Chemical Reactions and Equations
1. A chemical reaction involves a chemical change in which substances react to form new substances
with entirely new properties.
2. Substances that react or take part in the reaction are known as reactants, and the substances formed
are known as products.
3. Important characteristics of chemical reactions
i. Evolution of a gas
ii. Change in temperature
iii. Formation of a precipitate
iv. Change in colour
v. Change of state
4. Exothermic reaction
If heat is evolved during a reaction, then such a reaction is known as an exothermic reaction.
5. Endothermic reaction
If heat is absorbed during a reaction, then such a reaction is known as an endothermic reaction.
6. Chemical equation
The symbolic representation of a chemical reaction is called a chemical equation.
7. Features of a chemical equation
i. The reactants are written on the left hand side and the products are written on the right hand side
with a plus sign between them.
ii. An arrow separates the reactants from the products. The arrow head points towards the products
and indicates the direction of the reaction.
8. Skeletal chemical equation
A chemical equation which simply represents the symbols and formulae of reactants and products
taking part in the reaction is known as a skeletal chemical equation for a reaction. Example: For the
burning of magnesium in the air, Mg + O2 → MgO is the skeletal equation.
9. A balanced chemical equation is a chemical equation in which the number of atoms of each element
is equal on both sides of the equation.
10. As per the law of conservation of mass, the total mass of the elements present in the products of a
chemical reaction is equal to the total mass of the elements present in the reactants.
11. Balancing of chemical equations
The process of equating the number of atoms on both the sides of a chemical equation is known as
balancing of a chemical equation.
12. While balancing a chemical equation, the molecular formulae of the reactants and products should not
change. The molecular formulae are simply multiplied by suitable coefficients.
13. A chemical equation can be made more informative by using the following:
i. By indicating the physical states of the reactants and products, i.e. solid, liquid, gas and aqueous
solution.
ii. By indicating the heat changes taking place in the reaction, i.e. by writing '+Heat' on the products'
side in the equation of an exothermic reaction and by writing '+Heat' on the reactants' side in the
equation of an endothermic reaction.
iii. By indicating the conditions under which the reaction takes place, i.e. presence of catalyst,
temperature, pressure or heat conditions required for the reaction.
14. Types of chemical reactions
i. Combination reaction
ii. Decomposition reaction
iii. Displacement reaction
iv. Redox reaction
15. A combination reaction is a reaction in which two or more substances combine to give a single
product. A combination reaction can be between two elements, between an element and a compound
or between two compounds.
16. A decomposition reaction is a reaction in which a single reactant decomposes to give two or more
products. Decomposition reactions require energy in the form of heat, light or electricity.
17. A decomposition reaction is just the opposite of a combination reaction.
18. Uses of decomposition reactions
Decomposition reactions carried out by electricity are used to extract several metals from their
naturally occurring compounds such as chlorides or oxides. The digestion of food in our body is an
example of a decomposition reaction.
19. A displacement reaction is a reaction in which a more active element displaces a less active element
from its salt solution.
20. A reactivity series is a list of metals arranged in the order of decreasing reactivity. The most reactive
metal is placed at the top and the least reactive metal is placed at the bottom.
21. A double displacement reaction is a chemical reaction in which there is an exchange of ions
between the reactants to give new substances.
22. A precipitation reaction is any reaction in which an insoluble solid known as precipitate is formed.
23. Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.
24. Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.
25. If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised and is called
a reducing agent.
26. If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced and is called
an oxidising agent.
27. In terms of electronic concept, oxidation is defined as a loss of electrons, while reduction is defined as
a gain of electrons.
28. Redox reaction is a reaction in which oxidation and reduction take place simultaneously in a reaction.
29. Corrosion is the slow eating up of metals by the action of air and moisture on their surfaces.
Corrosion of iron is known as rusting. Chemically, rust is hydrated ferric oxide (Fe2O3.xH2O).
30. Advantages of corrosion
Although corrosion is undesirable, it can be advantageous in case of aluminium which on exposure to
air, gets coated with a protective layer of aluminium oxide. This protects the metal underneath from
further corrosion and damage.
31. Rancidity
When oils and fats or foods containing oils and fats are exposed to air, they get oxidised because of
which the food becomes stale and gives a bad taste or smell. This is called rancidity.
32. Rancidity can be prevented by
i. Adding antioxidants, i.e. substances which prevent oxidation in foods containing fats and oils.
ii. Refrigeration of food.
iii. Storing the food in air-tight containers.
