of strong acid is NH 4 Cl in liquid ammonia • Eg of strong base is Na. NH 2 in ammonia

Brønsted-Lowry theory • Taking and giving protons • States that an acid is any substance that can donate a proton and a base is any substance that can accept a proton.

• Give example of Bronstead-Lowry theory, pp 220.

Half reaction • Acid = H+ + base • Acid and base of a half-reaction are called conjugate pairs. • There must be a combination of two half reaction

LEWIS THEORY • Taking and giving electrons • It states that: • an acid is a substance that can accept an electron pair and • a base is a substance that can donate an electron pair. • Assumes a donation (sharing) of electrons from a base to an acid.

• Give example of Lewis acid-base.

ACID-BASE EQUILIBRIA IN WATER • When acid or base is dissolved in water, it will dissociate • Amount of ionization depends on the strength of the acid • Strong electrolyte completely dissociated • Weak electrolyte dissociated partially

• Example: a) Hydrochloric acid (strong acid) a) Acetic acid (weak acid) • Equilibrium constant • is thermodynamic activity constant

• Pure water ionizes slightly (autoprotolysis) • Equilibrium constant • is thermodynamic autoptorolysis, self-ionization, constant

• H+ will be used instead of H 3 O+ for simplification • Molar concentration will be replaced by [] • Ka and Kw are molar equilibrium constants • [H+]=1. 0× 10 -7 M = [OH-]

• Give example 7. 1