Some Important Chemical Reactions - Redox Reactions | Physical Chemistry PDF Download

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
(green)           (black)

 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 .

1. NH₄Cl  → NH₃ + HCl

2. (NH₄)₂SO₄  → 2NH₃+ H₂SO₄

3.  (NH₄)₃PO₄  → 3NH₃+ H₃PO₄

4. (NH₄)₂CO₃  → 2NH₃+ CO₂ +H₂O

5. 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↑  

CuCO₃→CuO+CO₂ ↑  

(pale green)(black)

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.

1. MgCl₂.6H₂O does not get completely dehydrated because MgCl₂ is hydrolysed by water to give basic MgCl₂.
   MgCl2.6H₂O  → Mg(OH)Cl + 5H₂O + HCl

2. Al₂Cl₆.12H₂O  → Al₂O₃ + 6HCl + 9H₂O

3. SnCl₂.2H₂O undergoes hydrolysis to form basic chloride

SnCl₂.2H₂O →Sn(OH)Cl + H₂O + HCl

1. 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^- + 5C₂O₄^2- + 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 =