CBSE Class 10 Science Chemical Reactions And Chemical Equations Notes Set 04

QUICK RECAP

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 coal, cooking food, etc.

Chemical reactions : The process involving a chemical change is known as a chemical reaction. The chemical substances taken initially are called reactants and the chemical substances which are formed during a chemical reaction are called products. Thus, 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.
\( 2Mg + O_2 \rightarrow 2MgO \)
Reactants Product

Characteristics of chemical reactions : The following observations help us to determine whether a chemical reaction has taken place or not.

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.
\( 2H_{2(g)} + O_{2(g)} \xrightarrow[\text{room temp.}]{\text{electric spark}} 2H_2O_{(l)} \)
Hydrogen Oxygen Water

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.
\( 2Pb_3O_{4(s)} \xrightarrow{\text{heat}} 6PbO_{(s)} + O_{2(g)} \uparrow \)
Lead tetraoxide (red colour) Lead monoxide (yellow colour) Oxygen

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.
\( Zn_{(s)} + H_2SO_4 (\text{dil.}) \rightarrow ZnSO_{4(aq)} + H_{2(g)} \uparrow \)
Zinc Sulphuric acid Zinc sulphate Hydrogen

Change in temperature : Some chemical reactions occur with change in heat energy or with change in temperature.

• Reactions which result in rise in temperature i.e., in which heat is evolved are called exothermic reactions e.g.,
  \( C_6H_{12}O_{6(aq)} + 6O_{2(g)} \rightarrow 6CO_{2(g)} + 6H_2O_{(l)} + \text{Heat} \)
  Glucose
• Reactions which result in fall in temperature i.e., in which heat is absorbed are called endothermic reactions e.g.,
  \( CaCO_{3(s)} + \text{Heat} \rightarrow CaO_{(s)} + CO_{2(g)} \)
  Lime stone Quick lime or lime Carbon dioxide

Chemical equation : A method of representing a chemical reaction in terms of words or symbols and formulae of the substances participating in the chemical reaction 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 \( \rightarrow \) Zinc chloride + Hydrogen
Reactants Products

In terms of symbols and formulae : Chemical reaction between granulated zinc and hydrochloric acid can be written in terms of symbols and formulae as :
\( Zn + 2HCl \rightarrow ZnCl_2 + H_2 \)
Reactants Products

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 (\( \rightarrow \)) separates the reactants from the products.
• The arrowhead 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.
  \( 2Mg + O_2 \rightarrow 2MgO \)
  A balanced chemical equation must obey the law of conservation of mass.
• Unbalanced chemical equation : The equation in which the number of atoms of each element on both sides of the arrow are not equal is called an unbalanced chemical equation.
  \( H_2 + O_2 \rightarrow H_2O \)

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 left hand side and the products on the right hand side. Separate them by an arrow (\( \rightarrow \)) with head pointing from the reactants to products.
Iron + Steam \( \rightarrow \) Iron (II, III) oxide + Hydrogen

Step-II : Write down the symbols and formulae of the various reactants and products which gives skeletal chemical equation.
\( Fe + H_2O \rightarrow Fe_3O_4 + H_2 \)

Step-III : List number of atoms of different elements.

Step-IV : Select the compound with maximum number of atoms to start balancing. It may be a reactant or a product. In that compound, balance the element with maximum number of atoms (e.g., oxygen in the given equation).

Step-V : To balance the atoms of an element, put a whole number coefficient before the formula of the compound. 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.

Step VI : For checking the correct balanced equation, we count atoms of each element on both sides of the equation. As the number of atoms of each element on both sides of the equation are equal, the equation is balanced.
\( 3Fe_{(s)} + 4H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + 4H_{2(g)} \)

How can a chemical equation be made more informative?

A chemical equation can be made more informative by adding some extra information to the chemical equation which may be summarised as :

• Write the state symbols for the reactants and products taking part in a chemical reaction. Use symbols (g) for gases, (s) for solids, (l) for liquids and (aq) for aqueous solutions.
• Indicate the gas evolved in the reaction by (\( \uparrow \)).
• Indicate the precipitate obtained in the reaction by (\( \downarrow \)).
• Mention the heat evolved by (+) sign and heat absorbed by (–) sign on the product side.
• Mention the reaction conditions, temperature as t °C or K, pressure as atm, catalyst, etc. above or below the arrow.
• When reactants are converted into products and products cannot be converted back to reactants then the reaction is called an irreversible reaction and is represented by (\( \rightarrow \)). On the other hand, if in a chemical reaction, reactants are converted into products in forward direction and again products are converted back into reactants in backward direction then, it is called a reversible reaction and is represented by (\( \rightleftharpoons \)).

