Science : Chapter 1 Chemical Reactions and Equations Summary | Class 10th

Chemical Reaction :

A chemical reaction is a process in which one or more substances, also called reactants, are converted to one or more different substances, known as products.

Some Examples of Chemical Reaction :

• Burning of paper and log of wood.
• Digestion of food.
• Boiling an egg.
• Chemical battery usage.
• Electroplating a metal.

To determine whether a chemical reaction has taken place:

1. Change in state
2. Change in colour
3. Evolution of a gas
4. Change in temperature

Reactant: A substance that takes part in and undergoes change during a reaction.

Product: A substance that is present at the end of a chemical reaction.

Chemical Equation:

Chemical equations are symbolic representations of chemical reactions in which the reactants and the products are expressed in terms of their respective chemical formulae.

It involves uses of symbol of elements or chemical formula of reactant and product with mention of physical state.

The necessary conditions such as temperature, pressure or any catalyst should be written on arrow between reactant and products.

Example : Magnesium is burnt into air to form magnesium oxide can be represented as Mg + O₂ →Mgo

Note : Physical state of the reactant and products are mentioned to make chemical reaction more informative. for e.g : we use (g) for gas, (l) for liquid, (s) for solid and (aq) for aqueous.

Balancing Chemical Equations :

A balanced chemical equation is an equation where the number of atoms of each type in the reaction is the same on both reactants and product sides.

Balancing of Chemical Equations follow the "Law of Conservation of Mass".

How to balance a Chemical Equations:

To balance the given or any chemical equation, follow these steps:
Fe + H₂O → Fe₃O₄ + H₂
Step 1: Write the number of atoms of elements present in reactants and in products in a table as shown here.

Name of atom	No. of atoms in the reactant	No. of atoms in the product
Iron	1	3
Hydrogen	2	2
Oxygen	1	4

Step 2: Balance the atom which is maximum in number on either side of a chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen, one needs to multiply the oxygen on the LHS by 4, so that, the number of oxygen atoms becomes equal on both sides.
Fe + 4 × H₂O → Fe₃O₄ + H₂
Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 × H₂O → Fe₃O₄ + 4 × H₂
Step 3: After that, the number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.
3 × Fe + 4 × H₂O → Fe₃O₄ + 4 × H₂
Step 4: Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.

Name of atom	No. of atoms in the reactant	No. of atoms in the product
Iron	3	3
Hydrogen	8	8
Oxygen	4	4

Step 5: After balancing, the above equation can be written as follows:
3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Types of Chemical Reactions:

There are several types of chemical reaction based on different factors. However, in chemical reactions and equations notes some of them are mentioned as:

Classification of Chemical Reactions based on Chemical Change:

1. Combination Reaction: The reaction in which two or more substances combine to form a new single substance.
   Eg. C_(s) + O_2(g) →CO_2(g)

2. Decomposition reaction: The reaction in which a single substance decomposes to give two or more substances.
   A → B + C

Decomposition reactions can be of three types:

• Thermal Decomposition: When a decomposition reaction is carried out by heating.
  Eg. CaCO3 (s)     →    CaO (s) + CO₂ (g)
  (calcium carbonate)    (quick lime)  (carbon dioxide)

• Electrolytic Decomposition : When decomposition is carried out by passing electricity.
  e.g., 2H₂O + electric current → 2H₂ + O₂

• Photolytic Decomposition : When decomposition is carried out in presence of sunlight.
  e.g., 2AgCl (s) + sunlight→2Ag (s) + Cl₂ (g)

3. Displacement Reaction: The chemical Reaction in which an element displaces another element from its solution.
   Eg. Fe(s) + CuSO₄(aq)  →  FeSO₄ + Cu(s)

4. Double Displacement Reaction: The reaction in which two different atoms or group of atoms are mutually exchanged.
   Eg. Na₂ SO₄ + BaCl₂ → BaSO_4(s) + 2NaCl

5. Precipitation reaction: The reaction which involves formation of precipitate (insoluble substance) is known as precipitation reaction.
   Eg. Pb(NO₃)₂(aq) + 2KI(aq) → 2KNO₃(aq) + PbI₂(s)

6. Oxidation reaction: The reaction in which substance gains oxygen or loses hydrogen is called as oxidation reaction.
   Eg. 2Cu(s) + O₂(g)+ Heat  → 2CuO(s)

7. Reduction reaction: The reaction in which substance gains hydrogen or loses oxygen is called as reduction reaction.
   Eg. C(s) + 2H₂(g)  → CH₄(g)

8. Oxidation-reduction reaction (Redox reaction):The reaction in which one reactant gets oxidised and the other reactant gets reduced is called as oxidation-reduction reaction or redox reaction. In this reaction, simultaneous oxidation and reduction reactions take place.
   Eg. ZnO + C → Zn + CO
   MnO₂ + 4HCl →  MnCl₂ + 2H₂O + Cl₂

Classification of chemical reactions based on energy change:

The classification of reactions is the prime part of chemical reactions and equations notes:

• Exothermic Reactions: Reaction in which heat is released along with formation of products.
  Eg. CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(g) + Heat

• Endothermic reactions: The reactions which require energy in the form of heat, light or electricity to break reactants are called endothermic reactions.
  Eg. 2Pb(NO₃)_2(s) 2PbO_(s) + 4NO_2(g) + O_2(g)

Effects of Oxidation in Daily Life:

Oxidation has a damaging effect on metals as well as on food. The damaging effect of oxidation on metals is studied as corrosion, and that on food is studied as rancidity. Thus, two common effects of oxidation reactions observed in daily life are corrosion of metals and rancidity of food.

1. Corrosion : Corrosion is the process by which a metal surface is attacked by a certain substance such as air or water. The most common occurence of corrosion is the rusting of iron by air and moisture to form a reddish-brown layerof triferric tetraoxide(Fe₃O₄)
2. Rancidity :When fats and oils stay in open for long, they get oxidized and become rancid (old and stale), and their smell and taste change. This process is known as rancidity.

Methods to prevent Rancidity:

• Adding antioxidants (substances that prevent oxidation) to food.
• Storing food in airtight containers to slow the process of rancidification.
• Refrigerating food also helps to slow down rancidification.
• Replacing oxygen in the containers with another gas.