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Chapter 1 - Chemical Reactions and Equations
Chemical change: - is a change in which one or more new substances are formed
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• Energy changes are involved
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• Permanent change takes place
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Rusting of iron is a chemical change because
❖ A new substance iron oxide is formed
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❖ There is an increase in mass when rust forms
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Chemical changes are also known as chemical reactions
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Examples:

The amount of heat (energy) produced is written along with the products
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Endothermic Reactions - A chemical reaction which is accompanied by absorption of heat
energy is known as exothermic reaction
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This indicates that heat
is absorbed
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(a) Formula of one molecule of water is H2O
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(b) Formula of one molecule of sulphuric acid is H2SO4
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Chemical Equations - Representation of a chemical change in terms of symbols and formulae of
the reactants and products is known as chemical equation of the reaction
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The arrow may be read as "to yield" or "to form" or "to give" and shows the direction of the
reaction
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The different reactants as well as products are connected by a plus sign (+)
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Step II: Balancing of Chemical Equation
An equation in which number of atoms of each element is equal on both the sides of the equation
is known as balanced chemical equation
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Equations must always be balanced
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If there are 4H2O, then 4 × 2 = 8 hydrogen atoms
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e
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 On the reactant side, there must be an even number in front of HCl
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As a result, the number of chlorine atoms will also be even
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e
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i
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, 1 + 2 +
2 = 5)
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Let us change it into the simplest even
number possible i
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, 2
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In 2 KMnO4, there are 2 K, 2 Mn, and 8 O
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If there are 8 H2O on the product side, there should be 16 H (8 x 2) on the reactants side as well
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On the left hand side, there are 16 Cl
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Subsequently 10 more Cl atoms are to be accounted for
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This type of balancing the chemical equations is known as the Hit and trial method
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Names of various reactants and products
2
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Relative number of moles of the reactants and products
4
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Relative volumes of gaseous reactants and products
Limitation of a Chemical Equation
• It does not mention the state of the substances
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Example: CaCO3 (s)+ HCl (aq)
CaCl2 (s) + H2O (l) + CO2(g)
• The reaction may or may not be complete
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• It does not give any information regarding the speed of the reaction
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In some cases, terms like diluted (dil)
and concentrated (conc) may be added
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This is overcome by
mentioning these above or below the arrow
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g
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•

It does not give any indication regarding the production or absorption of heat
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C + O2
CO2 + Heat
2C + O2
2CO + Heat

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Some reactions are reversible
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Chemical reactions that proceed with the absorption of heat energy are called endothermic
reactions
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Balancing: To make the number of atoms of all the elements equal on both the sides in a skeletal
equation
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Steps involved in balancing a Chemical Equation:
Step 1: Write the correct skeleton equation
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Step 3: Balance elements that appear only once on each side of the arrow first
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Step 4: Elementary substances are balanced last of all
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Balancing of Ionic Equations:
A balanced ionic equation must satisfy mass as well as charge balance
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Number of moles of
reactants and products can be calculated from a chemical reaction
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Types of Combination reactions:
1
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2
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2CO (g) + O2(g)
2CO2 (g)
3
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Decomposition reactions: are opposite to combination reactions
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When a substance decomposes due to heat it is called thermal decomposition, while
decomposition due to electricity, is called electrolytic decomposition
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Photolysis: The decomposition of a compound with light is called photolysis
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Mercuric oxide, when heated, undergoes thermal decomposition, to give mercury and oxygen
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Similarly, if blue crystals of copper nitrate are heated, they undergo thermal decomposition to
give black colored copper oxide, reddish brown fumes of nitrogen dioxide, and a colorless
gas of oxygen
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When water taken in an electrolytic cell, is acidified with a small quantity of sulphuric acid
and a direct current passed through it undergoes electrolytic decomposition to yield hydrogen
and oxygen
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If an electric current is passed through molten lead bromide, it decomposes to give lead and
bromine
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A substitution or displacement reaction is a chemical change in which atoms of one element
replace the atoms of another element from the molecules of a compound
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More electro
positive elements displace lesser electropositive elements
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For e
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,

The iodine so liberated, dissolves in the chloroform, giving it a purple color
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Types of Double displacement reactions:
1
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Neutralization Reactions
1
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This
insoluble substance formed is known as a precipitate
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2
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The positive charge of the hydrogen ion of the acid, and
the negative charge of hydroxyl ions or oxide ions of the base, lose their electrical charge, and
become covalent molecule of water
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e
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Addition of Oxygen - Oxygen adds on to magnesium to become magnesium oxide
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"Reduction is a reaction in which oxygen is removed from a substance or hydrogen is added to a
substance
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Addition of Hydrogen - Hydrogen adds to chlorine to form hydrogen chloride gas
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Example of Redox Reaction

Example of Non-redox Reaction

Redox reactions are the reactions in which oxidation and reduction takes place simultaneously
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Reducing Agents: A substance that brings about reduction
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Reduction: The process of addition of electropositive element or radical or removal of
electronegative element or radical
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Reduction: The process in which there is gain of electrons
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One must have observed iron articles that are shiny when new, but get coated with
a reddish brown powder when left for some time
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The problem with iron (as well as many other metals) is that oxidation takes place and the
oxide formed does not firmly adhere to the surface of the metal causing it to flake off easily
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Hence metal is attacked by substances around it, it is said to corrode and this process is called
corrosion
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What happens to copper vessels or artifacts when exposed to air and water? They slowly get
tarnished by acquiring a thin green oxide layer
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The heaviest metal lead also tarnishes in moist weather
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Rancidity
Have you ever tasted or smelt the fat/oil containing food materials left for a long time? This
unpleasant change in the flavor and odour of a food is called rancidity
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When an
oxygen atom replaces hydrogen atom in the fatty acid molecule it destabilizes the molecule
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Usually substances which retard fat oxidation or rancidity are called antioxidants (such as BHT,
BHA, vitamin E, and vitamin C, and spices such as sage and rosemary)
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Keeping food in air tight containers or air
tight wrapping also helps to slow down oxidation
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At times, to
avoid the presence of oxygen altogether, vacuum packaging is used in some processed foodstuff

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