Revision Notes of Chemical Reaction and Equations

Revision Notes of Chemical Reaction and Equations?

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Class 10th – Science (Chemistry)

Chapter-1

Chemical Reaction 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.

  → 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 in the form of symbols and formulae, wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side.
  Magnesium + oxygen → magnesium oxide
              (Reactant)                     (Product)

Representation of physical states of reactants and products:
       •  For  solids it is “(s)”.
       •  For  liquids it is “(l)”.
       •  For  gases it is “(g)”.
       •  For aqueous solutions it is  “(aq)”.
       •  For gas produced in the reaction it is “(↑)”.
       •  For precipitate formed in the reaction it is  “(↓)”.
       •  Direction of reaction is indicated by “(→)”.

 Balanced chemical equation

→ The chemical equation that shows the chemical reaction needs to be balanced. A balanced chemical equation occurs when the number of the atoms involved 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)

 

Steps involved in balancing of chemical equation:
       •  Determining the reactants and products in a reaction.
       •  Counting the number of atoms of each element on both sides of the equation.
       •  Selecting the elements that occur for the least number of times in the equation.
       •  Balance atoms of each element on both sides of the reaction.
       •  Always leave hydrogen and oxygen for last to balance.
       •  Balance the hydrogen atoms lastly followed by balancing of oxygen atoms.

Example:
Formation of Ammonia:
         N₂ + H₂ → NH₃   (Un-balanced equation)
Step 1: 2-Nitrogen atoms, 2-Hydrogen atoms → 1 –Nitrogen atom, 3-Hydrogen atoms

Step 2: In the above equation number of Nitrogen atoms on both sides are not equal, multiply with suitable integer to balance the Nitrogen atoms on both sides.

So, multiply with “2” on the product side.
        N₂ + H₂ → 2NH₃

In the above equation the number of Nitrogen atoms were balanced.

Step 3: Hydrogen atoms on both sides were not balanced. So, multiply with suitable integer.

Multiply with “3” to Hydrogen on reactant side.
         N₂ + 3H₂ → 2NH₃

Now the above equation is balanced.

Types of chemical reaction

Combination reaction:

Such a reaction in which 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 vigorously with 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. It is interesting to note that the chemical formula for marble is also CaCO₃.
Ca(OH)₂ (aq) + CO₂ (g)   →   CaCO₃ (s) + H₂O (l)
(slaked lime)                   (calcium carbonate)

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)

Decomposition reactions:

The Reaction in which a single reactant beak down into tow or more simpler products. Decomposition reactions are the opposite of combination reactions.  Decomposition reactions are of three types:

•    Thermal decomposition
•    Electrolysis
•    Photolysis

Decomposition by the application of heat on a substance is called thermal decomposition. 
Example: 
Thermal decomposition of calcium carbonate.
CaCO₃ + Δ (Heating) → CaO + CO₂

Decomposition of any substance by passing current through it is called electrolysis. 
Example: 
Electrolysis of water 

2H₂O → 2H₂ + O₂

The decomposition reaction resulting from action of light energy is called photolysis. 
Example: 
Photolysis of silver chloride
2AgCl  → 2Ag + Cl₂

Displacement reaction: 

Single displacement reaction is the one in which one element substitutes or displaces another element in a compound to give new compound. Generally in a single displacement reaction,  less active metal is displaced by a more active metal.

It is important to note that in a single displacement reaction, one of the reactants is always an element. 
Example: 
The reaction of magnesium with copper chloride
Mg + CuCl₂ →  MgCl₂ + Cu

Double displacement reaction:

The reactions which involves exchange of ions (cations and anions) between the reactants are called double displacement reactions.
AB + CD → AC + BD
Example:

Double displacement reactions classified into different types. They are: 

• Neutralization reactions
• Precipitation reactions
• Gas forming reactions

Neutralization reaction:

Reaction in which hydrogen ions of an acid react with hydroxide ions of a base to form water is an neutralization reaction. 
Example :

The reaction of hydrochloric acid with sodium hydroxide.
H⁺Cl^–  + Na⁺OH^– → Na⁺Cl^– + H₂O

Precipitation reaction:

Reactions which proceeds through the formation of precipitation are called precipitation reactions.
Example:
In the reaction of barium chloride with sodium sulphate produces precipitate of barium sulphate along with formation of sodium chloride.
BaCl_2 (aq) + Na₂SO_4 (aq) → BaSO_4 (s) + NaCl _(aq)

Gas forming reaction:

In these reactions gas is produces as one of the product during reaction.

Example:
Na₂CO_3 (s) + 2HCl _(aq) → 2NaCl _(aq) + H₂O _(l) + CO_2 (g)

Based on energy differences between reactants and products the chemical reactions are classified into two types. They are exothermic reaction and endothermic reaction.

Exothermic reaction:
Chemical reactions in which heat energy released are known as exothermic reactions.
Burning of methane gas in air produces heat energy.
CH_4 (g) + 2O₂ (g) → CO₂ (g) +  2H₂O (l)+ heat
All combustion reactions are exothermic reactions.

Endothermic reaction:
Reactions which involves absorption of energy are known as endothermic reactions.
Example:
Formation of nitric oxide from nitrogen and oxygen.
N_2 (g)+ O_2 (g) + Heat → 2NO 

Combustion reaction:
Burning of a substance in the presence of oxygen which resulting release of energy is called combustion.
Example:
Combustion of ethylene gas.
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

→ Redox reaction:

An oxidation-reduction (Redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.
Oxidation: This process involves gain of oxygen or loss of hydrogen.
Reduction: This process involves gain of hydrogen or loss of oxygen.

 CuO + H₂ + Heat→   Cu + H₂O

→ Oxidizing Agent

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

→ Reducing Agent
It is the substance which gives hydrogen or gains oxygen.
Or it the substance which is oxidized itself and reduces other.
Oxidation is the process which involves loss of electrons but reduction is the process which involves gain of electrons.

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.

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

 Methods to Prevent Rusting

• Painting on the surface of metals
• Greasing and oiling on the surface of metals
• By galvanisation process

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.

Methods to Prevent Rancidity

• Store cooking oils from direct sunlight.
• Food should be placed at low temperature.
• By adding antioxidants food can be protected from rancidity.
• Packing material should replace the air with nitrogen.
• Minimize the use of salts in fried foods.

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Revision Notes of Chemical Reaction and Equations??

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