Chemical reactions and equations

Chemical Reactions
and Equations
Ms. Bandana Pradhan
M.Sc-Physics, MFA, M.Ed
Ms. Bandana Pradhan. M.Sc, MFA, M.Ed
Class - X

How does changes happened?
Whenever lime is mixed with water we get
a clear solution with and on touching the
tube it feels hot. That is lime when mixed with
water undergoes certain physical changes
that makes it evolve heat and give a
clear solution.
Similarly, the iron articles undergoes
physical changes and develops a layer
of rust on its surface. It reacts with the
oxygen present in air and moisture and
develops rust (hydrated iron (III) oxide).
These are all chemical reactions.

Chemical Reaction
• Chemical reaction
The reaction in which the original state of the particles changes and it cannot be
reversed by simple physical means. Whenever a chemical reaction takes place
following changes occurs:
• Change in colour
• Change in state
• Change in odour
• Change in temperature
• Change in colour
During rusting the iron articles undergoes
change in colour. We have also observed
that silver gets tarnished when kept for a
long time. The metallic silver reacts with
hydrogen sulphide or sulphur present in
air and gets tarnished.

• Change in state
On heating liquid changes its state and
converts to vapour due to reaction within
the particles caused by heat.
• Change in odour
Due to chemical reaction food gets spoiled
and smells really bad.

• Change in temperature
Lime when mixed with water undergoes
certain physical changes that makes it
evolve heat and give a clear solution.
Such reactions are called
exothermic reactions.
• Release of gas
During Aerobic cellular respiration in human body energy molecules combines
with inhaled oxygen and releases energy needed by the cells. It also releases
carbon dioxide gas and water. Here is the overall equation for aerobic cellular
respiration:
C6H12O6 + 6O2 6CO2 (RELEASED GAS) + 6H2O + ENERGY

Difference between Exothermic and Endothermic reaction .
Exothermic reaction : Endothermic reaction
Heat is given out during the reaction . Heat is absorbed during the reaction
Heat liberated is indicated by a positive sign
on the product side
Heat observed is indicates by negative sign
on the product side
An exothermic reaction is represented
reactants→ product + heat
An endothermic reaction is represented
reactants→ product − heat
Energy of the reactant is greater than the
product.
Energy of the reactant is lesser than the
product.

Chemical equation
Describing all of the chemical reactions that
occurs around us in words is quite impossible.
Like if you want to depict rusting of iron.
Instead of writing the phenomenon in
sentences, you can simply write it as
• This is known as word equation. And it itself describes the whole phenomenon
of rusting. Through this we can conclude that iron on reacting with oxygen and
water present in atmosphere undergoes chemical transformation and gets
converted to ferric oxide i.e. Rust.
• Points to be kept in mind
• while writing chemical reaction.

Types of arrows used in chemical equation
• In a word equation the reactants are always
written on the left hand side with plus sign
in between the reactants.
• The product is written on the left hand side.
• In between reactants and products lies the
arrow sign.
• The arrow head points towards the
direction of reaction.
• You can also add the conditions and
catalysts required to perform the reaction
above and below the arrow sign.

Balanced chemical equation
• It is always necessary to balance a chemical equation. In an unbalanced equation the
mass is not the same on both sides of the equation which leads to violation of law of
conservation of mass which states that mass can neither be created nor destroyed. So
balancing a chemical equation balances the amount of reactant and product on both
side of arrow. An unbalanced chemical equation is called a skeletal chemical
equation.
• For instance, Chemical equation for rusting of iron is given by
• According to this equation amount of reactants and products on both sides is as
follows:
• Ferrous – 4
• Oxygen- 8
• Hydrogen-4 Ms. Bandana Pradhan. M.Sc, MFA, M.Ed

How to balance a chemical equation
• We take the reaction of formation of water.
• H2 + O2 H2O (unbalanced equation for formation of water)
• This equation can be balanced by using certain steps as mentioned
below.
• Step I: list the number of atoms of each element present on both
sides of arrow.
Reactants Products
Hydrogen 2 2
Oxygen 2 1

