Chap 9A - Acid-base equilibria A

Describe arrhenius theory 

• (Def.): An acid is a substance that has H in its formula and dissociates in water to yield H3O+ and A base is a substance that has OH in its formula and dissociates in water to yield OH

• Eg. HCI, NaOH 

• Chemists use H+(aq) and H3O+(aq) interchangeably = solvated H+ ion

• Limitation: restricted to aqueous solutions 

• Rationale: 

  • H+ is a bare proton -> very small and has a relatively high charge density -> attracts any molecule with lone pair of electrons such as H2O

  • In aqueous solution, a water molecule forms a dative covalent bond (coordinate bond) to the H+  ion to produce hydronium ion, H3O+

  • H2O(l) + H+(aq) -> H3O+(aq)

Describe bronsted-lowry theory 

• Acid : Any species which donates a proton , must thus contain H in its formula

• Base : Any species which accepts a proton, H+ , must contain a lone pair of electrons to bind the H+ ion

• A Bronsted-Lowry acid-base reaction involves the transfer of a proton from an acid to a base (can occur between gases too) 

Describe conjugate acid and base

• Conjugate base: Species formed after acid has donated its proton 

• Conjugate acid: Species formed after base has accepted a proton 

• Conjugate acid-base pair: 2 species that differ from each other by 1 proton 

Describe lewis theory Eg. Ai(OH)3 + OH- -> Ai(OH)4

• Acid : species that accepts an electron pair 

• Base : species that donates an electron pair 

• A Lewis acid-base reaction can be viewed as a transfer of a pair of electrons from the base to the acid

Ai(OH)3 + OH- -> Ai(OH)4-

• AI(OH)3 accepts a lone pair of electrons from OH- to form AI(OH)4- -> AI(OH)3 is a lewis acid 

NOTE:

Bronsted-Lowry and Lewis theories are often used to describe specific acid-base reactions whereas Arrhenius theory is used to describe whether isolated substances are acids, bases or neither

An Arrhenius acid / base also be a Bronsted-Lowry acid / base

A Bronsted-Lowry acid / base also be a Lewis acid / base

Draw formation of H3O+ from H+ and H2O

Describe degree of dissociation and state its equation

• Strong acid and base: a = 1 (normal arrow in equation) 

• Weak acid and base: a < 1 (reversible arrows in equation) 

Define pH and state its equation

• pH (Def.): negative logarithm to base 10 of H3O+ concentration in mol dm^-3, it is the measure of the contentation of H3O+ ions in a solution (no units) 

Define pOH and its equation

• pOH (Def.): negative logarithm to base 10 of OH- concentration in mol dm^-3, it is the measure of the contentation of OH- ions in a solution (no units) 

Describe relationship between pH, H+ and OH-

Describe pH meter

• Electronic instrument that measures the potential difference (voltage) between a reference electrode and an electrode whose voltage varies with the concentration of H+ (aq) in an aqueous solution

• The measured voltage is converted to pH values so that the scale of the meter can be read directly in pH

Describe Ka and state its equation

• Units: mol dm^-3 

• Constant at constant temperature 

• Measures strength of acid 

  • Larger the Ka (smaller the pKa) -> stronger the acid 

Describe [H2O] in dilute aq solutions

• [H2O] is constant (~55.6 mol dm^-3) in dilute aqueous solutions and has been incorporated into the equilibrium constant Ka -> omitted from the equilibrium constant expression

Describe Ka and the 4 acids u need to know

Describe Kb and its equation

• Units: mol dm^-3 

• Constant at constant temperature

• Measures strength of base

  • Larger the Ka (smaller the pKa) -> stronger the base

Describe Kb and the 4 bases u need to know

Is pH or Ka/Kb more reliable?

Kb and Ka more reliable method of finding strength of acid or base than pH: pH changes with concentration, Kb and Ka constant at all dilutions (only affected by temperature)