Ch 1 Chemical Reactions and Equations Notes

Class 10th Science

    Ch 1 Chemical Reactions and Equations       

Topics in the Chapter

•  Introduction
• Chemical reaction
• Chemical equation
• Balanced chemical equation
• Types of chemical reaction
• Corrosion
• Rancidity

Introduction

→ Most of the substance around us undergoes various changes. Some of these changes are temporary with no new substance being formed. They are called physical changes.

→ Chemical changes: In certain other changes the new substance formed in which the reactant or the parent loses its identity to form new substance called product. These changes are permanent changes as we won’t get the reactant back.

Chemical Reaction

→ Chemical reaction is the process by which two or more substance react with each other to form new substance with different properties.

→ These are the following changes to determine that the chemical reaction has taken place:

(i) Change in state

(ii) Change in colour

(iii) Evolution of gas

(iv) Change in temperature

Chemical Equation

• A chemical equation is the symbolic representation of a chemical reaction.
• the reactants are given on the left-hand side and the products on the right-hand side.     

Magnesium + oxygen → magnesium oxide              (Reactant)                     (Product)

Balanced chemical equationI

In balanced chemical equation the number of the atoms  in the reactants side is equal to the number of atoms in the products side.

Zn + H2SO4  →  ZnSO4  + H2

3Fe (s) + 4H2O (g) →  Fe3O4  (s) + 4H2 (g)

Types of chemical reaction

→ Combination reaction: a single product is formed from two or more reactants is known as a combination reaction.

CaO (s) + H2O (l) → Ca(OH)2 (aq)

• Calcium oxide reacts vigorouslywith water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
• A solution of slaked lime produced by the reaction is used for white washing walls.
• Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls.
• Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. 

Ca(OH)2 (aq) + CO2 (g)   →   CaCO3 (s) + H2O (l)

It is interesting to note that the chemical formula for marble is also CaCO3.

Burning of coal: C (s) + O2 (g) → CO2 (aq)

Formation of water: H2 (g) + O2 (g) →  H2O (aq)

Burning of natural gas (Methane): CH4 (g) + O2 (g) →  CO2 (g) + H2O (l)

→ Exothermic reaction: Heat energy produced in the reaction is called exothermic reaction.

 carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. This reaction is respiration is an exothermic reaction.

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

(glucose)

→ Endothermic reaction: Heat Energy absorbed by the reaction is called endothermic reaction.

The decomposition of vegetable matter into compost is also an example of an endothermic reaction.

Decomposition reaction: single reactant breaks down to give two or more products. This is a decomposition reaction.

>White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

The above reactions are used in black and white photography.

Displacement reaction:  More reactive element displaces a less reactive element from its compound.

>Reaction of iron nails with copper sulphate solution.

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

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

Double Displacement reaction: Exchange of ions between the reactants is called double displacement reaction.

>Insoluble substance formed is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.

Na2(SO)4 (aq) + BaCl2 (aq) →  BaSO4 (s) + NaCl (aq)

Redox reaction: An oxidation-reduction (Redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species.

the copper(II) oxide is losing oxygen and is being reduced.

>Reduced substance - CuO, 

The hydrogen is gaining oxygen and is being oxidised.

>Oxidized substance - H2

Oxidation: This process involves gain of oxygen or loss of hydrogen.

Ex: 2Cu + O2 --> 2CuO

The surface of copper powder becomes coated with black copper(II) oxide. This is because oxygen is added to copper and copper oxide is formed.

Reduction: This process involves gain of hydrogen or loss of oxygen.

Ex: ZnO + C →Zn+ CO

MnO2 + 4HCl --> MnCl2 + 2H2O + Cl2 

→ Oxidizing Agent is the substance which gives oxygen or gains hydrogen. Or it the substance which is reduced itself and oxidizes other.

→ Reducing Agent is the substance which gives hydrogen or gains oxygen. Or it the substance which is oxidized itself and reduces other.

Corrosion 

• The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion.

>The black coating formed on the surface of silver articles due to corrosion is silver scarbonate 

>green coating on copper is copper carbonate

• Rusting:  Iron when reacts with oxygen and moisture forms red substance called rust. 

Rancidity 

• The taste and odour of food materials containing fat and oil changes when they are left exposed to air for long time. This is called rancidity. 
• It is caused due to oxidation of fat and oil present in food material.

• It can be prevented by using various methods such as by adding antioxidants to the food materials, Storing food in air tight container and by flushing out air with nitrogen. 

Click here 👉 Chapter 2 Acids, Bases, and Salts Notes