11. SOME IMPORTANT CHEMICAL REACTIONS
There are some chemical reactions, which a student should remember in order to solve problems on stoichiometry. These are categorized are
11.1COMBINATION OF ELEMENTS WITH OXYGEN
(i) Heating mercury at its boiling point in air 2Hg + O2 → 2HgO ( red mercuric oxide )
(ii) Heating magnesium in air. It forms mostly magnesium oxide and some magnesium nitride
2Mg + O2 → 2MgO , 3Mg + N2 → Mg3N2
(iii) Calcium behaves similarly
2Ca + O2 → 2CaO , 3Ca + N2 → Ca3N2
(iv) Silver does not combine with oxygen, as Ag2O is unstable to heat.
(v) Many non-metals burn in O2 forming their respective oxides.
2H2 + O2 → 2H2O ; S + O2 → SO2
C + O2 → CO2 ; P4 + 5O2 → P4O10
Cl2 , Br2 and I2 do not directly combine with oxygen.
11.2 ACTION OF HEAT ON CERTAIN OXIDES :
(i) Mercuric oxide and silver oxide are unstable to heat and decompose readily.
(ii) Various higher oxides, dioxides, mixed oxides and peroxides are decomposed to oxygen and a lower oxide.
2Pb3O4 → 6PbO + O2 ; 2PbO2 → 2PbO + O2
3MnO2 → Mn3O4 + O2 ; 2BaO2 → 2BaO + O2
11.3 COMPOUNDS RICH IN OXYGEN DECOMPOSE TO GIVE OXYGEN
(i) 2KNO3 → 2KNO + O2↑
(ii) 2KMnO4 → K2MnO4 + MnO2 + O2↑
( purple ) ( green ) ( black )
(iii) 4K2Cr2O7 →4K2CrO4 + 2Cr2O3 + 3O2↑
(orange-red) (yellow) (green)
Potassium chlorate when heated just above its melting point decomposes into potassium perchlorate and potassium chloride. This reaction is called disproportionation or auto-oxidation and auto-reduction. On heating further, KClO4 decomposes to KCl and oxygen.
(iv) 4KClO3 →3KClO4 + KCl
(ii) KClO4 → KCl + 2O2
11.4 ACTION OF HEAT ON NITRATES
Generally heavy metal nitrates decompose to metal oxide, reddish brown nitrogen dioxide gas and oxygen.
1. Lead nitrate decomposes to PbO, NO2 , O2
2Pb(NO3)2 →2PbO + 4NO2
Litharge or lead (II) oxide (red when hot and yellow when cold )
2. Cupric nitrate decomposes to CuO, NO2 and O2
2Cu(NO3)2 →2CuO + 4NO2 + O2
3. Zinc nitrate decomposes ZnO, NO2 and O2
(zinc oxide, yellow when hot and white when cold )
Knowing the colors of some oxides will be useful in qualitative analysis.
4. Nitrates of mercury and silver, whose oxides are unstable, decompose into the metal, NO2and O2. Hg(NO3)2 → Hg + 2NO2 + O2
2AgNO3 →2Ag + 2NO2 +O2
5. Alkali metal nitrates decompose to give the metal nitrite and O2( No reddish brown NO2 gas)
2KNO3 →2KNO3 + O2
2NaNO3 →2NaNO2 + O2
6. Ammonium nitrate on heating leaves no residue and forms nitrous oxide and steam.
NH4NO3 →N2O + 2H2O
11.5 ACTION OF HEAT ON AMMONIUM COMPOUNDS
Generally an ammonium compound decomposes into ammonia and an acid or acidic oxide if the acids is unstable to heat .
NH4Cl → NH3 + HCl
(NH4)2SO4 → 2NH3+ H2SO4
(NH4)3PO4 → 3NH3+ H3PO4
(NH4)2CO3 → 2NH3+ CO2 +H2O
Ammonium compounds which do not give ammonia on heating are ammonium nitrate, ammonium nitrite and ammonium dichromate.
NH4NO3→ N2O + 2H2O
NH4NO2 →N2 + 2H2O
(NH4)2Cr2O7 →N2 + 4H2O + Cr2O3 (green fluffy chromic oxide )
11.6 ACTION OF HEAT ON METALLIC CARBONATES
(I)Generally metallic carbonates decompose to give metal oxide and CO2.
CaCO3 →9000C →CaO +CO2↑
MgCO3 →MgO +CO2↑
PbCO3 →PbO(yellow) + CO2
ZnCO3 →ZnO +CO2 ↑
(II) Carbonates of strongly electropositive metals ( alkali metals except lithium ) do not decompose on heating.
(III) Silver carbonate decomposes to give the metal, CO2 and O2
2Ag2CO3 →4Ag + 2CO2 + O2
(IV) Ammonium carbonate ( smelling salt ) decomposes to give NH3, H2O and CO2. All the products are in gaseous phase and there is no residue left.
(NH4)2CO3→ 2NH3 + H2O + CO2
11.7 ACTION OF HEAT ON METALLIC BICARBONATES
Only NaHCO3 and KHCO3 are solids; others are known in solution. All Bicarbonates decompose to give the metal carbonate, H2O and CO2.
