At a school laboratory:
(i)An acid may be defined as a substance that turn litmus red.
(ii)A base may be defined as a substance that turn litmus blue.
Litmus is a lichen found mainly in West Africa. It changes its colour depending on whether the solution it is in, is basic/alkaline or acidic.It is thus able to identify/show whether
1. An acid is a substance that dissolves in water to form H+/H3O+ as the only positive ion/cation. This is called the Arrhenius definition of an acid. From this definition, an acid dissociate/ionize in water releasing H+ thus:
HCl(aq) -> H+ (aq) + Cl– (aq)
HNO3(aq) -> H+ (aq) + NO3– (aq)
CH3COOH(aq) -> H+ (aq) + CH3COO–(aq)
H2SO4(aq) -> 2H+ (aq) + SO42-(aq)
H2CO3(aq) -> 2H+ (aq) + CO32-(aq)
H3PO4(aq) -> 3H+ (aq) + PO43-(aq)
2.A base is a substance which dissolves in water to form OH– as the only negatively charged ion/anion.
This is called Arrhenius definition of a base.
From this definition, a base dissociate/ionize in water releasing OH– thus:
KOH(aq) -> K+(aq) + OH–(aq)
NaOH(aq) -> Na+(aq) + OH–(aq)
NH4OH(aq) -> NH4+(aq) + OH–(aq)
Ca(OH)2(aq) -> Ca2+(aq) + 2OH–(aq)
Mg(OH)2(aq) -> Mg2+(aq) + 2OH–(aq)
3. An acid is a proton donor.
A base is a proton acceptor.
This is called Bronsted-Lowry definition of acids and bases.
From this definition, an acid donates H+ .
H+ has no electrons and neutrons .It contains only a proton.
Examples
I. From the equation:
HCl(aq) + H2O(l) === H3O+(aq) + Cl– (aq)
(a)(i)For the forward reaction from left to right, H2O gains a proton to form H3O+ and thus H2O is a proton acceptor .It is a Bronsted-Lowry base
(ii) For the backward reaction from right to left, H3O+ donates a proton to form H2O and thus H3O+ is an ‘opposite’ proton donor. It is a Bronsted-Lowry conjugate acid
(b)(i)For the forward reaction from left to right, HCl donates a proton to form Cl– and thus HCl is a proton donor .
It is a Bronsted-Lowry acid
(ii) For the backward reaction from right to left, Cl– gains a proton to form HCl and thus Cl– is an ‘opposite’ proton acceptor.
It is a Bronsted-Lowry conjugate base.
Every base /acid from Bronsted-Lowry definition thus must have a conjugate product/reactant.
II. From the equation:
HCl(aq) + NH3(aq) === NH4+(aq) + Cl– (aq)
(a)(i)For the forward reaction from left to right, NH3 gains a proton to form NH4+ and thus NH3 is a proton acceptor .
It is a Bronsted-Lowry base
(ii) For the backward reaction from right to left, NH4+ donates a proton to form NH3 and thus NH4+ is an ‘opposite’ proton donor.
It is a Bronsted-Lowry conjugate acid
(b)(i)For the forward reaction from left to right, HCl donates a proton to form Cl– and thus HCl is a proton donor .
It is a Bronsted-Lowry acid
(ii) For the backward reaction from right to left, Cl– gains a proton to form HCl and thus Cl– is an ‘opposite’ proton acceptor.
It is a Bronsted-Lowry conjugate base.
4. Acids and bases show acidic and alkaline properties/characteristics only in water but not in other solvents e.g.
(a)Hydrogen chloride gas dissolves in water to form hydrochloric acid Hydrochloric acid dissociates/ionizes in water to free H+(aq)/H3O+(aq) ions. The free H3O+(aq) / H+(aq) ions are responsible for:
(i)turning blue litmus paper/solution red.
(ii)show pH value 1/2/3/4/5/6
(iii)are good electrolytes/conductors of electricity/undergo electrolysis.
