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Chemical Reactions and Equations
Physical changes : Those changes which are accompanied with change in physical properties of the substances but no new substance is formed are called physical changes e.g., melting of ice, boiling of water, etc.
Chemical changes : Those changes in which the original substances lose their nature and identity to form new chemical substances with different properties are called chemical changes e.g., burning of candle, cooking food, etc.
Chemical reactions : The process involving a chemical change is known as a chemical reaction. e chemical substances taken initially are called reactants and the chemical substances which are formed during a chemical reaction are called products. us, chemical reaction is a process in which breaking of chemical bonds (present in the reactant molecules) and making of new chemical bonds (in the product molecules) occur e.g., burning of magnesium ribbon in air.
Mg + O2 → MgO
Characteristics of chemical reactions : The following observations help us to determine whether a chemical reaction has taken place or not
Characteristics of chemical reactions
The following observations help us to determine whether a chemical reaction has taken place or not as following
- Change in state : Certain chemical reactions are accompanied with the change of state e.g., when a mixture of hydrogen gas and oxygen gas is ignited with an electric spark at room temperature, liquid water is formed.
2H2(g) + O2(g) → 2H2O(l)
- Change in colour : Certain chemical reactions are accompanied with the change of colour e.g., when red lead oxide is heated yellow lead monoxide is formed.
2Pb3O4(s) → 6PbO(s) + O2(g) ↑
- Evolution of a gas : Some chemical reactions are accompanied with the evolution of a gas e.g., reaction between a metal (like zinc, magnesium or iron) and dilute sulphuric acid produces hydrogen gas.
Zn(s) + H2SO4 (dil.) → ZnSO4(aq)
- Change in temperature : Some chemical reactions occur with change in heat energy or with change in temperature. The reaction produced heat after reaction is complete this type of reaction called exothermic reactions.
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) + Heat
The reaction is required heat to complete those reaction this type of reaction are called endothermic reactions.
CaCO3(s) + Heat → CaO(s) + CO2(g)
A method of representing a chemical reaction in terms of symbols and formulae of the substances participated is known as chemical equation. There are two ways to represent a chemical equation
- In terms of words : When a chemical equation is written in terms of words, it is called a word equation. The chemical reaction between granulated zinc and hydrochloric acid can be written in terms of words as
Zinc + Hydrochloric acid →Zinc chloride + Hydrogen
2. In terms of chemical formula : Chemical reaction between granulated zinc and hydrochloric acid can be written in terms of symbols and formulae as
Zn + 2HCl → ZnCl2 + H2
Conventions used in chemical equations :
- The reactants are written on the left hand side along with plus (+) sign between them.
- Similarly, products are written on the right hand side along with plus (+) sign between them.
- An arrow ( →) separates the reactants from the products.
- The arrow head points towards the products and tells about the direction of the reaction.
Balanced and unbalanced chemical equations
- Balanced chemical equation : : THe equation which contains an equal number of atoms of
each element on both sides of the arrow is called a balanced chemical equation. A balanced chemical equation must obeys the law of conservation of mass.
2Mg + O2 → 2MgO
Unbalanced chemical equation : The equation in which the number of atoms of each element on both sides of the arrow is not equal.
H2 + O2 → H2O
How to Balancing a chemical equation
Balancing of a chemical equation means to equalise the number of atoms of each element on both sides of the equation. Consider the following steps for balancing the chemical equation : For example, iron reacts with water (steam) to form iron(II, III) oxide and hydrogen.
Step-I : Write the chemical equation in the form of a word equation. Keep the reactants on the le hand side and the products on the right hand side. Separate them by an arrow (→ ) with head pointing from the reactants to products.
Iron + Steam → Iron (II, III) oxide + Hydrogen
Step-II : Write down the symbols and formulae of the various reactants and products which gives skeletal chemical equation.
Fe + H2O → Fe3O4 + H2
Step-III : Listing number of atoms of dierent elements.
|Elements||Number of atoms on LHS||Number of atoms on RHS|
Step-IV : Select the compound with maximum number of atoms to start balancing. In that compound, balance the element with maximum number of atoms (e.g., oxygen in the given equation). It may be a reactant or a product.
|Atoms of oxygen||In Reactant (H2O)||In Product (Fe3O4)|
Fe + 4H2O → Fe3O4 + H2
Step-V : To balance the atoms of an element, put a whole number coecient before the formula of the compound. If selection of the biggest formula appears inconvenient, balance the atoms of that element which occurs at minimum number of places on both sides of the equation. Atoms of the element which occur at maximum places are balanced at last.