2NaHCO3 →Na2CO3 + H2O + CO2 ↑
Ca(HCO3)2 →CaCO3 + H2O + CO2 ↑
Mg(HCO3)2 →MgCO3 + H2O + CO2 ↑
11.8 ACTION OF HEAT ON CERTAIN HYDRATED CHLORIDES
Hydrated halides on heating are converted to oxides, H2O and halo acids.
MgCl2.6H2O does not get completely dehydrated because MgCl2 is hydrolysed by water to give basic MgCl2.
MgCl2.6H2O → Mg(OH)Cl + 5H2O + HCl
Al2Cl6.12H2O → Al2O3 + 6HCl + 9H2O
SnCl2.2H2O undergoes hydrolysis to form basic chloride
SnCl2.2H2O →Sn(OH)Cl + H2O + HCl
On heating, certain halides of metal ions in higher oxidation state changes to halides of lower oxidation state.
2FeCl3 →2FeCl2 + Cl2
2CuCl2 →Cu2Cl2 + Cl2
11.9 ACTION OF HEAT ON SOME OTHER COMPOUNDS
When sodium sulphite is heated, it undergoes disproportionation reaction.
4Na2SO3 →3Na2SO4 + Na2S
Sodium thiosulphate Na2S2O3.5H2O loses water of hydration and becomes anhydrous salt, which on further heating gives a mixture of sodium sulphate, sodium sulphide and sulphur.
Na2S2O3.5H2O →Na2S2O3 + 5H2O
4Na2S2O3 → 3Na2SO4 + Na2S + 4S
When hydrated copper sulphate ( blue vitriol ) is heated, CuO and SO2 are formed.
Gypsum CaSO4.2H2O, when heated to 120-1300 C forms a hemihydrate called Plaster Of Paris.
CaSO4.2H2O →CaSO4.1/2H2O + 3/2H2O
If heated above 2000C, it forms anhydrous calcium sulphate which does not set with water.
Green vitriol FeSO4.7H2O, when heated forms Fe2O3 , SO2 , SO3 and H2O
FeSO4.7H2O→ Fe2O3 +SO2 + SO3 + 14H2O
11.10 ACID-BASE REACTIONS
A strong acid liberates a relatively weaker acid from its salt. The common strong acids are perchloric acid, sulphuric acid, hydrochloric acid and nitric acid. The weaker acids are carbonic acid, sulphurous acid, hydrocyanic acid and most of the organic acids. Thus conc. sulphuric acid displaces most other acids from their salts.
KCl + H2SO4 →KHSO4 + HCl
KNO3 + H2SO4 →KHSO4 + HNO3
Ca3(PO4)2 + 3H2SO4 →3CaSO4 + 2H3PO4
CH3CO2Na + HCl →CH3CO2H + NaCl
Al most all the acids displace carbonic acid from carbonates and bicarbonates. Since carbonic acid is unstable , it decomposes liberating CO2 with effervescence ( Test For Acids ).
Na2CO3 + 2HCl →2NaCl + H2O + CO2 ↑
Na2CO3 + H2SO4 →Na2SO4 + H2O + CO2 ↑
KHCO3 + HNO3 →KNO3 + H2O + CO2 ↑
A strong base can displace a weak base from a salt of strong acid and weak base.
+ NaOH →NaCl + NH4OH
A salt of strong acid and strong base do not react with any acid or base.
11.11 SOME OTHER USEFUL REACTIONS
O3 + 2KI + H2O →2KOH + I2+ O2
BaCO3 + 2HCl → BaCl2 + CO2 + H2O
BaCl2 + H2CrO4 → BaCrO4 + 2HCl
2BaCrO4 + 6KI + 8H2SO4 →3I2 + 2BaSO4 + 3K2SO4 + Cr2(SO4)3 + 8H2O
2Cu SO4 + 4KI → Cu2I2 +I2 + 2K2SO4
2MnO4- + 5C2O42- + 16H+ → 2Mn+2 + 10CO2 + 8H2O
2KMnO4+ 10FeSO4 + 4H2SO4 → 2MnSO4 + 5Fe2(SO4)3 + K2SO4 + 8H2O
Mn3O4 + 2FeSO4 + 4H2SO4 → 3MnSO4 + Fe2(SO4)3 + 4H2O
MnO4- + 5Fe2+ + 8H+ → Mn+2+5Fe3+ + 4H2O
KMnO4 + 5 KI + 4H2SO4 → 3K2SO4 + MnSO4 +5/2 I2 + 4H2O
K2Cr2O7 + 6KI + 7 H2SO4 →4 K2SO4 + Cr2(SO4)3 + 3I2 + 7H2O
11.12 HARDNESS OF WATER
The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of Ca and Mg. The temporary hardness is due to the bicarbonates and permanent hardness is due to chlorides and sulphates of Ca and Mg. The extent of hardness is known as degree of hardness defined as the number of parts by weight of CaCO3 present in one million parts by weight of water.
Hardness of water =