Chemical Changes and Reactions|Class 9

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Chemical Changes and Reactions

A chemical reaction can occur by two means-

  1. Reactant+ Reagent → Product
  2. Reactant + Reagent → Intermediate → Product

A reaction can take place only when –

  1. The molecules collide among themselves.
  2. The energy of the reactants should cross the threshold energy by absorbing activation energy.
  3. The molecules should orient themselves in a proper orientation.

Conditions necessary for a chemical change-

Mixing or close contact-

  • P4 + 6I2 → 4PI3
  • Pb(NO3)2 + 2KI → 2KNO3 + PbI2

In molten state or in aqueous solution:

  • (COOH)2 + Na2CO3 → (COONa)2 + CO2 + H2O
  • NaCl + AgNO3 → AgCl + NaNO3
  • Any double decomposition reaction.

Heat:

  • NH4NO3 → N2O + 2H2O
  • NH4NO2 → N2 + 2H2O
  • 4HNO3 → 2H2O + 4NO2 + O2
  • 2NaNO3 → 2NaNO2 + O2
  • 2Ca(NO3)2 → 2CaO + 4NO2 + O2
  • 2Zn(NO3)2 → 2ZnO + 4NO2 + O2
  • 2Pb(NO3)2 → 2PbO + 4NO2 + O2
  • 2Cu(NO3)2 → 2CuO + 4NO2 + O2
  • 2AgNO3 → 2Ag + 2NO2 + O2
  • Hg(NO3)2 → Hg + 2NO2 + O2
  • ZnCO3 → ZnO + CO2
  • CuCO3 → CuO + CO2

Light :

  • 2AgNO3 → 2Ag + 2NO2 + O2
  • 2H2O2→2H2O + O2
  • H2 + Cl2 → 2HCl
  • 2Cl2 + 2H2O →4HCl + O2
  • CH4 + Cl2 → CH3Cl + HCl

Electricity:

  • 2NaCl → 2Na + Cl2
  • PbBr2 → Pb + Br2
  • 2H2O → 2H2 + O2

Pressure:

  • N2 + 3H2 2NH3   200 atm
  • HgCl2 + 2KI → HgI2 + 2KCl
  • C6H5Cl + NaOH → C6H5ONa + HCl

Catalyst:

Chemical substances which can increase or decrease the speed of a reaction but itself remain intact in its mass.

Positive catalyst:

Chemical substances which can increase the speed of a reaction but itself remain intact in its mass.

Preparation ofReactionPositive catalyst
Ammonia(NH3) [Haber’s process]N2 + 3H2  2NH3Fe
Nitric Acid (HNO3 ) [Ostwald process]4NH3 + 5O2 →6H2O + 4NOPt
Sulphuric acid(H2SO4) [Contact process]2SO2 + O2 → 2SO3V2O5
Ethane(C2H6)C2H4 + H2 → C2H6Pt or Pd or Ni
Chlorobenzene(C6H5Cl)C6H6 + Cl2 → C6H5Cl + HClFe
Oxygen2KClO3 → 2KCl + 3O2MnO2
Hydrogen (Bosch Process)(CO + H2) + H2O → CO2 + 2H2Fe2O3
Decomposition of hydrogen peroxide2H2O2    2H2O + O2Pt, Ag, Co, Fe, Cu, Au

Negative catalyst:

Chemical substances which can decrease the speed of a reaction but itself remain intact in its mass.

 ReactionNegative catalyst
Decomposition of hydrogen peroxide2H2O2    2H2O + O2Glycerine, acetanilide, phosphoric acid, urea, alcohol
Oxidation of chloroformCHCl3 + O2 → COCl2Alcohol

Promoters:

Chemical substances that increase the efficiency of the catalyst.

  1. Mo or Al2O3 acts as a promoter of Fe during Haber’s Process.
  2. Chromic oxide acts as a promoter of Fe2O3 during the Bosch Process.

Catalyst Poison:

Chemical substances that can decrease the efficiency of the catalyst.