To balance H-atoms on both sides :
|Atoms of hydrogen||In Reactant (H2O)||In Product (H2)|
|Initial||4 × 2 = 8||2|
|For Balance||8||4 × 2|
Fe + 4H2O →Fe3O4 + 4H2
To balance Fe-atoms on both sides :
|Atoms of Iron||In Reactant (Fe)||In Product (Fe3O4)|
|For Balance||1 × 3 = 3||3|
So, the equation would be 3Fe + 4H2O → Fe3O4 + 4H2
For checking the the reaction is balanced or not, we count atoms of each element on both sides of the equation. hence there is equall no of every atom in both sides.
Types of chemical reactions
As we know, in chemical reactions, bonds present in reactants break and new bonds form in the products. is exchange of species can take place in a number of ways resulting in different types of reactions, which can be explained as follows :
1. Combination reactions : The reactions in which two or more substances combine to form a single substance under suitable conditions.
Examples : – Combustion of coal
C(s) + O2(g) CO2(g)
Combination of nitric oxide with oxygen to form nitrogen dioxide.
2NO(g) + O2(g) 2NO2(g)
2. Decomposition reactions : The reactions in which a single substance breaks down to give two or more smaller substances under suitable conditions. three types of decomposition reactions are as follows : –
- thermal decomposition reactions : these reactions occur in presence of heat.
2FeSO4(s) + heat →Fe2O3(s) + SO2(g) + SO3(g)
- Electrolytic decomposition reactions : These reactions occur in presence of electric current.
2H2O(l) → 2H + O2
- Photodecomposition reactions : These reactions occur in presence of sunlight.
2AgBr(s) → 2Ag(s) + Br (g)
3. Displacement reactions : The chemical reactions in which one element takes the position of another element present in the compound. Examples :
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Note : All displacement reactions are exothermic reactions
4. Double displacement reactions : The reactions in which two compounds react by exchange of ions to form two new compounds, are called double displacement reactions. Example :
AgNO3(aq) + NaCl(aq) → AgCl(s)↓ + NaNO3(aq)
There are two types of double displacement reactions :
- Precipitation reactions : The reactions in which aqueous solution of two compounds on mixing react to form an insoluble compound which further separates out as a precipitate are called precipitation reactions. Examples : FeCl2(aq) + 2NaOH(aq) → Fe(OH)2(s)↓ + 2NaCl(aq)
- Neutralisation reactions : ose reactions of acids and bases in which product formed is neutral to litmus are known as neutralisation reactions.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O
Oxidation : The reactions in which the addition of oxygen to a substance or removal of hydrogen from a substance takes place are called oxidation reactions. On the other hand, the substance which either gives oxygen or removes hydrogen in an oxidation reaction is known as an oxidising agent. Reduction : The reactions in which addition of hydrogen to a substance or removal of oxygen from a substance takes place are called reduction reactions. On the other hand, the substance which either gives hydrogen or removes oxygen in a reduction reaction is known as reducing agent
Effect of oxidation reactions in everyday life : As oxygen is the most essential element for sustaining life, it is involved in a variety of reactions which has a wide range of effects on our daily life. The two effects are discussed below :
- Corrosion : It is a process in which metals are decayed gradually by the action of air, moisture and acids on their surface. Basically, it is caused by oxidation of metals by oxygen present in the air. Example : Rusting of iron,
4Fe(s) + 3O2(g) + 2xH2O(l) →2Fe2O3·xH2O(s)
Corrosion causes damage to car bodies, iron railings, ships and to all objects made up of metals, specially those of iron
Prevention of corrosion : Corrosion can be prevented
by coating the surface by a layer of another metal which does not corrode e.g., coating of iron with zinc.
by coating surface with grease, paint or oil, etc.
2. Rancidity : The slow oxidation of oils and fats present in food materials resulting in compounds with unpleasant smell is known as rancidity. Vacuum packing, refrigeration of food materials, placing of food materials away from direct sunlight will slow down the process of rancidity