Himachal Pradesh PSC: Oxidation and Reduction

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Oxidation and Reduction

  • Removal of hydrogen atom is oxidation while addition of hydrogen atom is reduction.

  • Addition of oxygen atom is oxidation while removal of oxygen atom is reduction.

  • Increase in valence of an element is oxidation while decrease in valence of an element is reduction.

  • Addition of an electronegative element is oxidation and removal is reduction.

  • Loss of electrons is oxidation and gain of electrons is reduction.

  • Increase in oxidation number is oxidation while decrease in oxidation number is reduction.

Oxidizing and Reducing Agents

  • Compounds having higher oxidation number will be more acidic and act as oxidizing agent and compounds having lower oxidation number will be less acidic and act as reducing agent.

  • Generally, compounds with oxygen atom are called oxidizing agent and compounds with hydrogen atom are called reducing agent.

  • H2O2 acts as a reducing agent when it is oxidized to O2 or O3.

  • H2S acts as a reducing agent when it is oxidized to sulphur.

  • Halogens act as oxidizing agent and they are reduced to halogen acids.

Chemical Changes/Reactions in Chemistry

Combination Reaction

Combination reaction occurs when two substances unite to form a third substance.

For example, combining magnesium (Mg) and oxygen (O2) result in the production of magnesium oxide (MgO)

2Mg + O2 → 2MgO

Decomposition Reaction

Decomposition reaction occurs when a single compound breaks down into two or more simpler substances.

In the decomposition of mercuric oxide (HgO), the elements mercury (Hg) and oxygen (O2) are produced

2HgO→ 2Hg + O2

Displacement Reaction

When one element replaces another in a compound, it is known as a displacement reaction.

For example, iron (Fe) may displace copper (Cu) in a solution of cupric sulphate (CuSO4)

Fe + CuSO4→ FeSO4 + Cu

Double Decomposition Reaction

When two compounds interact to form two other compounds, it is known as a double decomposition reaction.

For example, sodium iodide (Nal) and lead nitrate (PbNO3) react to form lead iodide (PbI2) and sodium nitrate

2NaI + Pb(NO3)2→ Pbl2 + 2NaNO3


Hydrolysis is a double decomposition reaction in which water reacts with a second substance.

When ammonium chloride (NH4Cl) is combined with water (H2O), it undergoes hydrolysis, yielding ammonium hydroxide (NH4OH) and hydrochloric acid (HCl).

NH4C1 + H2O → NH4OH + HCl

Neutralization Reaction

Neutralization is the interaction of an acid with an equivalent quantity of a base.

If the process is carried out in an aqueous solution (dissolved in water), the products are water and a salt.

For example, hydrochloric acid (HCl) and sodium hydroxide (NaOH) neutralize each other when dissolved in water, forming sodium chloride (NaCI), a salt, and water

HCl + NaOH→NaCI + H2O

Substitution Reaction

Substitution reaction occurs when an element, such as chlorine (Cl), replaces one or more hydrogen atoms in a hydrocarbon, such as methane (CH4)

CH4 + Cl2 → CH3Cl + HCl

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