PbSO4+ S+ Quiniline acts as a poison during the preparation of acetaldehyde from Acetyl chloride.CH3COCl + H2 → CH3CHO + HCl

Characteristics of Chemical Reactions:

Evolution of gas:

  • Zn + 2HCl → ZnCl2 + H2
  • Na2CO3+ 2HCl → 2NaCl + H2O + CO2
  • Na2SO3 + 2HCl → 2NaCl + H2O + SO2
  • Na2S+ 2HCl → 2NaCl + H2S↑
  • MnO2 + 4HCl → MnCl2 + Cl2↑ + 2H2O
  • 2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3
  • Cu + 4HNO3 → Cu(NO3)2 + 2H2O + 2NO2

Change of colour:

  • Fe + CuSO4(Blue solution) → FeSO4 (Green solution)+ Cu(reddish brown residue)
  • Zn + CuSO4(Blue solution) → ZnSO4 (Colourless solution)+ Cu(reddish brown residue)

Formation of precipitation:

ReactionColour of precipitation
Pb(NO3)2 + 2HCl → PbCl2 ↓+ 2HNO3White
Hg2(NO3)2 + 2HCl → Hg2Cl2 ↓ + 2HNO3White
AgNO3 + HCl → AgCl ↓ + HNO3White
BaCl2 + H2SO4 → BaSO4 ↓ + 2HClWhite
Pb(NO3)2 + H2SO4 → PbSO4 ↓ + 2HNO3  White
CaCl2 + 2NaOH → Ca(OH)2 ↓+ 2NaClWhite
ZnCl2 + 2NaOH → Zn(OH)2 ↓+ 2NaClGelatinous white
Pb(NO3)2 + 2NaOH → Pb(OH)2 ↓ + 2NaNO3Chalky white
FeSO4 + 2NaOH → Fe(OH)2 ↓+ Na2SO4Dirty green
FeCl3 + 3NaOH → Fe(OH)3 ↓+ 3NaClReddish brown
CuSO4 + 2NaOH → Cu(OH)2↓ + Na2SO4Pale blue
CuSO4 + H2S→ CuS ↓+ H2SO4Black

Change of state:

NH3 (g)+ HCl (g)→ NH4Cl(s)

Types of chemical reactions:

Synthesis or Direct combination:

Type of reaction where two or more substances combine to form a single product.

  • NH3 + HCl → NH4Cl
  • 2NO + O2 → 2NO2
  • 2NH3 + H2SO4 → (NH4)2SO4
  • 6NH3 + P2O5 + 3H2O → 2(NH4)3PO4
  • N2 + 3H2 2NH3
  • Pb + S → PbS
  • Fe + S → FeS
  • 2Fe + 3Cl2 → 2FeCl3

Decomposition:

The type of reaction where is compound is broken down into two or more elements or compounds.

Thermal decomposition:

The type of reaction where is compound is broken down into two or more elements or compounds by applying heat.

  • NH4NO3 → N2O + 2H2O
  • NH4NO2 → N2 + 2H2O
  • 4HNO3 → 2H2O + 4NO2 + O2
  • 2NaNO3 → 2NaNO2 + O2
  • 2Ca(NO3)2 → 2CaO + 4NO2 + O2
  • 2Zn(NO3)2 → 2ZnO + 4NO2 + O2
  • 2Pb(NO3)2 → 2PbO + 4NO2 + O2
  • 2Cu(NO3)2 → 2CuO + 4NO2 + O2
  • 2AgNO3 → 2Ag + 2NO2 + O2
  • Hg(NO3)2 → Hg + 2NO2 + O2
  • ZnCO3 → ZnO + CO2
  • CuCO3 → CuO + CO2
  • MgCO3 → MgO + CO2
  • 2Ag2CO3 → 4Ag + O2 + 2CO2
  • (NH4)2Cr27→ Cr2O3 + 4H2O + N2
  • Pb(OH)2 → PbO + H2O
  • Zn(OH)2 → ZnO + H2O
  • Ca(OH)2 → CaO + H2O
  • Cu(OH)2 → CuO + H2O
  • 4AgOH → 4Ag + O2 + 2H2O

Photochemical Decomposition:

The type of reaction where is compound is broken down into two or more elements or compounds by applying light.

  • 2H2O2 → 2H2O + O2
  • 2AgNO3 → 2Ag + 2NO2 + O2

Electrochemical Decomposition:

The type of reaction where is compound is broken down into two or more elements or compounds by passing electricity.

  • 2NaCl → 2Na + Cl2
  • PbBr2 → Pb + Br2
  • 2H2O → 2H2 + O2

Displacement:

The type of chemical reaction where a more active element displaces a less active metal from its salt solution or aqueous solution.

  • Zn + H2SO4 → ZnSO4 + H2
  • Zn + CuSO4 → ZnSO4 + Cu
  • Cl2 + 2KI → 2KCl + I2
  • Fe + 2HCl → FeCl2 + H2

Double decomposition:

The type of reaction where the two compounds mutually exchange their radicals to form two new compounds in aqueous solution.

1.Precipitation Reaction:

The type of reaction where two compounds mutually share their radicals to form two compounds out of which one is an insoluble salt or precipitation in aqueous solution.