(iv)react with metals to produce /evolve hydrogen gas and a salt. i.e.
Ionically:
-For a monovalent metal: 2M(s) + 2H+(aq) -> 2M+(aq) + H2(g)
-For a divalent metal: M(s) + 2H+(aq) -> M2+(aq) + H2(g)
-For a trivalent metal: 2M(s) + 6H+(aq) -> 2M3+(aq) + 3H2(g)
Examples:
-For a monovalent metal: 2Na(s) + 2H+(aq) -> 2Na+(aq) + H2(g)
-For a divalent metal: Ca(s) + 2H+(aq) -> Ca2+(aq) + H2(g)
-For a trivalent metal: 2Al(s) + 6H+(aq) -> 2Al3+(aq) + 3H2(g)
(v)react with metal carbonates and hhydrogen carbonates to produce /evolve carbon(IV)oxide gas ,water and a salt. i.e.
Ionically:
-For a monovalent metal: M2CO3(s)+ 2H+(aq) -> 2M+(aq) + H2O (l)+ CO2(g)
MHCO3(s)+ H+(aq) -> M+(aq) + H2O (l)+ CO2(g)
-For a divalent metal: MCO3(s)+ 2H+(aq) -> M2+(aq) + H2O (l)+ CO2(g)
M(HCO3) 2(aq)+2H+(aq) ->M2+(aq)+2H2O(l)+2CO2(g)
Examples:
-For a monovalent metal: K2CO3(s)+ 2H+(aq) -> 2K+(aq) + H2O (l)+ CO2(g)
NH4HCO3(s)+ H+(aq) -> NH4+(aq) + H2O (l)+ CO2(g)
-For a divalent metal: ZnCO3(s)+ 2H+(aq) -> Zn2+(aq) + H2O (l)+ CO2(g)
Mg(HCO3) 2(aq)+2H+(aq) ->Mg2+(aq)+2H2O(l)+2CO2(g)
(vi)neutralize metal oxides/hydroxides to salt and water only. i.e.
Ionically:
-For a monovalent metal: M2O(s) + 2H+(aq) -> 2M+(aq) + H2O (l)
MOH(aq) + H+(aq) -> M+(aq) + H2O (l)
-For a divalent metal: MO(s) + 2H+(aq) -> M2+(aq) + H2O (l)
M(OH) 2(s) + 2H+(aq) -> M2+(aq) + 2H2O(l)
-For a trivalent metal: M2O3(s) + 6H+(aq) -> 2M3+(aq) + 3H2O (l)
M(OH) 3(s) + 3H+(aq) -> M3+(aq) + 3H2O(l)
Examples:
-For a monovalent metal: K2O(s) + 2H+(aq) -> 2K+(aq) + H2O (l)
NH4OH(aq) + H+(aq) -> NH4+(aq) + H2O (l)
-For a divalent metal: ZnO (s) + 2H+(aq) -> Zn2+(aq) + H2O (l)
Pb(OH) 2(s) + 2H+(aq) -> Pb2+(aq) + 2H2O(l)
(b)Hydrogen chloride gas dissolves in methylbenzene /benzene but does not dissociate /ionize into free ions.
It exists in molecular state showing none of the above properties.
(c)Ammonia gas dissolves in water to form aqueous ammonia which dissociate/ionize to free NH4+ (aq) and OH–(aq) ions.
This dissociation/ionization makes aqueous ammonia to:
(i)turn litmus paper/solution blue.
(ii)have pH 8/9/10/11
(iii)be a good electrical conductor
(iv)react with acids to form ammonium salt and water only.
NH4OH(aq) + HCl(aq) -> NH4Cl(aq) + H2O(l)
(d)Ammonia gas dissolves in methylbenzene/benzene /kerosene but does not dissociate into free ions therefore existing as molecules
6. Solvents are either polar or non-polar.
A polar solvent is one which dissolves ionic compounds and other polar solvents.
