Chemical Reactions - Meaning, Characteristics, Types, Equations, FAQs

Characteristics of Chemical Reactions

Chemical reactions include the following general characteristics of chemical reactions

1. Evolution of Gas

Some chemical reactions produce a gas, which may be observed as bubbles or effervescence.
Example:

$\mathrm{Zn}+2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2+\mathrm{H}_2 \uparrow$

2. Formation of a Precipitate

In certain reactions, an insoluble solid called a precipitate is formed when two aqueous solutions react.
Example:

$\mathrm{AgNO}_3+\mathrm{NaCl} \rightarrow \mathrm{AgCl} \downarrow+\mathrm{NaNO}_3$

3. Change in Colour

A visible change in colour indicates that a chemical reaction has taken place.
Example:

$\mathrm{Fe}+\mathrm{CuSO}_4 \rightarrow \mathrm{FeSO}_4+\mathrm{Cu}$

4. Change in Temperature (Energy Change)

Chemical reactions may absorb or release heat.

• Exothermic reaction: Heat is released

• Endothermic reaction: Heat is absorbed

Example (Exothermic):

$\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2+$ Heat

5. Change in State

Some reactions involve a change in physical state (solid, liquid, gas).
Example:

$2 \mathrm{H}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}$

gases combine to form a liquid

6. Emission of Light or Sound

Certain chemical reactions produce light and/or sound.
Example:
Burning of magnesium ribbon produces bright white light:

$2 \mathrm{Mg}+\mathrm{O}_2 \rightarrow 2 \mathrm{MgO}$

7. Change in Smell

A new substance with a different smell may be formed.
Example:
Formation of ammonia gas has a sharp, pungent smell.

Chemical Equations

Because there are so many chemical reactions going on around us, a nomenclature was established to help us express in a chemical reactions in the form of a chemical equation. A chemical equation is nothing more than a mathematical statement that represents the product production from reactants while expressing particular conditions for how the reaction was carried out.

The reactants are on the left, while the products are created on the right, and they are connected by one-headed or two-headed arrows. As an example, consider a reaction.

$A+B \rightarrow C+D$

The reactants in this case are A and B, which react to generate the products C and D. Reactants are denoted by their chemical formula in a real-world chemical equation. A chemical equation must be balanced to ensure the law of conservation of mass, which means that the number of atoms on both sides must be equal. This is the equation's balancing.

For example:

hydrogen + oxygen⟶ water
iron + oxygen ⟶ rust
potassium and chlorine gas ⟶ chloride

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Types of Chemical Reactions

The product created, the changes that occur, the reactants involved, and so on constitute the basis for various types of reactions.

1. Combustion reaction

2. Decomposition reaction

3. Neutralization reaction

4. Redox reaction

5. Precipitation or Double-Displacement Reaction

6. Synthesis reaction

1. Combustion Reaction:

A combustion reaction is a reaction between a combustible material and an oxidiser that results in an oxidised product. An oxidiser is a chemical that a fuel requires to burn, typically oxygen. Consider the burning of magnesium metal.

$2 \mathrm{Mg}+\mathrm{O}_2 \rightarrow 2 \mathrm{MgO}+$ Heat

2. Decomposition Reaction

A decomposition reaction occurs when a single component degrades into several products. Certain changes in energy in the environment, such as heat, light, or electricity, must be made to break the bonds of the molecule. Consider the disintegration of calcium carbonate, which produces CaO (Quick Lime), a significant component of cement.

$\mathrm{CaCO}_3(\mathrm{~s}) \rightarrow \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$

3. Neutralisation Reaction

A neutralisation reaction is essentially a reaction between an acid and a base that produces salt and water as byproducts. The water molecule is created by combining OH^– ions with H⁺ ions. When a strong acid and a strong base are neutralised, the overall pH of the products is 7. Consider the neutralisation reaction between hydrochloric acid and sodium hydroxide, which produces sodium chloride (common salt) and water.

$\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}$

4. Redox Reaction

A reduction-oxidation reaction occurs when electrons are transferred across chemical species. Consider the following electrochemical cell-like redox process between Zinc and Hydrogen.

$\mathrm{Zn}+2 \mathrm{H}+\rightarrow \mathrm{Zn}_2^{+}+\mathrm{H}_2$

5. Precipitation or Double-Displacement Reaction

It is a sort of displacement reaction in which two compounds react, and their anions and cations switch positions, resulting in the formation of two new products. Consider the reaction between silver nitrate and sodium chloride as an example. After the double-displacement process, the products will be silver chloride and sodium nitrate.

$\mathrm{AgNO}_3+\mathrm{NaCl} \rightarrow \mathrm{AgCl}+\mathrm{NaNO}_3$

6. Synthesis Reaction

A synthesis reaction is one of the most fundamental types of reactions in which numerous simple molecules combine under specific physical circumstances to produce a complex product. The end result is invariably a compound. Consider the sodium chloride synthesis process with the reactants solid sodium and chloride gas.

$2 \mathrm{Na}(\mathrm{s})+\mathrm{Cl}(\mathrm{g}) \rightarrow 2 \mathrm{NaCl}(\mathrm{s})$

Also check-

• NCERT Exemplar Class 11th Chemistry Solutions
• NCERT Exemplar Class 12th Chemistry Solutions
• NCERT Exemplar Solutions for All Subjects

Some Solved Examples

Question 1: Which of the following observations necessarily indicates that a chemical reaction has occurred?
A. Change in colour
B. Change in state
C. Evolution of gas
D. Formation of a new substance

Solution:

Physical changes (state, colour, gas release like boiling) may occur without chemical change.
A chemical reaction is confirmed only when a new substance is formed.
Hence, the correct answer is option D.

Question 2:

Which of the following reactions is endothermic?

A. Combustion of methane
B. Neutralization of HCl and NaOH
C. Dissociation of CaCO₃ into CaO and CO₂
D. Formation of water from H₂ and O₂

Solution:

Endothermic reactions absorb heat.

$\mathrm{CaCO}_3 \xrightarrow{\Delta} \mathrm{CaO}+\mathrm{CO}_2$

This requires a continuous heat supply.

Hence, the correct answer is option C.

Question 3:

Q3. The reaction

$\mathrm{Fe}+\mathrm{CuSO}_4 \rightarrow \mathrm{FeSO}_4+\mathrm{Cu}$

is an example of:

A. Combination reaction
B. Decomposition reaction
C. Displacement reaction
D. Redox reaction only

Solution:

Iron displaces copper from its salt solution.
It is a single displacement reaction.

(Note: It is also a redox reaction, but the classification asked is reaction type.)

Hence, the correct answer is option C.