Types of chemical reactions :

As we know, in chemical reactions, bonds present in reactants break and new bonds form in the products. This exchange of species can take place in a number of ways resulting in different types of reactions, which can be explained as follows :

Combination reactions : The reactions in which two or more substances combine to form a single substance under suitable conditions. Examples :

Combustion of coal.
\( C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)} \)

• Combination of nitric oxide with oxygen to form nitrogen dioxide.
  \( 2NO_{(g)} + O_{2(g)} \rightarrow 2NO_{2(g)} \)
  Nitric oxide Nitrogen dioxide (brown gas)
• Combination of ammonia with hydrogen chloride gas to form a white solid mass of ammonium chloride.
  \( NH_{3(g)} + HCl_{(g)} \rightarrow NH_4Cl_{(s)} \)
  Ammonium chloride (white)

Decomposition reactions : Those 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.
\( 2FeSO_{4(s)} \xrightarrow{\text{heat}} Fe_2O_{3(s)} + SO_{2(g)} + SO_{3(g)} \)
Ferrous sulphate (green), Ferric oxide (reddish brown), Sulphur dioxide, Sulphur trioxide

Electrolytic decomposition reactions : These reactions occur in presence of electric current.
\( 2H_2O_{(l)} \xrightarrow{\text{electric current}} 2H_{2(g)} + O_{2(g)} \)

Photodecomposition reactions : These reactions occur in presence of sunlight.
\( 2AgBr_{(s)} \xrightarrow{\text{sunlight}} 2Ag_{(s)} + Br_{2(g)} \)
Silver bromide Silver Bromine

Displacement reactions : Those chemical reactions in which one element takes the position of another element present in the compound. Examples :
\( Zn_{(s)} + CuSO_{4(aq)} \rightarrow ZnSO_{4(aq)} + Cu_{(s)} \)
\( Pb_{(s)} + CuCl_{2(aq)} \rightarrow PbCl_{2(aq)} + Cu_{(s)} \)
Note : All displacement reactions are exothermic reactions.

Double displacement reactions : Those reactions in which two compounds react by exchange of ions to form two new compounds, are called double displacement reactions. Example :
\( AgNO_{3(aq)} + NaCl_{(aq)} \rightarrow AgCl_{(s)} \downarrow + NaNO_{3(aq)} \)
Silver nitrate Sodium chloride Silver chloride Sodium nitrate
There are two types of double displacement reactions :

Precipitation reactions : Those 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 :
\( FeCl_{2(aq)} + 2NaOH_{(aq)} \rightarrow Fe(OH)_{2(s)} \downarrow + 2NaCl_{(aq)} \)
Ferrous hydroxide (dirty green ppt.)
\( Na_2SO_{4(aq)} + BaCl_{2(aq)} \rightarrow BaSO_{4(s)} \downarrow + 2NaCl_{(aq)} \)
(Barium sulphate) (white ppt.)

Neutralisation reactions : Those reactions of acids and bases in which product formed is neutral to litmus are known as neutralisation reactions. Examples :
\( NaOH_{(aq)} + HCl_{(aq)} \rightarrow NaCl_{(aq)} + H_2O \)
Base Acid Salt
\( CuO_{(s)} + 2HCl_{(aq)} \rightarrow CuCl_{2(aq)} + H_2O \)
Base Acid Salt

Redox reactions : Reactions in which oxidation and reduction take place simultaneously are called redox reactions.

Oxidation : Those reactions in which the addition of oxygen to a substance or removal of hydrogen from a substance takes place are called oxidation reactions. The substance which either gives oxygen or removes hydrogen in an oxidation reaction is known as an oxidising agent.

Reduction : Those reactions in which addition of hydrogen to a substance or removal of oxygen from a substance takes place are called reduction reactions. The substance which either gives hydrogen or removes oxygen in a reduction reaction is known as reducing agent.

Examples :
\( ZnO + C \xrightarrow{\Delta} Zn + CO \)
ZnO : Oxidising agent, C : Reducing agent
\( 2H_2S + SO_2 \xrightarrow{\Delta} 3S + 2H_2O \)
\( SO_2 \) : Oxidising agent, \( H_2S \) : Reducing agent

Effect of oxidation reactions in everyday life :

As oxygen is the most essential element for sustaining life, it is involved in variety of reactions which has 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,
\( 4Fe_{(s)} + 3O_{2(g)} + 2x H_2O_{(l)} \rightarrow 2Fe_2O_3 \cdot xH_2O_{(s)} \)
Iron Air Moisture Hydrated Iron(III) oxide

• \( 2Cu_{(s)} + CO_{2(g)} + O_{2(g)} + H_2O_{(l)} \rightarrow CuCO_3 \cdot Cu(OH)_2 \)
  Air Moisture Basic copper carbonate (green)
• 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 with a layer of another metal which does not corrode, e.g., coating of iron with zinc.
  • by coating the surface with grease, paint or oil, etc.
• 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.