• Step II: Begin balancing with the compound having maximum number of atoms.
Following this rule we select H2O. Here there are 2 oxygen atoms on the reactant
side but only 1 oxygen atom on the product side. So equation is unbalanced. In
order to balance we need to do following step.
• So the equation stands out to be
H2 + O2 2H2O
Reactants Products
Initial Oxygen atoms 2 1
To balance 2 1 X 2

• Step III: After balancing oxygen the equation is H2 + O2  2H2O.
So now we need to balance hydrogen atom.
• So the final equation stands out to be
2H2 + O2  2H2O
Reactant Product
Initial Hydrogen atom 2 4
To balance 2 X 2 4

Types of reaction
• A chemical reaction can be categorised into several types.
• In some reaction two reactants combine to form one product. Whereas in other
many reactants undergoes reaction to give a product. In some reactions the
reactants displaces the other from their solution. Whereas in some other a
single reactant undergoes reaction and favourable conditions to decompose
into many products.

Combination reaction
• When two or more reactants undergoes chemical reaction to form a single
product, the reactions are called combination reactions i.e. A + B  AB.
• Consider the following combination reactions.
Burning of coal Formation of water
C + O2 CO2 2H2 + O2 2H2O
Formation of hydrochloric acid
H2+ Cl2 2HCl

• Formation of sodium chloride
2 Na + Cl2 2NaCl
• Reaction of lime with water

•Decomposition reaction
• When reactants on application of energy either in the form of heat, light or
electricity breaks down into products, the reaction is termed as decomposition
reaction i.e. AB + Heat  A + B. Such reactions involving the absorption of heat
are known as endothermic reactions.
• Consider the following decomposition reactions.
Silver chloride turns grey in sunlight.
This is due to the decomposition of
silver chloride into silver and chlorine by light.
• Decomposition of calcium carbonate to calcium oxide and carbon dioxide on
application of heat. Calcium oxide also known lime or quick lime finds great use
in the manufacture of cement. This reaction involves heating and hence is
termed as thermal reaction.

•Displacement reaction
In displacement reaction a more
reactive metal displaces a less
reactive metal but a less
reactive metal cannot displace
a more reactive metal.

• Lets take 5beakers
• In beaker A Zn being more reactive than
copper replaces copper from the solution
of copper sulphate and forms zinc sulphate.
CuSO4 + Zn ZnSO4 + Cu
• In beaker B iron being more reactive than
copper replaces it from its solution and forms
iron sulphate.
CuSO4 + Fe FeSO4 + Cu
• In beaker C there would be no reaction as less reactive metal (copper) cannot displace more reactive
metal (zinc) from its solution.
• In beaker D there would be no reaction as less reactive metal (copper) cannot displace more reactive
metal (iron) from its solution.
• In beaker E there would be no reaction as less reactive metal (iron) cannot displace more reactive
metal (zinc) from its solution.

Double displacement reaction
• A reaction involving the exchange of ions between the reactants is termed as
Double displacement reaction.
• Consider the following double displacement reaction.
Chemical reaction between Na2S and HCl.
Na2S + HCl NaCl + H2S
• Here Na and H are positive ions i.e. Na+ and H+, whereas S and Cl are negative
ions S2- and Cl-. Hence,

• Reaction of sodium sulphate solution with barium chloride solution.
Na2SO4(aq) + BaCl2(aq) BaSO4(s) + 2NaCl(aq)
• The reaction involves formation of precipitation. The white precipitate of BaSO4
is formed by the reaction of SO4
2- and Ba2+.
This type of reaction that involves formation of precipitation are called
precipitation reaction.

Oxidation- Reduction reaction
• A reaction in which one reactant undergoes oxidation whereas the other gets
reduced during the course of reaction are termed as oxidation-reduction
reactions or redox reactions.
• Oxidation refers to the loss of electrons or increase in oxidation state by a
molecule, atom, or ion.
• Reduction refers to the gain of electrons or decrease in oxidation state by a
molecule, atom, or ion.
Consider the following redox reaction.
• Burning sugars, such as glucose (C6H12O6) and the fatty acids in the fats we eat.
C6H12O6(aq) + 6 O2(g) 6 CO2(g) + 6 H2O(l)

• Reaction of Manganese dioxide with hydrochloric acid involves redox reaction.
• Reaction of zinc oxide and carbon.
ZnO + C Zn + CO
• In this reaction carbon is oxidised to CO and ZnO is reduced to Zn.

