Chemical Reactions and Equations

Chemical Reactions and Equations: Chemical reactions, Chemical equation, Balanced chemical equation, types of chemical reactions: combination, decomposition, displacement, double displacement, precipitation, endothermic exothermic reactions, oxidation and reduction.

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,

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.

Mg + O₂ → MgO
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.

2H₂(g) + O₂(g) —electric spark→ 2H₂O(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.

2Pb₃O₄(s) —Δ→ 6PbO(s) + O₂(g)
Lead oxide → Lead monoxide + Oxygen (colour change: yellow)

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) + H₂(g)
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.
C(s) + O₂(g) → CO₂(g) + 61 kJ
Reactions which result in fall in temperature i.e. in which heat is absorbed are called endothermic reactions.
CaCO₃(s) + Heat → CaO(s) + CO₂(g)
Limestone → Quick lime + Carbon dioxide

Chemical equation:

A method of representing a chemical reaction in terms of symbols and formulae of the substances participated is known as a 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.
Example:
Zinc + Hydrochloric acid → 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 → ZnCl₂ + H₂

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 (→) 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 + O₂ → 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 is not equal.

H₂ + O₂ → H₂O

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(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 (→) with head pointing from the reactants to products.

Iron + Steam → Iron (III) oxide + Hydrogen

Step-II: Write down the symbols and formulae of various reactants and products which gives skeletal chemical equation.

Fe + H₂O → Fe₂O₃ + H₂

Step-III: Listing number of atoms of different elements:

Elements	Number of atoms on LHS	Number of atoms on RHS
Fe	1	3
H	2	2
O	1	4

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 reactants	In products
(i) Initial	1 (in H₂O)	4 (in Fe₂O₃)
(ii) To balance	1 × 4	4

Fe + 4H₂O → Fe₂O₃ + H₂

Step-V: To balance the an element, put a whole number coefficient 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 reactants	In products
(i) Initial	8 (in 4H₂O)	2 (in H₂)
(ii) To balance	8	2 × 4

Fe + 4H₂O → Fe₂O₃ + 4H₂

To balance Fe-atoms on both sides:

Atoms of Fe	In reactants	In products
(i) Initial	1 (in Fe)	3 (in Fe₂O₃)
(ii) To balance	1 × 3	3

So, the equation would be:

3Fe + 4H₂O → Fe₂O₃ + 4H₂

Step-VI: For checking the correct balanced equation, we count atoms of each element on both sides.

Elements	Number of atoms on LHS	Number of atoms on RHS
Fe	3	3
H	8	8
O	4	4

As the number of atoms of each element on both sides of the equation are equal, the equation is balanced.

3Fe(s) + 4H₂O(g) → Fe₂O₃(s) + 4H₂(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 (↑).
Indicate the precipitate obtained in the reaction by (↓).
Mention the heat evolved by (+) sign and heat absorbed by (–) sign on the product side.

Mention the reaction conditions, temperature as °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 (→). 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 (⇌).

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) + O₂(g) → CO₂(g)
Carbon + Oxygen → Carbon dioxide
To form nitrogen dioxide:
2NO(g) + O₂(g) → 2NO₂(g)
Nitric oxide + Oxygen → Nitrogen dioxide (brown gas)
Combination of ammonia with hydrogen chloride gas to form a white solid mass of ammonium chloride:
NH₃(g) + HCl(g) → NH₄Cl(s)
Ammonia + Hydrogen chloride → Ammonium chloride

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.

2FeSO₄(s) —heat→ Fe₂O₃(s) + SO₂(g) + SO₃(g)
Ferrous sulphate (green) → Ferric oxide (reddish brown) + Sulphur dioxide + Sulphur trioxide

Electrolytic decomposition reactions:

These reactions occur in the presence of electricity.

2H₂O(l) —electric current→ 2H₂(g) + O₂(g)

Photodecomposition reactions:

The reactions occur in presence of sunlight.

2AgBr(s) —sunlight→ 2Ag(s) + Br₂(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) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Pb(s) + CuCl₂(aq) → PbCl₂(s) + Cu(s)

Double displacement reactions:

Those reactions in which two compounds react by exchanging ions to form two new compounds, are called double displacement reactions.

Example:
AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
Silver nitrate + Sodium chloride → Silver chloride + Sodium nitrate

There are two types of these 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:

FeCl₃(aq) + 2NaOH(aq) → Fe(OH)₃(s)↓ + 2NaCl(aq)
Ferric chloride + Sodium hydroxide → Ferric hydroxide (brown ppt) + Sodium chloride
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
Sodium sulphate + Barium chloride → Barium sulphate (white ppt) + Sodium chloride

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) → NaCl(aq) + H₂O
Base + Acid → Salt + Water

CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O
Black → Acid → Blue-green

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.

In other words, 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.

In other words, the substance which either gives hydrogen or removes oxygen in a reduction reaction is known as a reducing agent.

Examples:

Oxidation
ZnO + C → Zn + CO

Here, ZnO: Oxidising agent
C: Reducing agent

Reduction
2H₂S + SO₂ → 3S + 2H₂O

Here, SO₂: Oxidising agent
H₂S: Reducing agent

Effect of oxidation reactions in everyday life:

If a substance gets oxidised, then it is known as undergoing oxidation reaction which has wide range of effects in our daily life. The two effects are discussed below:

Corrosion:

It is a process in which metals are destroyed 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
Iron + Air + Moisture → Hydrated iron (III) oxide (rust)

2Cu(s) + CO₂(g) + O₂(g) + H₂O(l) → CuCO₃·Cu(OH)₂
Copper + Air + Moisture → Basic copper carbonate (green)

Corrosion causes damage to car bodies, iron railings, ships and to all objects made up of metals, 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.

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.

MNEMONICS

Concept: Types of decomposition reaction

Mnemonics: PET

Interpretations:

Photolytic reaction, Electrolytic reaction, Thermal reaction

Concept: Oxidation and reduction reaction

Mnemonics: OIL RIG

Interpretations:

Oxidation Is Loss of electrons, Reduction Is Gain of electrons

Concept: Types of chemical reactions

Mnemonics: ROCD³

Interpretations:

Reduction, Oxidation, Combination, Decomposition, Displacement, Double Displacement

Concept: Preventive ways of rusting

Mnemonics: POGG

Interpretations:

Painting, Oiling, Greasing, Galvanising