Note:Water is polar .It is made up of :
Oxygen atom is partially negative and two hydrogen atoms which are partially positive.
They surround the free H+ and Cl– ions.
A non polar solvent is one which dissolved non-polar substances and covalent compounds. If a polar ionic compound is dissolved in non-polar solvent ,it does not ionize/dissociate into free ions
7. Some acids and bases are strong while others are weak.
(a)A strong acid/base is one which is fully/wholly/completely dissociated / ionized into many free H+ /OH– ions i.e.
I. Strong acids exists more as free H+ ions than molecules. e.g.
HCl(aq) H+(aq) + Cl– (aq) (molecules) (cation) (anion)
HNO3(aq) H+(aq) + NO3–(aq) (molecules) (cation) (anion)
H2SO4(aq) 2H+(aq) + SO42-(aq) (molecules) (cation) (anion)
II. Strong bases/alkalis exists more as free OH– ions than molecules. e.g.
KOH(aq) K+(aq) + OH– (aq) (molecules) (cation) (anion)
NaOH(aq) Na+(aq) + OH–(aq) (molecules) (cation) (anion)
(b) A weak base/acid is one which is partially /partly dissociated /ionized in water into free OH– (aq) and H+(aq) ions.
I. Weak acids exists more as molecules than as free H+ ions. e.g.
CH3COOH(aq) H+(aq) + CH3COO– (aq) (molecules) (cation) (anion)
H3PO4(aq) 3H+(aq) + PO43-(aq) (molecules) (cation) (anion)
H2CO3(aq) 2H+(aq) + CO32-(aq) (molecules) (cation) (anion)
II. Weak bases/alkalis exists more as molecules than free OH– ions. e.g.
NH4OH(aq) NH4+(aq) + OH– (aq) (molecules) (cation) (anion) Ca(OH)2(aq) Ca2+(aq) + 2OH–(aq) (molecules) (cation) (anion)
Mg(OH)2(aq) Mg2+(aq) + 2OH–(aq) (molecules) (cation) (anion)
8. The concentration of an acid/base/alkali is based on the number of moles of acid/bases dissolved in a decimeter(litre)of the solution.
An acid/base/alkali with more acid/base/alkali in a decimeter(litre) of solution is said to be concentrated while that with less is said to be dilute.
9. (a) (i)strong acids have pH 1/2/3 while weak acids have high pH 4/5/6.
(ii)a neutral solution have pH 7
(iii)strong alkalis/bases have pH 12/13/14 while weak bases/alkalis have pH 11/10 /9 / 8.
(b) pH is a measure of H+(aq) concentration in a solution.
The higher the H+(aq)ions concentration ;
-the higher the acidity
-the lower the pH
-the lower the concentration of OH–(aq)
-the lower the alkalinity
At pH 7 , a solution has equal concentration of H+(aq) and OH–(aq).
Beyond pH 7,the concentration of the OH–(aq) increases as the H+(aq) ions decreases.
10.(a) When acids /bases dissolve in water, the ions present in the solution conduct electricity.
The more the dissociation the higher the yield of ions and the greater the electrical conductivity of the solution.
A compound that conducts electricity in an electrolyte and thus a compound showing high electrical conductivity is a strong electrolyte while a compound showing low electrical conductivity is a weak electrolyte.
(b) Practically, a bright light on a bulb ,a high voltage reading from a voltmeter high ammeter reading from an ammeter, a big deflection on a galvanometer is an indicator of strong electrolyte(acid/base) and the opposite for weak electrolytes(acids/base)
11. Some compounds exhibit/show both properties of acids and bases/alkalis.
A substance that reacts with both acids and bases is said to be amphotellic.
The examples below show the amphotellic properties of:
(a) Zinc (II)oxide(ZnO) and Zinc hydroxide(Zn(OH)2)
(i)When ½ spatula full of Zinc(II)oxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the oxide shows basic properties by reacting with an acid to form a simple salt and water only.
