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Chemical reactions and equations
• Following observations helps us to determine whether a chemical reaction has taken place :
o change in state
o change in color
o evolution of a gas
o change in temperature
• Balanced chemical equation
o Law of conservation of mass states that mass can neither be created nor be destroyed in a chemical
reaction. In other words, the number of atoms of each element remains the same before and after a
chemical reaction. That’s why we need to balance a chemical equation.
o Steps involved in balancing a chemical equation : Consider a chemical reaction taking place when iron
comes in contact with steam :
Fe + H 2 O Fe 3 O 4 + H 2 (Fe 3 O 4 is FeO.Fe 2 O 3 )
• Step 1 : Draw boxes around each formula. Do not change anything inside the boxes while balancing
the equation.
Fe + H 2 O Fe 3 O 4 + H 2
• Step 2 : List the number of atoms of different elements present in the unbalanced equation :
Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 1 3
H 2 2
O 1 4
• Step 3 : It is often convenient to start balancing with the compound that contains maximum number
of atoms. It may be a reactant or a product. In that compound select the element which has the
maximum number of atoms. In the above equation, Fe 3 O 4 is the compound with maximum atoms
and O is the element that has maximum atoms. So let's try to balance O on both sides. On LHS there
is only 1 O atom and on RHS there are 4 O atoms. Hence our partly balanced equation is :
Fe +4 H 2 O Fe 3 O 4 + H 2 ……………………… (Partly balanced equation)
(We have written 4 H 2 o and not H 2 O 4 )
Rewrite the number of atoms of different elements present in the partly balanced equation :
Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 1 3
H 8 2
O 4 4
• Step 4 : Now let us balance F atoms on both sides. F on LHS = 1, F on RHS = 3. Let's balance them :
3 Fe +4 H 2 O Fe 3 O 4 + H 2 ……………………… (Partly balanced equation)
Rewrite the number of atoms of different elements present in the partly balanced equation :
Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 3 3
H 8 2
O 4 4
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• Step 5 : Finally let's balance H atoms. H on LHS = , H on RHS = 2. Let's balance them :
3 Fe +4 H 2 O Fe 3 O 4 +4 H 2 …………… …………… (balanced equation)
Rewrite the number of atoms of different elements present in
the balanced equation :
o
Element
Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe
3
3
H
8
8
O
4
4
• Step 6 : Write symbols of physical states : gaseous ‐ (g), liquid ‐ (l), solid ‐ (s), aqueous (aq). (aq) is
written when a reactant or a product in present as a solution in water.
3 Fe (s) + 4 H 2 O (g) Fe 3 O 4 (s) + 4 H 2 (g)
(g) is used for H 2 O to indicate water is in the form
of steam.
Entities like presence of catalysts, pressure, temperaturee are written above or below the arrow in the
equation like :
CO (g) + 2H 2 CH 3 OH (l) ( ………………… Pressure 340 atm is indicated)
• Types of chemical reactions : During a chemical reaction, atoms of one element do not change into those of another
element. Nor do the atoms
disappear from the mixture or appear from elsewhere. Actually
the chemical reactions
involve making and breaking of bonds between atoms to produce
new substances.
o Combination reaction : When two or more substances (elements or compounds) combine to form a single
product, the reactions are called combination reactions.
• Examples :
•
A solution of slaked lime produced by the
reaction above 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 2‐3 days of white washing and
gives a shiny finish to the walls. It is interesting to know that chemical formula for marble
(and chalk too) is CaCO 3 .
• Burning of coal :
• Formation of water from hydrogen and oxygen.
• Exothermic reactions : Reactions in which heat is
released along with the formation of products are
called exothermic reactions.
• Examples :
o
o
Burning
of natural gas.
We need energy to stay alive. We
get this energy from the
food we eat. During
digestion, food is broken down into simpler substances (e. g. carbohydrates into
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glucoses). This glucose combines
with the oxygen in the cells and provides energy.
This reaction is called respiration
and is an exothermic reaction.
o
o The decomposition
of vegetable matter is also
an exothermic reaction.
o Reaction of slaked lime with water is also an example of exothermic reaction.
Decomposition reaction : A chemical reaction in which a single reactant breaks down to gove simpler
products is called a decomposition reaction.
• Examples :
• Decomposition
by heating
o When ferrous sulphate crystals ( FeSO 4 .7H2O) are heated FeSO 4 decomposes to give
ferric oxide (solid) and sulphur dioxide and sulphur trioxide (gases).
o
When calcium carbonate (limestone) is heated it decomposes to calcium oxide
(quicklime) and carbon dioxide (g). This reaction is used in various industries.
o
Another example of
decomposition by heat is
:
• Decomposition
by sunlight
o Silver chloride turns
grey in sunlight.
o
Silver bromide also behaves in the same way.
The above reactions
are used in black and white photography.
• Decomposition
by electricity
o When electric current is passed through water hydrogen and oxygen gases are
released at electrodes.
o
H 2 O (l)
H 2 (g) + O 2 (g)
• All decomposition reactions require energy in some form (heat, light, electricity). Reactions in whichh
energy is absorbed are known as endothermic reactions.
Displacement reactions : In a displacement reaction, an element or ion moves out
of one compound and
into another. (One
element is replaced by another in a compound.)
• Examples :
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• When iron nail is dipped in aqueous copper sulphate solution, Copper is displaced by iron.
• Other examples of displacement reactions are :
o
• Zinc and lead are more reactive elements
than copper. They displace copper from its
compounds.
Double displacement reactions
: Chemical reactions in which there is an exchangee of ions between the
reactants are called double displacement reactions.
o
•
Oxidation and reduction reactions : If a substance gains oxygen during a reaction, it is said to be oxidized. If
a substance loosess oxygen during a reaction, it is said to be reduced.
• Oxidation reactions :
•
•
•
• Redox reactions : A chemical reaction in which one reactant gets oxidized and another reactant gets
reduced is
called a redox reaction.
•
•
(Copper is reduced and hydrogen
is oxidized)
(Zinc is reduced and carbon
is oxidized)
•
• Effects of oxidation in daily life :
(Manganese is reduced, hydrogen is oxidized)
• When new, iron
articles are shiny but over time, they are coated with reddish brown
powder. This is due to rusting of iron (formation of Fe
3 O 4 (i.e. FeO. .Fe 2 O 3 )).
• Some other metals like silver and copper
are also attacked by the substances around (like
moisture, acids
etc.). Due to
such attack chemical reaction takes place and a coating of
metal compounds appears on metal surfaces (black in
case of silver, green in case of
copper). This coating tarnishes the metal
surface (surface looses shine). This process is
called corrosion. Corrosion causes damage to car bodies, bridges, iron railings, ships etc.
• When fats and oils present in food items come in contact with oygen for a long time, they
are oxidized. Food becomes
rancid and smell and taste change in an undisirable
manner. So
antioxidants (substances which avoid oxidation) are used, or, food stuff is packed in air tightt
containers. Potato chip manufacturers fills nitrogen gas in chip packings to avoid oxidation.
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