Three theories for acids and bases
1. Arrhenius Theory
2. Bronsted-Lowry Theory of acids and bases
3. Lewis Acids and BasesIonisation
Molecular compounds that are polar can undergo breaking up into ions with the help of water molecules (Undergoes hydration)Hydronium Ions
The process of hydration or ionization leads to the formation of Hydronium ions
HA + H2O => H3O+ + A-
Dissociation
Ionic compounds break up into ions in aqueous solution this is referred to as dissociation eg. NaCl, KNO3 etc
Acids and Bases
Arrhenius Theory of acids and Bases:
Arrhenius Acids:
Acids are substances which dissolve in water to increase the hydrogen ion or hydronium ion concentration.
Eg.
HCl(aq) => H+(aq) + Cl- (aq)
H+ ions being very small cannot exist by itself, they exist as hydronium ion or [H3O+]
HNO3 =>
H2SO4 =>
Strong acids
Strong acids are those acids which have a > 99 % ionization
Weak Acid
Acetic Acid
Hydro cyanic Acid
Hydrofluoric Acid
Methanoic Acid
Ethanoic Acid
Arrhenius Bases:
According to this theory Arrhenius bases are substances which produce OH- ions when dissolved in water or in aqueous solution.
NaOH, KOH, Ca(OH)2 etc
Strong bases
Bases that undergo 100 % ionisation are called strong bases
Examples:
Strong Bases:
Sodium Hydroxide
Potassium Hydroxide
Barium Hydroxide
Weak Bases
Ammonium Hydroxide
Sodium Carbonate
Substances that dissolve in water to increase the hydroxide ion concentration are considered to be bases.
Eg.
NaOH => Na+(aq) + OH- (aq)
Ca(OH)2 => Ca2+ (aq) + 2 OH- (aq)
Degree of ionization
Degree of ionization is an indicator of weather the acid is strong and weak
Neutralization reactions:
The reaction between an acid and a base is referred to a neutralisation.
An acid and a base react to give salt and water.
The reaction is exothermic.
The enthalpy of neutralisation for a strong acid and a strong base is found to be –57kj/mol
Write the reaction and equations involved in the reaction between Acids and Bases?
Example: Nitric acid and Sodium Hydroxide,
Sulphuric acid and Potassium hydroxide,
Phosphoric acid and Sodium Hydroxide,
Ethanoic acid and Calcium hydroxide,
pH scale
The scale was introduced by Sorensen.
May be defined as the –log of the Hydrogen ion concentration written as
pH = -log [H+]
The strength of an acid or base can be deduced from the pH value
pH = -log [H+] or [H+] = 10-pH
pH Range 0 to 14
7 neutral
< 7 Acidic , >7 Basic
If the difference of pH between two acids is 1 what does it mean?
If the difference in pH between two liquids is 2 what does it mean?
Check Summary of Acids and Bases page 385
Lowry Bonsted concept of Acids and Bases
An Acid is a proton donor. A Base is a proton Acceptor
Conjugate Acid and Conjugate base:
Examples:
HCl + H2O => H3O+ + Cl- All are aqueous solutions
Acid Base Conjugate Acid Conjugate Base
NH3 + H2O => NH4+ + OH- All are aqueous solutions
Base Acid Conjugate Acid Conjugate Base
Identify the acid, base, conjugate acid & conjugate base in the following reactions
H2SO4 + HCO3- =>
H3PO4 + OCl- =>
H3O+ + HSO3- =>
Amphiprotic
Some general aspects of acids and basesA substance that can either accept or donate a proton is referred to as an amphiprotic compound eg. Water is an example.
Illustrated with water as an example as in the above examples
HCO3- + H3O+ => H2CO3 + H2O All are aqueous solutions
Base Acid Conjugate Acid Conjugate Base
HCO3- + OH- => CO32-‑ + H2O All are aqueous solutions
Acid Base Conjugate Base Conjugate Acid
An acid and a conjugate base differ by a proton.
Similarly a base and a conjugate acid differ by a proton.
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