Chapter:- 1
Chemical Reactions and Equations
Chemical Reactions
When reactants combines together and forms the product, this process is called chemical reaction.
Reactant + Reactant → Product
Physical and chemical changes
Chemical change – one or more new substances with new physical and chemical properties are formed.
Physical change – change in colour or state occurs but no new substance is formed.
Example: Water changes to steam on boiling but no new substance is formed(Even though steam and water look different when they are made to react with a piece of Na, they react the same way and give the exact same products).
Characteristics of Chemical Reactions....
Chemical Equations
The method of representing a chemical reactions with the help of symbols and formulae of the substance involved in it os calles chemical equation.Word equation:-
symbols equation:-
Why balancing of a chemical reaction is needed ?
To follow the rule of low of conservation of mass we should need to balance any chemical reaction.
Type of Chemical Reactions
Chemical reactions can be classified in following types :
● Combination
● Decomposition
● Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation
(i) Combination Reaction: Reactions in which two or more reactants combine to form one product are called Combination Reactions.
Example:-
(ii) Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reaction. A decomposition reaction is just the opposite of combination reaction.
Example:-
on heating
(iii) Displacement Reaction: The chemical reactions in which a more reactive element displaces a less reactive element from a compound is known as Displacement Reactions.
Example:-
(iv) Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called Double Displacement Reactions.
Example:-
(v)Redox reaction:- A redox reaction occurs when the oxidation states of the substrate change. The loss of electrons or an increase in the oxidation state of a chemical or its atoms is referred to as oxidation. The gain of electrons or a decrease in the oxidation state of a chemical or its atoms is referred to as reduction.
Oxidation and reduction take place simultaneously.
Oxidation: Substance loses electrons or gains oxygen or loses hydrogen.
Reduction: Substance gains electrons or loses oxygen or gains hydrogen.
Oxidising agent – a substance that oxidises another substance and self-gets reduced.
Reducing agent – a substance that reduces another substance and self-gets oxidised.
Exothermic reaction – heat is evolved during a reaction. Most of the combination reactions are exothermic.
CH4 + 2O2 → CO2 + 2H2O + heat
Endothermic – Heat is required to carry out the reaction.
6CO2 + 6H2O + Sunlight → C6H12O6 + 6O2
Most of the decomposition reactions are endothermic.
Effect of oxidation reaction in everyday life
Corrosion
Gradual deterioration of a material, usually a metal, by the action of moisture, air or chemicals in the surrounding environment.
Rusting:
4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3.xH2O(rust)
Corrosion of copper:
Cu(s) + H2O(moisture) + CO2(from air) → CuCO3.Cu(OH)2(green)
Corrosion of silver:
Ag(s) + H2S (from air) → Ag2S(black) + H2(g)
Rancidity
It refers to the oxidation of fats and oils in food that is kept for a long time. It gives foul smell and bad taste to food. Rancid food causes stomach infection on consumption.
Prevention:
(i) Use of air-tight containers
(ii) Packaging with nitrogen
(iii) Refrigeration
(iv) Addition of antioxidants or preservatives