ACIDS and BASES

Properties of acids and bases

• Acids have a sour taste : e.g vinegar owes its taste to acetic acid, and lemons and other citrus fruits contain citric acids.

GENERAL PROPRTIES

Acids:

• Acids cause colour changes in plant dyes• E.g change the colour of litmus from blue to

red.• Acid react with certain metals e.g zinc,

magnesium, and iron to produce hydrogen gas.

• E.g: reaction between hydrochloric acid and magnesium:

• 2HCl (aq) + Mg (s) MgCl2 (aq) + H2 (g)

• Acids react with carbonates and bicarbonates such as Na2CO3, CaCO3, and NaHCO3 to produce carbon dioxide gas.

• Example :• 2HCl (aq) + CaCO3 (s) CaCl2 (aq) + H2O (l) + CO2 (g)

• HCl (aq) + NaHCO3 (s) NaCl (aq) + H2O (l) + CO2 (g)

• Aqueous acids solutions conduct electricity.

• Bases have a bitter taste.• Bases feel slippery; soaps which contains

bases, exhibit this property.• Bases cause colour changes in plant dyes e.g

change the colour of litmus from red to blue.• Aqueous base solutions conduct electricity.

Bases

• Ammonia, NH3

• Soluble carbonates, CaCO3

• Hydrogencarbonates, NaHCO3

Bases which are not hydroxide

Alkalis – bases that dissolve in water

• Strong acids are all strong electrolytes that ionize completely in water (dissociate) .

• Example ; hydrochloric acids, HClnitric acids, HNO3

Perchloric acids, HClO4

Sulphuric acids, H2SO4

STRONG & WEAK ACIDS and BASES

STRONG ACIDS

• HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)

• HNO3 (aq) + H2O (l) H3O+ (aq) + NO3- (aq)

• HClO4 (aq) + H2O (l) H3O+ (aq) + ClO4- (aq)

• H2SO4 (aq) + H2O (l) H3O+ (aq) + HSO4- (aq)

• Strong bases are all strong electrolytes that ionize (dissociates) completely in solution.

• Example; NaOH (aq) Na+ (aq) + OH- (aq)

KOH (aq) K+ (aq) + OH- (aq)

Ba(OH)2 (aq) Ba2+ (aq) + 2OH- (aq)

STRONG BASES

Hydroxides of alkali metals and alkaline earth metals e.g NaOH, KOH, Ba(OH)2.

• Weak acids are acids that ionize only to limited extent in water (partially).

• Example ;

• At equilibrium, aqueous solutions of weak acids contain mixture of nonionized acid molecule, H3O+ ion and conjugate base.

WEAK ACIDS

CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)

• Note : the strength of acid can vary greatly due to differences in extent of ionization.

• The limited ionization of weak acids is related to the equilibrium constant for ionization, Ka.

HF (aq) + H2O (l) H3O+ (aq) + F- (aq)

• Weak bases are bases that ionize only to a limited extent in water.

• Example ;

• At equilibrium, there is a mixture of nonionized NH3, NH4

+, and OH- ions.

WEAK BASES

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

CH3CH2NH2 (aq) + H2O (l) CH3CH2NH3+ (aq) + OH- (aq)

ACID – BASES THEORIES

Bronsted – lewry Theory

Lewis Theory

Bronsted – Lowrey

• Acids – proton donors• Bases – proton acceptor• Example ; • HCl (g) + H2O (l) H3O+ (aq) + Cl-

(g)

Using the concept of the conjugate acid-bases pair

? ?

• An acids becomes its conjugate base when it donates a proton.

• A base becomes its conjugate acid when it accepts a proton.

Acid + base conjugate acid + conjugate base

HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)

• HCl donates its proton completely to H2O.

• For a strong acid, the reverse reaction does not occur.

• Thus, Cl- ion is a weak conjugate base.• So, strong acids form weak conjugate bases.• Weak acids form strong conjugate bases.

A strong acid

• NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

(base) (acid) ( ? ) (? )

LOWRY – BRONSTED BASE IN AMMONIA, NH3

NH3 – base (accepts a proton from H2O)H2O – Bronsted – Lowrey Acid

EQUATION IS REVERSED ;NH 4

+ acidOH- base

NH3

• Weak base• Does not accept the proton from H2O

completely.

NH4+ ion

• strong conjugate acid

H2O• A weak acid

OH- • Strong conjugate base

When the equation reversed ; NH4

+ (aq) + OH- (aq) NH3 (aq) + H2O (l)

• NH4+ (aq) + OH- (aq) NH3 (aq) + H2O (l)

NH4+ ACID

OH- BASE

• The relative strength of an acid and its conjugate base, and a base and its conjugate acid.

• Explain why a strong acid or a strong base dissociates completely in aqueous solution?

• Explain why weak acid or a weak base dissociates partially in aqueous solution?

• A strong acid completely breaks apart to give ions in solution (100% dissociation) whereas a weak acid only slightly dissociates in solution (perhaps less than 1%).

• A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H+ (aq) ions from water.

• A weak acid will, however, only partially dissociate into ions, leaving a high percentage of unreacted molecules in the solution.

H2O amphoteric

• acting as a base in the presence of an acid• acting as an acid in the presence of a base

H2O + H2O H3O+ + OH-

(acid) (base)

Bronsted – Lowrey acids and bases are NOT limited to reactions with water

• Example ;• HCl (g) + NH3 (g) NH4

+ + Cl-

(acid) (base) (conjugate acid) (conjugate base)

Question ;Is it NaOH a Bronsted – Lowrey base ?

NaOH NOT a Bronsted – Lowrey

base because it does not accept a proton

NaOH (s) + H+ no reaction

• Identify Bronsted – Lowrey acid, Bronsted – Lowrey base, conjugate acid, or conjugate base, from each of the following equation.

• H2CO3 + H2O HCO3- + H3O+

• NH4+ + H2O NH3 + H3O+

• CH3NH2 + H2O CH3NH3+ + OH-

• CH3COOH + H2O CH3COO- + H3O+

Exercise ;

Lewis Theory

• Lewis acid is an atom, ion or molecule that accepts a pair of electrons to form a coordinate covalent bond.

• Lewis base is an atom, ion or molecule that donates a pair of electron to form a coordinate covalent bond.

Example ;

Lewis base Lewis acid

H+ ACTING AS A LEWIS ACID WHEN H ACCEPTS A PAIR OF ELECTRON FROM H2O TO FORM H3O+.

CONVERSELY ;H2O IS A LEWIS BASE SINCE IT DONATES A PAIR OF ELECTRONS TO H+.

AlCl3 is acting as a Lewis acid when it accepts an electrons pair from Cl- to form AlCl4

-.

Cl- is the Lewis base – donates the electron pair to AlCl3.

Worked example 1 ;

Why is hydroxide ion and ammonia are bases according to Lewis theory?

• NH3 is a Lewis base • Donates a pair of electrons to the Lewis acid

H+ when it forms NH4+ ions

Answer

• OH- Lewis base – donates a pair of electrons to the Lewis acid H+ to form H2O molecule

For the following reactions, identify the Lewis acids and the Lewis base:Question 2:

Answer :

Lewis base Lewis acid