Difference between Oxidation and Reduction
OXIDATION REDUCTION
Losing electrons Gaining electrons
Increase in oxidation number Decrease in oxidation number
For a given compound losing hydrogen For a given compound gaining hydrogen
This reaction releases energy This reaction stores energy
oxidizing agents: Ozone, Bleach, peroxide Common reducing agent is metal

BASIS OF
COMPARISON
REDUCING AGENT OXIDIZING AGENT
Description
A reducing agent is an element or compound that has ability
to lose or ‘donate’ an electron in a redox chemical reaction.
An oxidizing agent is a reactant that removes electrons from
other reactants during a redox reaction.
Alternative Name Electron donor Electron acceptor
Nature A reducing agent is usually a metal or a negative ion. An oxidizing agent is generally a non-metal or positive ion.
During Redox
During the reaction, a reducing agent is oxidized and loses
one or more electrons.
During the reaction, an oxidizing agent is reduced and gains one
or more electrons.
Strongest
Reducing/Oxidizing
Agent
Lithium is the strongest reducing agent in solution state
whereas cesium is the strongest reducing agent in dry state.
Fluorine is the strongest oxidizing agent.
Oxidation State
The oxidation state of a reducing agent increases during a
redox reaction.
The oxidation state of oxidizing agent decreases during a redox
reaction.
Example
Examples of reducing agents include the earth metals, formic
acid, oxalic acid and sulfite compound.
Examples of oxidizing agents include halogens, potassium nitrate
and nitric acid.
Chemical Reaction Reduction reaction stores energy. Oxidation reaction releases energy.
Generalization
All metals, hydrides and polymeric hydrides are reducing
agents.
Almost all non-metals are oxidizing agents.
Electronegativity
All the good reducing agents have the atoms which have low
electronegativity.
All the good oxidizing agents have the atoms which have high
electronegativity.

Applications Of Redox Reaction
• Bleaching of fabrics
• Purification of water
• Storage of energies in batteries
• Metabolism and photosynthesis
• Vulcanization of rubber i.e increasing the strength and the elasticity of rubber
• Combustion of fuel involves the use of an oxidizing agent
• Electroplating process
• Production of different bakery products
• Galvanization of steel
• Separation of metals from ores
• Cleaning of industrial products.

Corrosion
• We all must have observed newly bought
iron, silver or coper articles appears very
shiny but with passage of time they get dull.
This is due to the layer of metal oxide that
develops on their surface.
• Rusting of iron, silver jewellery getting
tarnished, or copper articles getting covered
by green layer.
• Metals reacts with atmospheric oxygen and
produces metal oxides that are basic in
nature because they react with water to
form bases.
In case of rusting of iron, the iron reacts with the oxygen present in air and
moisture and develops rust (hydrated iron (III) oxide).

• Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects
made of metals, specially those of iron.
• In case of rusting of copper, the metallic copper reacts with oxygen, carbon-dioxide
and atmospheric moisture and develops a green coloured coating of copper
hydroxide and copper carbonate.
Copper developing green coloured rust on exposure to moist air
2Cu + H2O + CO2 + O2 Cu(OH)2 + CuCO3
• In case of tarnishing of silver articles,
• the metallic silver reacts with hydrogen
• sulphide or sulphur present in air and
• gets tarnished.
•