Basic oxide + Acid -> salt + water
Examples:
Chemical equation
ZnO(s) + 2HNO3(aq) -> Zn(NO3) 2 (aq) + H2O(l)
ZnO(s) + 2HCl(aq) -> ZnCl2 (aq) + H2O(l)
ZnO(s) + H2SO4(aq) -> ZnSO4 (aq) + H2O(l)
Ionic equation
ZnO(s) + 2H+ (aq) -> Zn 2+ (aq) + H2O(l)
(ii) when reacting with sodium hydroxide, the oxide shows acidic properties by reacting with a base to form a complex salt.
Basic oxide + Base/alkali + Water -> Complex salt
Examples:
Chemical equation
1.When Zinc oxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxozincate(II) complex salt.
ZnO(s) + 2NaOH(aq) + H2O(l) -> Na2Zn(OH) 4(aq)
2.When Zinc oxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxozincate(II) complex salt.
ZnO(s) + 2KOH(aq) + H2O(l) -> K2Zn(OH) 4(aq)
Ionic equation
ZnO(s) + 2OH–(aq) + H2O(l) -> 2[Zn(OH) 4]2- (aq)
(ii)When Zinc(II)hydroxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the hydroxide shows basic properties. It reacts with an acid to form a simple salt and water only.
Basic hydroxide + Acid -> salt + water
Examples:
Chemical equation
Zn(OH) 2 (s) + 2HNO3(aq) -> Zn(NO3) 2 (aq) + 2H2O(l)
Zn(OH) 2 (s) + 2HCl(aq) -> ZnCl2 (aq) + 2H2O(l)
Zn(OH) 2 (s) + H2SO4(aq) -> ZnSO4 (aq) + 2H2O(l)
Ionic equation
Zn(OH) 2 (s) + 2H+ (aq) -> Zn 2+ (aq) + 2H2O(l)
(ii) when reacting with sodium hydroxide, the hydroxide shows acidic properties by reacting with a base to form a complex salt.
Basic hydroxide + Base/alkali -> Complex salt
Examples:
Chemical equation
1.When Zinc hydroxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxozincate(II) complex salt.
Zn(OH) 2 (s) + 2NaOH(aq) -> Na2Zn(OH) 4(aq)
2.When Zinc hydroxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxozincate(II) complex salt.
Zn(OH) 2 (s) + 2KOH(aq) -> K2Zn(OH) 4(aq)
Ionic equation
Zn(OH) 2 (s) + 2OH–(aq) -> 2[Zn(OH) 4]2- (aq)
(b) Lead (II)oxide(PbO) and Lead(II) hydroxide (Pb(OH)2)
(i)When ½ spatula full of Lead(II)oxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the oxide shows basic properties by reacting with an acid to form a simple salt and water only. All other Lead salts are insoluble.
Chemical equation
PbO(s) + 2HNO3(aq) -> Pb(NO3) 2 (aq) + H2O(l)
Ionic equation
PbO(s) + 2H+ (aq) -> Pb 2+ (aq) + H2O(l)
(ii) when reacting with sodium hydroxide, the oxide shows acidic properties by reacting with a base to form a complex salt.
Chemical equation
1.When Lead(II) oxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoplumbate(II) complex salt.
PbO(s) + 2NaOH(aq) + H2O(l) -> Na2Pb(OH) 4(aq)
2.When Lead(II) oxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoplumbate(II) complex salt.
PbO(s) + 2KOH(aq) + H2O(l) -> K2Pb(OH) 4(aq)
Ionic equation
PbO(s) + 2OH–(aq) + H2O(l) -> 2[Pb(OH) 4]2- (aq)
(ii)When Lead(II)hydroxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the hydroxide shows basic properties. It reacts with the acid to form a simple salt and water only.
Chemical equation
Pb(OH) 2 (s) + 2HNO3(aq) -> Pb(NO3) 2 (aq) + 2H2O(l)
Ionic equation
Pb(OH) 2 (s) + 2H+ (aq) -> Pb 2+ (aq) + 2H2O(l)
(ii) when reacting with sodium hydroxide, the hydroxide shows acidic properties. It reacts with a base to form a complex salt.