1. A chemical reaction involves a chemical change in which substances react to form new substances
with entirely new properties.
2. Substances that react or take part in the reaction are known as reactants, and the substances formed
are known as products.
3. Important characteristics of chemical reactions
i. Evolution of a gas
ii. Change in temperature
iii. Formation of a precipitate
iv. Change in colour
v. Change of state
4. Exothermic reaction
If heat is evolved during a reaction, then such a reaction is known as an exothermic reaction.
5. Endothermic reaction
If heat is absorbed during a reaction, then such a reaction is known as an endothermic reaction.
6. Chemical equation
The symbolic representation of a chemical reaction is called a chemical equation.
7. Features of a chemical equation
i. The reactants are written on the left hand side and the products are written on the right hand side
with a plus sign between them.
ii. An arrow separates the reactants from the products. The arrow head points towards the products
and indicates the direction of the reaction.
8. Skeletal chemical equation
A chemical equation which simply represents the symbols and formulae of reactants and products
taking part in the reaction is known as a skeletal chemical equation for a reaction. Example: For the
burning of magnesium in the air, Mg + O2 → MgO is the skeletal equation.
9. A balanced chemical equation is a chemical equation in which the number of atoms of each element
is equal on both sides of the equation.
10. As per the law of conservation of mass, the total mass of the elements present in the products of a
chemical reaction is equal to the total mass of the elements present in the reactants.
11. Balancing of chemical equations
The process of equating the number of atoms on both the sides of a chemical equation is known as
balancing of a chemical equation.
12. While balancing a chemical equation, the molecular formulae of the reactants and products should not
change. The molecular formulae are simply multiplied by suitable coefficients.
13. A chemical equation can be made more informative by using the following:
i. By indicating the physical states of the reactants and products, i.e. solid, liquid, gas and aqueous
solution.
ii. By indicating the heat changes taking place in the reaction, i.e. by writing '+Heat' on the products'
side in the equation of an exothermic reaction and by writing '+Heat' on the reactants' side in the
equation of an endothermic reaction.
iii. By indicating the conditions under which the reaction takes place, i.e. presence of catalyst,
temperature, pressure or heat conditions required for the reaction.
14. Types of chemical reactions
i. Combination reaction
ii. Decomposition reaction
iii. Displacement reaction
iv. Redox reaction
15. A combination reaction is a reaction in which two or more substances combine to give a single
product. A combination reaction can be between two elements, between an element and a compound
or between two compounds.
16. A decomposition reaction is a reaction in which a single reactant decomposes to give two or more
products. Decomposition reactions require energy in the form of heat, light or electricity.
17. A decomposition reaction is just the opposite of a combination reaction.
18. Uses of decomposition reactions
Decomposition reactions carried out by electricity are used to extract several metals from their
naturally occurring compounds such as chlorides or oxides. The digestion of food in our body is an
example of a decomposition reaction.
19. A displacement reaction is a reaction in which a more active element displaces a less active element
from its salt solution.
20. A reactivity series is a list of metals arranged in the order of decreasing reactivity. The most reactive
metal is placed at the top and the least reactive metal is placed at the bottom.
21. A double displacement reaction is a chemical reaction in which there is an exchange of ions
between the reactants to give new substances.
22. A precipitation reaction is any reaction in which an insoluble solid known as precipitate is formed.
23. Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.
24. Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.
25. If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised and is called
a reducing agent.
26. If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced and is called
an oxidising agent.
27. In terms of electronic concept, oxidation is defined as a loss of electrons, while reduction is defined as
a gain of electrons.
28. Redox reaction is a reaction in which oxidation and reduction take place simultaneously in a reaction.
29. Corrosion is the slow eating up of metals by the action of air and moisture on their surfaces.
Corrosion of iron is known as rusting. Chemically, rust is hydrated ferric oxide (Fe2O3.xH2O).
30. Advantages of corrosion
Although corrosion is undesirable, it can be advantageous in case of aluminium which on exposure to
air, gets coated with a protective layer of aluminium oxide. This protects the metal underneath from
further corrosion and damage.
31. Rancidity
When oils and fats or foods containing oils and fats are exposed to air, they get oxidised because of
which the food becomes stale and gives a bad taste or smell. This is called rancidity.
32. Rancidity can be prevented by
i. Adding antioxidants, i.e. substances which prevent oxidation in foods containing fats and oils.
ii. Refrigeration of food.
iii. Storing the food in air-tight containers.
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