X | Chemical Reactions and Equations | Revision Notes

X | Chemical Reactions and Equations | Revision Notes

Chemical Reactions and Equations

Change is the law of nature.

Changes can be classified in two categories

  1. Physical Change
    A change in which physical properties of a substance changes but chemical composition do not change.
    Examples:
    Freezing, Melting, Boiling, condensation etc.
  2. Chemical Change
    A change in which one or more substances changes into new substances with different chemical compostion.
    Examples:
    Burning of a candle, rusting of iron, calcination of lime stone etc.

When a chemical change occurs, a chemical reaction takes place.

Chemical Equation


  • The simple form of representation of a chemical reaction in words is known as a word equation.

Magnesium + Oxygen → Magnesum Oxide
(Reactants)                  (Products)

  • Representation of a chemical reaction with the help of chemical formulae is called a chemical equation.

2Mg(s) + O2 (g) → 2MgO
(Reactants)      (Products)

Writing Chemical Equations

  • In a chemical reaction, the reactants are written on the left hand side and the products on the right hand side of the equation.
  • An arrow (→) pointing towards the products is inserted between the reactants and the products. It also represents the direction of the reaction.
  • A single arrow (→) indicates the direction in which the reaction proceeds.
  • A double arrow (⇌) indicates a reversible reaction, i.e. the products recombine to form the reactants.
  • A plus sign (+) is inserted between two or more reactants or products formed.
  • If reactions are carried out under specific conditions of temperature, pressure, catalyst etc., then these conditions are mentioned on the arrow.
  • The chemical equation can be made more informative by mentioning the physical states of the reactants and products.
  • If gas is liberated as a product then it is represented by an arrow pointing upwards (↑). If the product formed is in the form of a precipitate, it is represented by an arrow pointing downwards (↓).

Balancing the Chemical Equations

  • In a balanced chemical equation, the total number of atoms of each element of the reactants on the left hand side of the equation is equal to the number of atoms of the products formed on the right hand side of the equation.
  • The total mass of the reactants is equal to the total mass of the products or the number of atoms of each element before the reaction and after the reaction is equal.

Types of Chemical Reactions


Combination Reaction

The reaction i which two or more substances combine to form a single new substance are called combination reaction.

For example:
In the laboratory, iron sulphide is prepared by mixing iron and sulphur.
Fe(s) + S(s) → FeS(s)

  • Endothermic Reaction: The reactions accompanied by the absorption of heat are called endothermic reactions.
  • Exothermic Reaction: The reactions accompanied by the evolution of heat are called endothermic reactions.

Decomposition Reaction

A chemical reaction in which a single compound breaks up into two or more simple substances is called a decomposition reaction.

For example:
When mercuric oxide is heated in a crucible, the orange-red powder begins to darken and a silver mirror begins to deposit on the cooler parts of the crucible.
2HgO(s) → 2Hg(s) + O2

  • Thermal Decomposition Reaction: The decomposition reactions carried out by heating are known as thermal decomposition reactions.
    2FeSO4(s) \xrightarrow[ ]{heat} 2Fe2O3(s) + SO2(g)+ SO3(g)
    CaCO3(s) \xrightarrow[ ]{heat} CaO(s) + CO2(g)
    2Pb(NO3)2(s) \xrightarrow[ ]{heat} 2PbO(s) + 4NO2(g)+ O2(g)
  • Photochemical reaction: The chemical reactions which proceed with the absorption of light energy are called photochemical reactions.
    2AgCl \xrightarrow[ ]{sunlight} 2Ag(s) + Cl2(g)
    2AgBr \xrightarrow[ ]{sunlight} 2Ag(s) + Br2(g)

Displacement Reaction

Reactions in which the more reactive element displaces the less reactive element from its compound are called displacement reactions.

For example:
Zinc displaces copper in copper sulphate to form zinc sulphate.
Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s)

Double Displacement Reaction

Reactions in which ions of the reactants exchange places to form two new compounds, are called double displacement reactions.

For example:
Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Oxidation Reactions

Reactions which involve the addition of oxygen or the removal of hydrogen are called oxidation reactions.

For example:
C(s) + 2H2 (g) → CH4 (g)

Reduction Reactions

Chemical reactions in which the reactants gain hydrogen or lose oxygen are reduction reactions.

For example:
Fe2O3 + 3CO → 2Fe + 3CO2
Ferric oxide Carbon monoxide Iron Carbon dioxide

Redox Reaction

The chemical reaction in which oxidation and reduction takes place simultaneously is known as a redox reaction.

Corrosion


The slow process of decay and destruction of metals due to the action of air, moisture or acids is called corrosion.

For example:
Iron combines with oxygen present in the air, in the presence of water, to form a red-brown flaky substance called rust. This process is commonly called the rusting of iron.
The chemical formula of rust is Fe2O3. x H2O.

Prevention of Corrosion

  • Corrosion damages buildings, bridges, ships, automobiles and other articles made of iron. Hence, prevention of corrosion is necessary. This will not only save money but can also prevent the occurrence of accidents.
  • It can be prevented by processes like galvanising and electroplating with other metals.

Rancidity


  • Oils and fats react with oxygen and get oxidised or turn rancid. This process is called rancidity.
  • Rancidity can be prevented by keeping food in air tight containers or by using antioxidants.
  • Antioxidants are used to prevent oxidation of food containing fats and oils.
  • Storage of food in air tight containers also decelerates oxidation.

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