1437566181chemicalreactionsandequationschap1

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Chemical Reactions and Equations


Chemical reaction:

The process of combination of different atoms to form different products is known as chemical

reaction. Chemical reaction involves a change in the physical and chemical properties like: Change in the colour of the substance

Change in the state of the substance

Change in heat energy - Absorption of energy or release of energy

Release of gas

Evolution of light and sound

Example:

Burning of coal: During the burning coal, the solid state coal burns with release of heat and gas.

Chemical equation:

A chemical equation is a symbolic representation of the reactants and the products using their

chemical formulae.

A chemical equation contains:

• Reactants

• Products

• An arrow separates the reactants and products

Reactants are the substances which take parts in the chemical reaction.

Products are the substances which produced during chemical reaction.

C + O2 → CO2

Reactants 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 "(↑)".

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• For precipitate formed in the reaction it is "(↓)".

• Direction of reaction is indicated by "(→)".

Example:

Zn (s) + dil.H2SO4 (aq) → ZnSO4 (aq) + H2 (g) (↑)

(Reactants) (Products)

A chemical equation is helpful to understand a chemical reaction in a easy way.

In a chemical equation the masses of reactants and products may or may not be equal. But

according to law of conservation of mass "the total mass of the reactants and the products should be

equal".

So in order maintain the law true it is necessary to balance a chemical equation.

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.


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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.

Reaction of potassium hydroxide with sulphuric acid:

Write down the equation

KOH + H2SO4 → K2SO4 + H2O (This is not balanced equation)

Check the number of atoms of each element on both sides

Potassium (K) Atoms on both sides are not equal, balance the Potassium atoms by multiplying with suitable

integer.

2KOH + H2SO4 → K2SO4 + H2O

Now Potassium atoms on both sides are balanced.

Number of Hydrogen atoms on both sides is not equal, balance hydrogen atoms by multiplying with suitable

integer.

2KOH + H2SO4 → K2SO4 + 2H2O

Now Hydrogen atoms on both sides are balanced. And Oxygen atoms on both sides get balanced.

This is balanced equation as atoms of all elements on both sides are balanced

2KOH + H2SO4 → K2SO4 + 2H2O


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Types of Chemical Reactions

SummaryLearnNext Lesson Video

HD 14:45

Table of Contents[Show] In general chemical reactions are classified into:

Combination reactions

Decomposition reactions

Single displacement reactions

Double displacement reactions

Combination reactions:

A combination reaction is one in which two or more reactants combine to form a single

product. Combination reactions are again of three types.

They are:

Combination reactions between elements.

Combination reactions between compounds.

Combination reactions between elements and compounds.

Combination reactions between elements:In these reactions elements were combined to form a product.

Examples :

Formation of calcium oxide by the combination elements calcium and oxygen.

2Ca + O2 → 2CaO

Formation of ammonia by the combination of elements nitrogen and hydrogen.

N2 + 3H2 → 2NH3

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Combination reactions between compounds:

In these reactions compounds were combined to form product.

Example:

Reaction of calcium oxide in water to form calcium hydroxide

CaO + H2O → Ca(OH)2

Combination reactions between elements and compounds:

In these reactions elements and compounds combined to form product.

Example:

Formation of sulphur trioxide by the combination of sulphur dioxide and oxygen.

2SO2 + O2 → 2SO3

Decomposition reaction:

Decomposition reactions are those in which a substance splits into two or more simpler

substances. 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

Single 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.


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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.

BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + NaCl (aq)

Gas forming reaction:


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

Example:

Na2CO3 (s) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + CO2 (g)

Based on energy differeneces 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.

CH4 (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.

N2 (g)+ O2 (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

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Oxidation reaction:

Oxidation reaction involves the addition of oxygen or the removal of hydrogen from a substance.

Example:

Rusting of iron

Rusting of iron:

Iron when reacted with both water and oxygen are present (moist air), corrodes. Its silvery colour changes

to a reddish-brown, because hydrated oxides are formed which is commonly called as rust.

Reaction showing the formation of rust when reacted with water in presence of oxygen

(atmosphere).

4Fe + 3O₂+ XH₂O → 2Fe₂O₃.XH₂O

Reduction reaction:

The reaction which involves addition of hydrogen or removal of oxygen from a substance is called

reduction reaction.

Example:

Photosynthesis is a reduction reaction.

Redox reaction:

The reactions in which both oxidation and reduction occurring together are known as redox reactions.

Example:

CuO + H2 → Cu + H2O

The above is a redox reaction as copper is reduced and hydrogen is oxidised.

1437566181chemicalreactionsandequationschap1

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