ReactionColour of precipitation
Pb(NO3)2 + H2S → PbS ↓ + 2HNO3Black
2AgNO3 + H2S →Ag2S ↓ + 2HNO3Black
ZnSO4+ H2S →ZnS ↓ + H2SO4White
Na2S2O3 + 2AgNO3 → Ag2S2O3↓+ 2NaNO3White → Yellow→Orange→Red→Brown→Black

2. Neutralization Reaction:

The type of reaction where an acid reacts with a base to form salt and water only.

  1. NaOH + HCl →NaCl + H2O
  2. H2SO4 + 2NaOH →Na2SO4 + 2H2O
  3. HNO3 + NaOH →NaNO3 + H2O

3. Hydrolysis:

The type of reaction where a salt react with water to form an acidic or basic solution.

ReactionNature of the final solution
Na2CO3 + 2H2O → 2NaOH + H2CO3Basic
CH3COONa + H2O → CH3COOH + NaOHBasic
K3PO4 + 3H2O → H3PO4 + 3KOHBasic
NH4Cl + H2O → HCl + NH4OHAcidic
(NH4)2SO4 + 2H2O → H2SO4 + 2NH4OHAcidic
CuSO4 + 2H2O → H2SO4 + Cu(OH)2Acidic
ZnSO4 + 2H2O → H2SO4 + Zn(OH)2Acidic
FeCl3 + 3H2O → 3HCl + Fe(OH)3Acidic
FeSO4 + 2H2O → H2SO4 + Fe(OH)2Acidic

Exothermic Reaction:

The type of reaction where heat is produced.

  1. C + O2 → CO2
  2. CH4 + 2O2→CO2 + 2H2O
  3. CaO + H2O →Ca(OH)2
  4. N2 + 3H2 2NH3

Endothermic reaction:

The type of reaction where heat is absorbed.

  1. C + 2S → CS2
  2. N2 + O2 → 2NO
  3. NaNO2 + HCl → NaCl + HNO2

Reversible reaction:

The type of chemical reaction that can be reversed by changing the conditions of the reaction.

  1. 2SO2 + O2 2SO3
  2. N2 + 3H2 2NH3
  3. 3Fe + 4H2O + Fe3O4 + 4H2

Few Important Reactions:

ReactionObservation
2Zn(NO3)2 → 2ZnO + 4NO2 + O2  When zinc nitrate is heated, a residue is formed which is yellow when hot and white when cold. A reddish brown gas is evolved.
2Pb(NO3)2 → 2PbO + 4NO2 + O2  When lead nitrate is heated, a residue is formed which is yellow when hot and cold. A reddish brown gas is evolved.
2Cu(NO3)2 → 2CuO + 4NO2 + O2  When copper nitrate is heated, a black coloured residue is formed. A reddish brown gas is evolved.
ZnCO3 → ZnO + CO2  When zinc carbonate is heated, a residue is formed which is yellow when hot and white when cold. A colourless gas is evolved which turns lime water milky.
(NH4)2Cr2O7 → Cr2O3 + 4H2O + N2When orange coloured ammonium dichromate is heated, green coloured residue is formed.
CuSO4.5H2O → CuSO4 + 5H2OWhen blue vitriol is heated, a white anhydrous powdery solid is formed.
CuCO3 → CuO + CO2  When copper carbonate is heated, a black coloured residue is formed. A colourless gas is evolved which turns lime water milky.

Few important observations:

Compound or elementObservation
H2 gasColourless odourless gas which burns with a pop sound
O2 gasColourless odourless gas which turns alkaline pyrogallol solution brown.
CO2 gasTurns lime water milky but have no effect on acidified orange potassium dichromate solution green.
SO2 gasi)Turns lime water milky ii)Turns acidified orange potassium dichromate solution green iii)Turns acidified pink potassium permanganate solution colourless.
H2S gasi)Rotten egg smell ii)Turns moist lead acetate paper black iii)Turns lead nitrate solution black.
Cl2 gasi)Greenish yellow gas ii) Turns moist starch iodide paper blue black
HCl gasi)Produce a dense white fumes when comes in contact with a glass rod dipped in NH4OH. ii)A curdy white ppt is formed when the gas is passed through silver nitrate solution.
NH3 gasi) Produce a dense white fumes when comes in contact with a glass rod dipped in conc.HCl. ii)Turns nessler’s reagent brown.
Water vapouri) Turns anhydrous copper sulphate blue from white. ii) Turns anhydrous cobalt chloride pink from blue.
NO2 gasi)Reddish brown gas ii)Turns acidified green ferrous sulphate solution brown.