Download Chemical Reactions and Equations

Bal Bharati Public School, Pitampura
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.
(e) Redox reaction : If a substance gains oxygen or looses hydrogen during a reaction it
is oxidized. On the other hand if a substance loses oxygen or gains hydrogen during a
reaction, it is reduced. Both oxidation and reduction take place simultaneously and
hence these are called redox reaction. Oxidation
ZnO
Reduction
Cl2
+
C
Zn + CO
Oxidation
+
H2S
2HCl + S
Reduction
(f) Exothermic reaction and endothermic reaction : On the basis of energy changes
during chemical reaction, they can be classified as
(i) Exothermic reaction- A chemical reaction in which heat energy is produced is called
exothermic reaction .For e.g. (i) C (s) + O2
CO2 (g) + Heat
(ii) Endothermic reaction- A chemical reaction in which heat energy is absorbed is called
endothermic reaction. CaCO3 + Heat
CaO + CO2
5. Effects of some oxidation reaction in everyday life:
(a) Corrosion – The process of slow conversion of metals into their undesirable
compounds due to their reaction with moisture and other gases present in the
atmosphere is called corrosion of metals. Some common examples of corrosion are
rusting of iron, formation of green layer on the surface of copper, tarnishing of silver etc.
Formation of brown layer on the surface of iron is called rusting of iron. Formula of rust
is Fe2O3.xH2o. Rusting of iron is a serious problem.
(b) Rancidity- The taste and odour of food materials containing fat and oil changes
when they are left exposed to air for a long time. This is known as rancidity. Rancidity is
caused due to oxidation of fat and oil present in food materials. Rancidity can be
prevented by using various methods such as by adding antioxidants to the food
materials, storing food in air tight container and by flushing out air with some inert gases
like nitrogen.