10th Class Science Chemical Reactions and Equations Chapter 1 Note
10th Class Science Chemical Reactions and Equations Chapter 1 Note
Chemical reaction:
It is a process in which one or more substances
react to form some other new substances with different properties.
Chemical equation:
It is the shorthand notation of actually occurring
chemical reaction in terms of the symbols, formulae and the ratio of the number
of various reactants and products involved.
Balanced chemical equation:
A chemical equation is said to be
balanced when the number of atoms of each element on the reactant side is equal
to the number of atoms of corresponding elements on the product side. Equations
must always be balanced according to law of conservation of mass.
A complete balanced chemical equation represents the
reactants, products and the conditions under which a reaction occurs.
Types of chemical reactions:
Chemical reactions can be
classified into combination, decomposition, displacement and double
displacement reactions.
1. Combination reaction:
A Reaction in which two or more substances combine to form a new single substance is called a combination reaction.
CaO(s) + H20 (1) --------------> Ca(OH)2 (aq)
Quick lime Water Slaked
lime
2. Decomposition reaction:
In Decomposition reaction, a single substance decomposes to give two or more
simpler substances. The reaction in which decomposition occurs by applying heat
is known as thermal decomposition.
--------------> CaC03 (s) Heat
CaO (s) + C02 (g)
Limestone Quick lime Carbon dioxide
The reaction in which decomposition
occurs by applying electricity is known as electric decomposition. 
2H2O (l) ----------------> 2H2 (g) + 02
(g)
The reaction in which decomposition
occurs in the presence of sunlight is called photo decomposition.
-------------> 2AgBr(s) Sunlight 2Ag(s) + Br2 (g) Silver bromide Silver Bromine
3. Displacement reaction:
The reaction in which a more reactive element displaces a less reactive element
from its salt solution is called displacement reaction. In displacement
reaction, one atom or a group of atoms of a compound is replaced by another
atom or group of atoms.
Fe (s) + CuS04
(aq) ---------> FeS04 (aq) + Cu
(s) Iron Copper sulphate
Iron sulphate Copper
4. Double displacement reactions:
The reactions in which two compounds exchange their ions to form two new
compounds are called double displacement reactions.
Na2SO4 (aq) + BaC12
(aq) -----------> BaS04(s) + 2NaC1 (aq)
Sodium sulphate Barium chloride Barium sulphate Sodium chloride
Exothermic reactions:
Reactions in which heat is given out
along with the products are called exothermic reactions.
Endothermic reactions:
Reactions in which heat is absorbed
are known as endothermic reactions.
Precipitation
reactions produce insoluble salts.
Redox (Oxidation-Reduction) Reactions:
1. Oxidation is a process which
involves gain of oxygen or loss of hydrogen.
2. Reduction is a process which
involves gain of hydrogen or loss of oxygen.
3. Redox reactions are those reactions in
which oxidation and reduction take place simultaneously.
4. Oxidising agent is a substance which
gives oxygen or gains hydrogen.
Alternatively, oxidising agent is the substance which
oxidises other substances and reduces itself
5. Reducing agent is a substance which
gives hydrogen or gains oxygen.
Alternatively, reducing agent is the substance which reduces
other substances and oxidises itself.
Corrosion:
The process of slowly eating up of the metals due to attack of atmospheric gases such as oxygen, carbon dioxide, hydrogen sulphide, water vapour, etc. on the surface of the metals so as to convert the metal into oxide, sulphide, carbonate, etc. is known as corrosion.
2Fe(s) + 
3/2O2 (g) + xH2O (1) -----------> Fe2O3 .xH2O
(s)
Iron moisture Hydrated ferric oxide (Rust)
------>
2Ag(s) + H2S (g) Ag2S(s) + H2
(g) Silver Hydrogen sulphide
Silver sulphide Hydrogen gas
(From air) (Black)
2Cu(s)
+ CO2 (g) + O2 (g) + H2O (1) --------> CuCO3.Cu (OH)
2 Copper From moist air
Basic
copper carbonate
(Green)
Methods to Prevent Corrosion:
Ø Painting
Ø Oiling and greasing
Ø Plastic coating
Ø Chromium plating
Ø Galvanising (zinc plating): This method allows a coating of a
more reactive metal (zinc) over the surface of iron or steel so that they can
be protected against rusting.
Ø Alloying: This method changes the properties of the metal.
Rancidity: We have often noticed
that a food containing oil or fat, if left for a long time, develops a bad
taste and smell. This is because the oils and fats present in such foods get
oxidised with the passage of time. The products (compounds) formed as a result
of this oxidation are volatile and have bad smell. Due to this reason, the
taste of the food also changes.
The oxidation of oils or fats in a food resulting into a bad
taste and smell is called rancidity.
Methods to Prevent Rancidity:
1. By adding anti-oxidants: Anti-oxidant is a
substance (or chemical) which prevents oxidation. In the food industry, a
number of special types of substances are added before packing which slow down
the process of oxidation. BHA (Butylated Hydroxy Anisole) is one of the example
of antioxidants.
2. Vacuum packing: In a number of cases,
after packing the food, the container is evacuated before sealing so that no
air/oxygen is available to bring about the oxidation of the food
3. Replacing air by nitrogen: In some food-stuffs, the
air present around them is replaced by nitrogen in the packet containing the
food-stuff. Thus, oxidation of the food-stuff is prevented. For example, the
plastic bags containing chips are flushed with nitrogen to prevent them from
oxidation.
4.
Refrigeration of the food-stuff: When the food is kept in
a refrigerator, the oxidation of fats and oils in it is slowed down due Co low
temperature, so the development of rancidity due to oxidation is retarded.
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