CHEMICAL EQUATIONSThe method of representing a chemical reaction with the help of symbols and formula of the substances involved in it, is known as a chemical equation.
REACTANTS
The substances which take part in a chemical reaction are called reactants.
The reactants are always written on the left hand side in an equation with a plus sign (+) between them.
PRODUCTS
The new substances produced as a result of chemical reaction are called products.
The products are always written on the right hand side in an equation with a plus sign (+) between them.
The arrow sign (→) is put in between the reactants and the products and always directed from left to right.
A chemical equation is a short-hand method to represent a chemical reaction.
TWO WAYS TO WRITE A CHEMICAL EQUATION
1. UNBALANCED CHEMICAL EQUATIONS.
2. BALANCED CHEMICAL EQUATIONS.
UNBALANCED CHEMICAL EQUATIONS
An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.
Example: H₂ + O₂ → H₂O
H atoms in Reactants = 2, in Products = 2
O atoms in Reactants = 2, in Products = 1
BALANCED CHEMICAL EQUATIONS
A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
Example: Zn + H₂SO₄ → ZnSO₄ + H₂
Zn atoms in Reactants = 1, in Products = 1
H atoms in Reactants = 2, in Products = 2
S atoms in Reactants = 1, in Products = 1
O atoms in Reactants = 4, in Products = 4
BALANCING OF CHEMICAL EQUATIONS
The process of making the number of different types of atoms equal on both the sides of an equation is called balancing of chemical equation.
(Steps are to be discussed in the class)
How to make a chemical equation more informative?
The chemical equation can be made more informative in three ways.
1. By indicating the physical states of the reactants and products.
2. By indicating the heat changes taking place in the reaction.
3. By indicating the conditions under which the reaction takes place.
1. To indicate the Physical States of the Reactants and Products in an Equation.
We write
For solids (s) ,
for liquids (l),
for gas (g),
for water solution or dilute (aq),
for evolving gas ↑,
for precipitates (insoluble solid product) ↓,
Like
Zn (s) + H₂SO₄(aq) → ZnSO₄ (aq) + H₂ (g) ↑
2. To indicate the heat changes in an equation
EXOTHERMIC REACTION
Those reactions in which heat is evolved are known as Exothermic Reactions.
Q. When calcium hydroxide reacts with carbon dioxide, white calcium carbonate precipitates and hydrogen gas evolved. Write the chemical reaction.
Ans. Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s)↓ + H₂O (l)
Q. What happens when we burn Methane ? Write its chemical reaction.
Ans. When Methane (natural gas) burns in the presence of Oxygen from air, Carbon Dioxide and Water is formed along with a large quantity of heat.
CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) + Heat energy
Burning of natural gas is an exothermic reaction. Infact, all combustion (burning) is exothermic reaction.
Q. What happens when respiration takes place?
Ans. When glucose undergoes a slow combustion by combining with oxygen in our cells to produce energy in a process called respiration, carbon dioxide along with water and Energy is produced.
C₆H₁₂O₆ (aq) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (l) + Enegy
Hence, Respiration is aalso an Exothermic process as energy is produced during the reaction.
Q. Write the chemical equation of burning of Magnesium wire in air. Is it an exothermic process?
Ans. When Magnesium wire burns in wire, Magnesium Oxide is produced along with heat and light energy. Hence, it is an exothermic reaction.
2Mg (s) + O₂(g) → 2MgO (s) + Heat Energy + Light Energy
Q. Fill in the blanks
Decomposition of vegetables into compost is an example of ___________ reaction.
Ans. Exothermic
ENDOTHERMIC REACTION
Those reactions in which heat is absorbed are known as Endothermic reactions.
Q. What happens when Nitrogen and Oxygen are heated to a very high temperature? What type of reaction it is?
Ans. When Nitrogen and Oxygen are heated to a very high temperature it forms Nitrogen monoxide which is an endothermic reaction.
N₂ (g) + O₂ (g) → 2NO (g)
"All the decomposition reactions are endothermic as they need energy to start the reaction"
(i). Like the decomposition of Calcium Carbonate is an endothermic reaction.
(ii). Photosynthesis is also an endothermic reaction.
(iii). Electrolysis of water to form hydrogen and oxygen is also a decomposition reaction.
3. To indicate the conditions under which the reaction takes place.
{If heat is required for a reaction to take place, then the heat sign delta (Δ) is put over the arrow of the equation. If the reaction takes place in the presence of a catalyst than the symbol or formula of the catalyst is also written above or below the arrow sign in the equation.}
1. When potassium chlorate (KClO₃) is heated in the presence of manganese dioxide (MnO₂) catalyst, it decomposes to form potassium chloride (KCl) and oxygen (O₂) gas.
2. Methanol (CH₃OH) is manufactured from carbon monoxide (CO) and hydrogen (H₂) under extreme pressure (300 atm) and temperature (300°C) and in the presence of ZnO + CrO₃
3. In photosynthesis, glucose is formed from Carbon Dioxide and Water under Sunlight and Chlorophyll as catalyst.
Q. Convey the following information in the form of a balanced chemical equation:
On adding an aqueous solution of sodium hydroxide to an aqueous solution of copper sulphate copper hydroxide is precipitated and sodium sulphate remains in solution.
Ans: NaOH (aq) + CuSO₄ (aq) ⟹ Cu(OH)₂ (s)↓ + Na₂SO₄ (aq)
It is an unbalanced equation. When balanced it becomes
2NaOH (aq) + CuSO₄ (aq) ⟹ Cu(OH)₂ (s)↓ + Na₂SO₄ (aq)
IRON OXIDES
There are two types of iron oxides.
Iron (II) oxide FeO, and Iron (III) oxide Fe₂O₃
Iron (II) oxide FeO is known as Ferrous oxide.
Iron (III) oxide Fe₂O₃ is known as Ferric Oxide.
Mixture of both the oxide is called Magnetic Iron Oxide (Fe₃O₄).
Q. Write a balanced equation for the chemical reaction when heated iron metal reacts with steam to form iron (II, III) oxide and hydrogen.
Ans. Fe (s) + H₂O (g) → Fe₃O₄ (s) + H₂ (g)
But it is an unbalanced equation, when balanced it becomes,
3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g)
Q1. Translate the following statement into chemical equation and then balance the equation:
Hydrogen gas combines with nitrogen to form ammonia.
Q2. Write the balanced chemical equation for the following reaction:
Sodium metal reacts with water to form sodium hydroxide and hydrogen.
Q3. Write the balanced chemical equation for the following reaction:
(i) Aluminium reacts with copper chloride to form aluminium chloride and copper.
(ii) Aluminium hydroxide reacts with sulphuric acid to form Aluminium sulphate and water.
(iii) Manganese oxide reacts with hydrochloric acid to form manganese chloride, chlorine gas and water.
Q4. When the solution of substance X is added to a solution of potassium iodide, then a yellow solid separates out from the solution.
a. What do you think substance X is likely to be?
b. Name the substance which the yellow solid consists of.
c. Which characteristic of chemical reactions is illustrated by this example?
d. Write a balanced chemical equation for the reaction which takes place. Mention the physical states of all the reactants and products involved in the chemical equation.
