Chemical Reactions and Equations notes

Ten Science NCERT CBSE

Chemical Reactions and Equations

Chemical Reaction:- The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc.

In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.
Following are the signs of chemical reaction:
(a) Change of state of substance.
(b) Change of colour of substance.
(c) Evolution of heat.
(d) Absorption of heat.
(e) Evolution of gas.
(f) Evolution of light.
(g) Evolution of sound, etc.

Reactant and Product:

Reactant: Substances which take part in a chemical reaction are called reactants.
Product:- New substance formed after a chemical reaction is called product.
Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.

Example:

In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.
In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations.
Chemical Equation is a way to represent the chemical reaction in concise and informative way.

Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.

Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.

Example:

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.
According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.
Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.

Example:

In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.

Balancing a chemical equation:

To balance the given or any chemical equation, follow these steps:
(1) Write the number of atoms of elements present in reactants and in products in a table; as shown here.
(2) Balance the atom which is the maximum in number; on either side of chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.
(3) Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
(4) After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.
(5) Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.
After balancing, the above equation can be written as follows.
Question: 1. Balance the given equations.

Writing the symbols of Physical States of substances in Chemical equation:

By writing the physical states of substances a chemical equation becomes more informative.
Gaseous state is represented by symbol ‘g’
Liquid state is represented by symbol ‘l’
Solid state is written by symbol ‘s’
Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.
Example: