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Chapter 1

Chemical Reactions and Equations - Study Notes

Chemical Reaction

It is defined as a process in which 2 or more substances react to form new substances.

Following changes can be observed for a chemical reaction:

  • Change in state
  • Change in color
  • Evaluation of a gas
  • Change in temperature

Chemical Equation

Chemical Equation is a symbolic representation of a chemical reaction in the form of symbols and chemical formulas where reactants are written on left hand side and products are written on right hand side.

For E.g.:

The burning of Magnesium in air:

Magnesium + Oxygen → Magnesium oxide
(Reactants) (Product)
  • Mg + O2 → MgO (ChemicalEquation)

Balanced Chemical Equation

A chemical equation is said to be balanced when the number of atoms of each element is equal on both sides of the chemical equation i.e.

Number of atoms of an element on reactant side = Number of atoms of element onproduct side

2Mg + O2→ 2MgO

The equation in the above chemical equation is completely balanced as the number of atoms in reactants and products are equal.

Type of Chemical Reactions

  1. Combination Reaction: When two or more substances combine to form a new single substance then it is called as combination reaction.

    Examples:

    1. Formation of slaked lime from quicklime
      CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
      (QuickLime) (Slaked lime)
    2. Burning of coal

      C(s) + O2(g) → CO2(g)

    3. Formation of water

      2H2(g) + O2(g) → 2H2O(l)

  2. Exothermic Reaction: Reaction in which heat is evolved is called an exothermic reaction.

    Examples:

    1. Burning of hydrocarbon

      CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

    2. Respiration

      C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + Energy

  3. Decomposition Reaction: When a substance decomposes to give two or more substances, then it is known as decomposition reaction.

    1. 2FeSO4(s) Heat→ Fe2O3(s) + SO2(g) + SO3(g)
    2. CaCO3(s) Heat→ CaO(s) + CO2(g)
    3. 2Pb(NO3)2(s) Heat→ 2PbO(s) + 4NO2(g) + O2

  4. Displacement Reaction:When an element displaces another element from its compound in a reaction, it is said to be displacement reaction.

    Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

    • (Fe is more reactive than Cu)

    Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

    • (Zn is more reactive than Cu)

    Pb(s)+ CuCl2(aq) → PbCl2(aq) + Cu(s)

    =>(Pb is more reactive than Cu)

    NOTE:-ReactivitySeries to determine the displacing elements:

    K >Na > Ca > Mg > Al > Zn > Fe >Pb >H > Cu >Hg > Ag > Au

  5. Double Displacement Reaction: When two different atoms or group of atoms are exchanged in a chemical reaction then it is called a double displacement reaction.

    Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl.(aq)

  6. Oxidation Reaction: The reaction in which gain of oxygen or loss of hydrogen atom takes place is known as oxidation. e.g.

    2Cu + O2 Heat→ 2CuO

  7. Reduction Reaction: The reaction in which the loss of oxygen or gain of hydrogen atom takesplace is known as reduction. e.g.

    CuO + H2 Heat→ Cu + H2O

  8. Redox Reaction: The reaction in which oxidation and reduction both takes place simultaneously is known as redox reaction.

    ZnO + C → Zn + CO

    MnO2 + 4HCl → MnCl2 + 2H2O + Cl2

Corrosion

When an element is attacked by some substances such as moisture, acid etc,the element gets corroded and this process is known as corrosion.

Example: Rusting of Iron, Black coating on silver, Green coating on copper

Reaction of Rusting of Iron:

4Fe + 3O2 → 2Fe2O3(Rust)

Rancidity

When food material containing fat and oil are left exposed to air for a long time, the taste and odour of food material changes. This is called Rancidity.

  • It is caused due to oxidation of fat and oil present in food material.

  • It can be prevented by adding antioxidants to the food material.