Ch - 1 Chemical Reactions & Equations Notes

Chemical Reaction

The process in which new substances with new properties are formed from one or more substances is called chemical reaction. E.g.,

• Digestion of food
• Respiration
• Formation of curd, etc.

A Chemical Reaction involves

• Change in state.
• Change in colour.
• Change in temperature.
• Evolution of gas.

1.1 Chemical Equation

Description of a chemical reaction in short form is called Chemical Equation. It involves name, symbol and chemical formula of Reactant & Product with mention of physical state.

1.1.1 Writing a Chemical Equation

1. Word Equation => Magnesium + Oxygen ⟶ Magnesium Oxide
   LHS (Reactant) ⟶ RHS (Product)
2. Formulae Equation => Mg + O₂ ⟶ MgO

In both cases on LHS, Magnesium (Mg) and Oxygen (O₂) are reactants while on RHS, Magnesium Oxide (MgO) is a product.
The necessary conditions such as temperature, pressure or any other catalyst should be written on the arrow between reactants and products.

1.1.2 Balanced Chemical Equation

Since the law of conservation of mass says “mass can neither be created nor destroyed in a chemical reaction”, which means the no. of atoms of each element remains the same, before and after a chemical reaction. i.e.,

1. Mg + O₂ ⟶ MgO
   is not a Balanced Chemical Equation, because the no. of atoms of oxygen on LHS is two while on RHS is one.
2. Zn + H₂SO₄ ⟶ ZnSO₄ + H₂
   is a Balanced Chemical Equation, because the no. of atoms of each element is equal on both LHS and RHS.

1.2 Types of Chemical Reaction

1.2.1 Combination Reaction

The reaction in which a single product is formed from two or more reactants is called Combination Reaction.
In other words when two or more substance combined to form a single product, the reaction is called Combination Reaction. i.e., A + B ⟶ C

E.g.

1. Burning of Coal: C + O₂ ⟶ CO₂
2. Formation of Water: 2H₂ + O₂ ⟶ 2H₂O
3. Formation of slaked lime : CaO(s) + H₂O(l) ⟶ Ca(OH)₂(aq)

✳️ White Washing of Walls:

The solution of slaked lime (Calcium hydroxide; Ca(OH)₂ ) is used for whitewashing of walls, when slaked lime applied on walls, after 2-3 days it react slowly with atmospheric Carbon dioxide and form a thin layer of Calcium carbonate which gives a shiny finish to the walls
i.e., Ca(OH)₂(aq) + CO₂(g) ⟶ CaCO₃(s) + H₂O(l)
Limestone, Chalk, Marble are different forms of Calcium Carbonate (CaCO₃).

✳️ Exothermic Reaction:

Reaction in which heat is released along with the formation of product is called Exothermic Reaction. E.g.,

1. Burning of natural gas: CH₄(g) + O₂(g) ⟶ CO₂(g) + 2H₂O + Heat
2. Respiration: C₆H₁₂O₆(aq) + 6O₂(g) ⟶ 6CO₂(aq) + 6H₂O(l) + Energy

✳️ Endothermic Reaction:

The reaction in which energy is required in the form of heat, light or electricity to break reactants, is called Endothermic Reaction.

1.2.2 Decomposition Reaction

The reaction in which a single compound break down to give simple products is called Decomposition Reaction. i.e., A ⟶ B + C

There are three types of Decomposition Reaction:
1. Thermal decomposition
   When decomposition reaction is carried out by heating is known as Thermal Decomposition.
   E.g.,
   1. FeSO₄ ⟶ Fe₂O₃(s) + SO₂(g) + SO₃(g)
   2. CaCO₃(s) ⟶ CaCo(s) + CO₂
2. Electrolytic decomposition
   When decomposition reaction is carried out by beating is known as Electrolytic Decomposition.
   E.g.,
   1. 2H₂O ⟶ 2H₂ + O₂
3. Photolytic decomposition
   When decomposition reaction is carried out in presence of sunlight is known as Photolytic Decomposition.
   E.g.,
   1. 2AgCl(s) ⟶ 2Ag(s) + Cl₂(g)
   2. 2AgBr(s) ⟶ 2Ag(s) + Br₂(g)
   💡 Photolytic Decomposition is also used in Black & White Photography.

1.2.3 Displacement Reaction

The reaction in which more reactive element displace or remove less reactive element from it’s aqueous solution/compound is called Displacement Reaction.
i.e., A + BC ⟶ AC + B

E.g.,
1. Fe(s) + CuSO₄(aq) ⟶ FeSO₄(aq) + Cu(s)
2. Zn(s) + CuSO₄(aq) ⟶ ZnSO₄(aq) + Cu(s)
Here Iron (Fe) and Zinc (Zn) are more reactive element than Copper (Cu). Thus they displace Copper from it’s Compound.

1.2.4 Double Displacement Reaction

The reaction in which there is an exchange of ions between the reactants are called Double Displacement Reaction.

E.g.,
1. Na₂SO₄(aq) + BaCl₂(aq) ⟶ BaSO₄(s) + 2NaCl(aq)

In this reaction, Barium Sulphate (BaSO₄) is a white pricipitate which is not soluble in water, any reaction that produce a pricipate is called Pricipitatation reaction.

1.2.5 Oxidation and Reduction

Oxidation

The reaction in which a substance gain Oxygen or loss Hydrogen, it is said to be oxidised and the reaction it called Oxidation Reaction.
i.e., 2H₂ + O₂ ⟶ 2H₂O ; Here Hydrogen gain oxygen.
E.g., 2Cu + 0₂ ⟶ 2CuO

Reduction

The reaction in which a substance gain Hydrogen or loss Oxygen, it is said to be reduced and the reation is called Reduction Reaction.
i.e., 2H₂ + O₂ ⟶ 2H₂O ; Here Oxygen gain hydrogen.
E.g., CuO + H₂ ⟶ Cu + H₂O

Redox Reaction

The reaction in which one reactant get oxidised while the other gets reduced during the reaction. Such reactions are called Oxidation-Reduction Reactions or Redox Reactions.
i.e., CuO + H₂ ⟶ Cu + H₂O

E.g.,
1. ZnO + C ⟶ Zn + CO
2. MnO₂ + 4HCl ⟶ MnCl₂ + 2H₂O + Cl₂

1.3 Effects of Oxidation Reaction in Everyday Life

1.3.1 Corrosion

When metal is attacked by its surrounding such as a moisture, gases, acids, etc for some times, it formes a thin layer of hydrated oxide, which weakens the metal and hence the metal is said to be corrode and the process is called Corrosion.

E.g.,
1. Rusting of iron.
2. Black coating on silver.
3. Green coating on copper.

Prevention:
1. By Galvanization.
2. Electroplating.
3. Painting.

1.3.2 Rancidity

The oxidation of fats and oils, when kept in air for some time is known as rancidity. It also leads to bad smell and bad taste of food.
E.g., When chips are kept in air for sometimes, it’s taste and crunchiness get change. beacause of rancidity.

Preventions:
1. By adding Antioxidents.
2. Keeping food in air-tight Container.
3. Replacing air by nitrogen.
4. Keeping food under refrigeration.