Ncert Cbse Class 10, Science (Chemistry)
Chemical Reactions and Equations
Science Notes
What is a ‘Reaction’?
Reaction is a term used for depicting a change or
transformation in which a substance decomposes, combines with other substances,
or interchanges constituents with other substances.
What is a ‘Chemical Reaction’?
A chemical change is always accompanied by a
chemical reaction. a chemical change or reaction can be confirmed by any or all
of the following observations:
=> change in state
=> change in colour
=> change in temperature
=> evolution of gas.
What is a ‘Chemical Equation’?
A Chemical Equation represents a chemical reaction.
A complete chemical equation represents the reactants, products, and their
physical states symbolically. A chemical equation also indicates a number of
atoms of each element involved in a reaction. A chemical equation can be
written as balanced or unbalanced. An unbalanced chemical equation is also
known as Skeletal Chemical Equation for a reaction. An equation having
an equal number of atoms of each element on both the sides is called a balanced
chemical equation.
Types of Chemical Reaction
Chemical Reactions are primarily of five types as
listed below:
1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Double displacement reactions
5. Oxidation and Reduction reactions
What are ‘Combination Reactions’?
When two or more substances (elements or compounds)
combine to form a single product, the reactions are called ‘Combination
Reactions’. Generally, Combination Reactions are exothermic in nature. The
following are some examples of Combination Reactions:
CaO (s) + H2O (l) -----> Ca(OH)2
(aq)
C (s) + O2 (g) ------> CO2
(g)
What are ‘Decomposition Reactions’?
Decomposition reactions are opposite to combination
reactions. In a Decomposition reaction, a single substance decomposes to give
two or more substances.
What are ‘Displacement Reactions’?
When an element displaces another element from its
compound, such reaction is called a displacement reaction e.g. -
Zn (s) + CuSO4 (aq) -----> ZnSO4
(aq) + Cu (s)
What are ‘Double Displacement Reactions’?
A double displacement reaction is a bimolecular
process in which parts of two compounds are exchanged to give two new
compounds. The general equation used to represent double displacement reactions
can be written as -
AB + CD -----> AD + BC
Examples of double displacement reactions are -
Na2SO4 (aq) + BaCl2
(aq) -----> BaSO4 (s) + 2NaCl (aq)
“Double Displacement Reactions” have two commoj
features -
1. Two compounds exchange their ions resulting in
the formation of two new compounds.
2. One of the new compounds formed would be
separated from the mixture in some way, commonly as a solid or gas.
Double displacement reactions can be further
classified as Precipitation, Gas formation, and Acid-base neutralization
reactions.
Sample Hot Questions
Q.1: Which of the
following is a Combination and which is a Displacement Reaction?
(a) Cl2 + 2KI ----->
2KCl + I2
(b) 2K + Cl2 ------>
2KCl
Ans:
(a) Displacement reaction
(b) Combination reaction
Q.2: What is a catalytic
decomposition?
Ans: Those reactions in which a
compound splits up into two or more simpler substances in the presence of
catalyst are called catalytic reactions. For example, digestion of food in
human body.
Q.3: What is the term used for the symbolic
representation of a reaction.
Ans: Chemical equation.
Q.4: Write two uses of decomposition
reactions.
Ans: Two uses of decomposition reactions -
(i) These reactions are used to extract
several metals from their oxides and salts. When the oxide or salt is
electrolyzed, metal is obtained as per the following equation:
2Al2O3 ------>
4Al + 3O2
(ii) Digestion of food in our body is
also an example of decomposition reaction. The starch (carbohydrates) present
in the body is decomposed to give simple substance (glucose) and the proteins
are broken down into amino acids.
Q.5: Explain the terms ‘oxidizing agent’
and reducing agent’.
Ans:
Oxidizing agent
The substance, which oxidizes elements or compounds, is
known as oxidizing agent e.g. -
4Fe + 3O2 ------> 2Fe2O3
Hence, oxygen is an oxidizing agent. In this equation it
oxidizes iron (FeII) to ferric oxide (Fe2O3).
Reducing agent
The substance, which oxidizes elements or compounds, is
known as oxidizing agent e.g. -
SnCl2 + 2FeCl3 -----> SnCl4
+ 2FeCl2
Here stannous chloride is a reducing agent. It reduces
ferric chloride to ferrous chloride.
Further
Study on Chapter 1, Chemical Reactions
and Equations
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