Chemical Reactions and Equations – Revision Notes (Class 10)

Chemical Reactions and Equations

Chemical Change
New substances formed.
Change in chemical properties.
Examples are as bellow:
(a) Curdling of milk.
(b) Rusting of iron.
(c) Candle Burning – Wax combines with atmospheric Oxygen to give Carbon dioxide.
(d) Combustion of Liquid Petrolume Gax.

Rancidity
Rancidity, condition produced by aerial oxidation of unsaturated fat present in foods and other products, marked by unpleasant odour or flavour.
Oxidation of oils and fats in food resulting in bad smell and taste.
How to avoid Rancidity
(a) This can be avoided by packing in airtight containers.
(b) This can be avoided by keeping vegetables, fruits in fridge.
(c) This can be avoided by flushing with Nitrogen gas.
(d) This can be avoided by using antioxidants (reducing agents)

Corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide.
Metals are eaten up by action of air, water and other substances.
Iron is corroded by atmospheric oxygen and humidity and form red-brown substance called rust.
How to avoid Corrosion
(a) This can be avoided by Painting.
(b) This can be avoided by Greasing.
(c) This can be avoided by Galvanization.
(d) This can be avoided by Electroplating.

Observations that help to determine a chemical reaction
(a) Evolution of a gas
(b) Formation of precipitate
(c) Change in state
(d) Change in colour
(e) Change in temperature

Chemical Reaction
One or more substances react to form new compounds with new properties.
Examples are as follows:
(a) Heating copper wire in presence of air.
(b) Magnesium ribbon is usually cleaned before burning in air as it has a coating of magnesium oxide due to a chemical reaction. This hinders the process of
burning.

Physical Change
(a) No new substances formed.
(b) Change of shape, size, color or density of original substance.
Eg:
(a) Melting of ice -> water
(b) Boiling of water ->water vapour
(c) Dissolution of salt into water

Balancing Chemical Equation
(a) Number of atoms of each element should be equal on both sides.
(b) Follows law of conservation of mass (Mass Reactants = Mass Products).
Eg: 2Pb(NO₃)₂ —-> 2PbO + 4NO₂ + O₂

Writing Chemical Equation
(a) Skeletal equation: number of atoms of each element on both sides are not equal
(b) Balanced chemical equation: number of atoms of each element on both sides are equal.
2H₂ + O₂->2H₂0

Chemical Equation
(a) It is a symbolic representation of chemical reaction through symbols and formulae.
(b) Physical state of an element is written as gas (g), liquid (I), solid (s) and when solution made in water called aqueous (aq).
(c) The reaction conditions such as temperature, pressure and catalyst are indicated above or below the arrow.

Combination Reaction : Two elements or compounds combine to form new compounds .(A combination reaction (also known as a synthesis reaction) is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product).
Example  
2Mg(s) + 0₂(g) –> 2Mgo(s)
CaO(s) + H₂0 –> Ca(0H)₂ + Heat

Exothermic Reaction : Heat is produced or released (An exothermic reaction is a reaction in which energy is released in the form of light or heat. Thus in an exothermic reaction, energy is transferred into the surroundings rather than taking energy from the surroundings as in an endothermic reaction. )
Eg:
Respiration during digestion. CO₂ in food breaks down to glucose. This glucose with O₂ releases energy.
CH₄(g) + 2Q₂(g)—>CO₂(g) + 2H₂O

Endothermic Reaction : Heat is aborbed. (Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products.)
Eg:
CaCO₄ (s) —> CaO(s) + CO₂(g)

Decomposition Reaction : One compound (or reactant) requires energy (heat, light or electricity) to break into two or more elements/compounds or products. (A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances.)
Eg:
FeSO₄(s) –> Fe₂O₃ + SO₂ + SO₃
Decomposition reactions are always endothermic.

Thermal Decomposition : A single compound breaks into two or more elements/compounds on heating. (Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat.)
Eg:
CaCO₃ –> CaO + CO₂

Electrolytic Decomposition : A reactant breaks down on applying electric current. (Electrolytic decomposition is a chemical decomposition in which a single reactant is decomposed into more than one products by the passage of electricity.)
Eg:
2H₂O(l)  –> 2H₂(g) + O₂(g)

Photochemical Decomposition : A reactant breaks down in presence of sunlight. (A decomposition reaction where decomposition takes place in the presence of light is known as photochemical decomposition reaction)
Eg:
2AgBr –> 2Ag + Br₂
(AgBr sale Black and White photography)

Oxidation Reaction (An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species.)
a) Gain of oxygen atoms
b) Loss of hydrogen atoms
Eg:
2Cu(s) + O₂ –> 2CuO(s)

Reduction Reaction (An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species.)
a) Gain of hydrogen atoms
b) Loss of oxygen atoms
Eg:
CuO + H₂ –> Cu + H₂O

Redox Reaction : Reaction in which both oxidation and reduction takes place. (An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species.)
Eg:
ZnO + C –> Zn + CO

Oxidizing Agent : Substance which gives oxygen or gains hydrogen or, that which reduces itself is an oxidizing agent. (Oxidising agent (oxidiser) is a substance that has the ability to oxidize other substances)

Reducing Agent : Substance which gives hydrogen or gain oxygen or that which oxidizes itself is a reducing agent. (A reducing agent (also called a reductant or reducer) is an element or compound that loses (or “donates”) an electron to an electron recipient (oxidizing agent) in a redox chemical reaction.)

Displacement Reaction : (A displacement reaction is the one wherein the atom or a set of atoms is displaced by another atom in a molecule. )
More reactive element displaces less reactive.
Eg:
Fe + CuSO₄ –> FeS0₄ + Cu
(Fe is more reactive than Cu) Fe changes CuSO₄ color to light green ~ FeSO₄

Double Displacement Reaction :  Two compounds exchange their ions to form new compounds. (Double displacement reactions may be defined as the chemical reactions in which one component each of both the reacting molecules is exchanged to form the products. )
Eg:
Na₂SO₄ + BaCl₂  –> BaSO₄ + 2NaCl
After a chemical reaction when one of the products formed in solid state called precipitation reaction.
Eg:
Pb(NO₃)₂ + 2KI –> Pbl₂   2KNO.
Takes place only when there is a formation of highly soluble salt.