How to Prevent Rust in Any Situation
• Use an Alloy: The use of alloys, like stainless steel, is one of the most common ways to prevent rust, or slow it down.
Stainless steel isn’t suitable or economical for all applications, but it will work for many.
• Apply Oil: A coating of oil will help to prevent rust or slow it down, since it inhibits moisture from reaching the iron in
the metal. However, an oily surface might be problematic for some tools or machines and poses environmental and
human health concerns.
• Apply a Dry Coating: Special rust preventive products dry with no residue and form a protective barrier over metal
parts and equipment. These are effective for products in use, in shipping, storage and more.
• Paint the Metal: A good quality paint will slow down rusting by preventing moisture from reaching the metal.
• Store Properly: Store metal parts or products in a low-moisture area, or inside a temperature and humidity-controlled
environment to significantly slow down rust. Use of desiccant drying agents in this storage are also helpful.
• Galvanize: Galvanizing coats iron or steel in zinc to protect from rust. Zinc corrodes at a much slower rate than iron or
steel, so it’s highly effective for slowing rust.
• Blueing: This process creates a layer of magnetite over the metal to prevent rust. The metal must be regularly oiled to
maintain rust resistance, and it will turn blue or black in the process.
• Powder Coating: A layer of acrylic, vinyl, epoxy or other substances will prevent moisture from reaching the metal,
thereby preventing rust.
• VCI Packaging: Vapour Corrosion Inhibitors(VCI) are a type of chemical compound that when infused into various
packaging materials, protect metals by emitting rust eradicating vapors into an enclosed air space to prevent
corrosion on a metal surface. Ms. Bandana Pradhan. M.Sc, MFA, M.Ed

Rancidity
• It is generally observed that when oily food items are kept for longer time
they start to smell very bad. This happens because the fats and oils get
oxidised and hence they become rancid that changes their taste and smell.
• To avoid such spoilage antioxidants are added to oily food items. Food
items can also be prevented from getting oxidised by keeping them in air
tight packets. Due to this reason packets of chips are flushed with gas such
as nitrogen that acts as antioxidant and prevents spoilage of the fried
chips.
•
Packet of chips flushed with nitrogen

Factors Affecting Rancidity
• Oxidation
• Hydrolysis
• Presence of Microorganisms – Microbial Lipase
• Presence of Unsaturation in Fatty Acid Chain
• Polyunsaturation
• Chemical Structure of Oils and Fats
• Temperature and pH
• Heat and Light

Prevention of Rancidity
Rancidity can be prevented by several ways
• Addition of Antioxidants
There are five types of antioxidants:
• (1) Natural antioxidants.
• (2) Synthetic antioxidants.
• (3) Semi-synthetic antioxidants – gallic acid, propylgallate.
• (4) Metal chelators – citric acid, phosphoric acid.
• (5) Oxygen scavengers – ascorbic acid.
• Addition of Sequestering Agents
• Proper Storage of Fats and Oil Food

Value based Questions
• A house wife wanted her yellow house to be
white washed. She bought 10 kg of quick lime
from the market and dissolved in 30 litres of
water. She noticed that water started boiling
even when it was not being heated. Give
reason for her observation.
• Write the corresponding equation
• Name the product formed.
• Which type of reaction is this? Exothermic or
Endothermic.

• One day, Sakshi's mother noticed that the water pipe line of her house was
getting rusted. She did not want excavation of the pipe. She asked her
daughter that as she was a student of science, she should suggest some
method so that further rusting stops.
Now, answer the following questions:
• (i) What values are expressed in the above paragraph?
• (ii) What do you think Sakshi must have suggested to her mother and why?

Mohan was working in a factory. He purchased a new cycle but kept it in the
open. After two months he found that the cycle chain and even the handles got
rusted. His friend advised him to apply a coating of rust proof paint to the cycle
and not to keep it in the open in future.
• Why was the cycle rusted ?
• What is the role of rust proof paint ?
• What values are associated with this gesture ?

• A student working in the laboratory prepared an aqueous
solution of silver nitrate and kept it in a glass beaker
overnight. Next morning, he found that the beaker has
developed black turbidity.
• Why did the solution develop black turbidity ?
• In your opinion, what precaution he should have taken ?

Sonia purchased a packet of potato chips from the shop. She opened the packet and
ate some of the chips and left the packet as such in one comer of her study room.
After a gap of about two weeks, she saw the packet and wanted to do so munching
again. At that time, her elder sister Pallavi, a science student of class eleven was
present in her room. She found that a foul smell was coming out from the packet.
She immediately threw it in the dustbin and did not allow her sister to eat the chips.
• Why did potato chips develop foul smell ?
• What was the cause of the spoilage of the chips ?
• What is the nature of the chemical reaction involved in it ?
• Why do not sealed packets develop foul smell even if kept for months ?
• What values are displaced by Pallavi ?

Chemical reactions and equations

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