Chemical equation
1.When Lead(II) hydroxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoplumbate(II) complex salt.
Pb(OH) 2 (s) + 2NaOH(aq) -> Na2Pb(OH) 4(aq)
2.When Lead(II) hydroxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoplumbate(II) complex salt.
Pb(OH) 2 (s) + 2KOH(aq) -> K2Pb(OH) 4(aq)
Ionic equation
Pb(OH) 2 (s) + 2OH–(aq) -> 2[Pb(OH) 4]2- (aq)
(c)Aluminium(III)oxide(Al2O3) and Aluminium(III)hydroxide(Al(OH)3)
(i)When ½ spatula full of Aluminium(III)oxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the oxide shows basic properties by reacting with an acid to form a simple salt and water only.
Chemical equation
Al2O3 (s) + 6HNO3(aq) -> Al(NO3)3 (aq) + 3H2O(l)
Al2O3 (s) + 6HCl(aq) -> AlCl3 (aq) + 3H2O(l)
Al2O3 (s) + 3H2SO4(aq) -> Al2(SO4)3 (aq) + 3H2O(l)
Ionic equation
Al2O3 (s) + 3H+ (aq) -> Al 3+ (aq) + 3H2O(l)
(ii) when reacting with sodium hydroxide, the oxide shows acidic properties by reacting with a base to form a complex salt.
Chemical equation
1.When Aluminium(III) oxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoaluminate(III) complex salt.
Al2O3 (s) + 2NaOH(aq) + 3H2O(l) -> 2NaAl(OH) 4(aq)
2.When Aluminium(III) oxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoaluminate(II) complex salt.
Al2O3 (s) + 2KOH(aq) + 3H2O(l) -> 2NaAl(OH) 4(aq)
Ionic equation
Al2O3 (s) + 2OH–(aq) + 3H2O(l) -> 2[Al(OH) 4]– (aq)
(ii)When Aluminium(III)hydroxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the hydroxide shows basic properties. It reacts with the acid to form a simple salt and water only.
Chemical equation
Al(OH) 3 (s) + 3HNO3(aq) -> Al(NO3)3 (aq) + 3H2O(l)
Al(OH)3 (s) + 3HCl(aq) -> AlCl3 (aq) + 3H2O(l)
2Al(OH)3 (s) + 3H2SO4(aq) -> Al2(SO4)3 (aq) + 3H2O(l)
Ionic equation
Al(OH)3 (s) + 3H+ (aq) -> Al 3+ (aq) + 3H2O(l)
(ii) when reacting with sodium hydroxide, the hydroxide shows acidic properties. It reacts with a base to form a complex salt.
Chemical equation
1.When aluminium(III) hydroxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoaluminate(III) complex salt.
Al(OH) 3 (s) + NaOH(aq) -> NaAl(OH) 4(aq)
2.When aluminium(III) hydroxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoaluminate(III) complex salt.
Al(OH) 3 (s) + KOH(aq) -> KAl(OH) 4(aq)
Ionic equation
Al(OH) 3 (s) + OH–(aq) -> [Al(OH) 4]– (aq)
Summary of amphotellic oxides/hydroxides
| Oxide | Hydroxide | Formula of simple salt from nitric (V)acid | Formula of complex salt from sodium hydroxide |
| ZnO | Zn(OH)2 | Zn(NO3)2 | Na2Zn(OH)4 [Zn(OH)4]2-(aq) Sodium tetrahydroxozincate(II) |
| PbO | Pb(OH)2 | Pb(NO3)2 | Na2Pb(OH)4 [Pb(OH)4]2-(aq) Sodium tetrahydroxoplumbate(II) |
| Al2O3 | Al(OH) 3 | Al(NO3)3 | NaAl(OH)4 [Al(OH)4]–(aq) Sodium tetrahydroxoaluminate(II) |
