Chemical Reaction : Definition, Equations, and Types

Chemical Reaction

Consider what happens when

• You blow the candle.
• You leave the cooked food in a hot place in the summer.
• An iron item like a door, nail, rod, or other is left exposed to a humid atmosphere.
• Add a spoon of curd to the warm milk and let it rest for some time.

Have you noticed, in the above cases, a change of nature and identity of the material you initially took? Yes! There occur some considerable physical and chemical changes due to some chemical reactions. You are already aware of them.

But, do you know, a chemical change is carried out by some chemical reactions? Can you answer what it is? 

This article will help you understand, their examples, and the types of chemical reactions.

What is a Chemical Reaction?

The simplest answer to the question, what is a chemical reaction, is a process that involves the rearrangement of a substance’s molecular or ionic structure, distinct from a change in physical form or a nuclear reaction. A chemical reaction takes place whenever there occur the following changes:

1. Change in state
2. Change in colour
3. Evolution of a gas
4. Change in temperature

If there is a change in any of the above four cases during a process, a change in chemical reaction occurs.

The rusting of iron objects on exposure to damp air, changing milk into curd, and digestion of food in your body are all daily life chemical reaction examples that you must have noticed.

It is represented in a short form by writing a chemical equation.

What is a Chemical Equation?

The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.

For example, when zinc metal reacts with dilute sulphuric acid, it forms zinc sulphate and hydrogen gas. In words,

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

On putting the symbols and formulae of all the substances in the above word-equation, the following chemical equation will be

Zn + H₂SO₄ → ZnSO₄ + H₂ ↑

Here, Zn and H₂SO₄ are reactants, and ZnSO₄ and H₂ are products.

Chemical Reaction Definition

The most basic chemical reaction definition is a chemical change that occurs when two or more substances combine to form a new substance. It is a process in which new substances with new properties are formed.

As per the chemical reaction definition, it involves breaking bonds in the atoms of reacting substances and making new bonds between the atoms of products. During chemical reactions, a large variety of rearrangements of atoms can take place to produce new substances. Some chemical reaction examples are,

A chemical reaction is when an electron bound to one atom becomes bound to another atom, or the bound electron becomes shared across another atom. The simplest example is two separate hydrogen atoms sharing their individual bound electron between the pair and forming a hydrogen molecule.

Types of Chemical Reactions

Chemists classified chemical reactions in several ways. The four main categories, they divided are:

1. Based on the type of product:

What type of products are formed during chemical reactions, are categories under this type of reaction like gas-forming reactions, precipitation reactions, etc.

2. Based on types of reactants:

It involves the nature of the reactants like acid-base reactions, oxidation-reduction reactions, etc.

3. Based on reaction outcome:

Reactions like decomposition, polymerisation, addition, elimination, and substitution reactions are involved in this category.

4. Based on reaction mechanism:

The reactions that detail how atoms are rearranged to form products are covered under this category. Chain reactions, photolysis reactions, etc., are these reactions.

Some important types of chemical reactions are:

1. Combination Reactions:

Reactions, where two or more substances are combined to form a product, are known as combination reactions. The general chemical equation for this reaction is

A + B → AB

Some chemical reaction examples are:

C + O₂ → CO₂

2Mg + O₂ → 2MgO

Cao + H₂O → Ca(OH)₂

2SO₂  + O₂ → 2SO₃

2. Decomposition Reactions:

Reactions, where a compound breaks into two or simpler substances, are known as decomposition reactions. These reactions are carried out by applying light, heat, or electricity. The general chemical equation for this reaction is

AB → A + B

Some chemical reaction examples are:

3. Displacement Reactions:

Reactions, where one element takes the place of another, are called displacement reactions. Generally, a more reactive element displaces the less reactive element and forms the product. The general chemical equation for this reaction is

A + BC → AC + B

Some chemical reaction examples are:

CuSO₄ (aq) + Mg(s) → MgSO₄ (aq) + Cu(s)
Here, Mg is more reactive than Cu. Hence, Mg displaced Cu.

2AgNO₃ (aq) + Cu(s) → Cu(NO₃)₂ (aq) + 2Ag(s)

Here, Cu is more reactive than Ag. Hence, Ag was displaced by Cu.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂ (g)

Here, sodium is more reactive than hydrogen.

4. Double Displacement Reactions (or Precipitation Reactions):

Reactions, where two compounds react by exchanging ions to form two new compounds, are called double displacement reactions. This reaction usually occurs in solution and one of the products, being insoluble, precipitates out (or separated as a solid). The general chemical equation for this reaction is

A + soluble salt B → Soluble salt C + Precipitate ↓

Some chemical reaction examples are:

AgNO₃ (aq) + NaCl (aq) → AgCl (s) ↓ + NaNO₃

Here, AgCl will form a white-coloured precipitate and settle in the test tube.

CuSO₄ (aq) + H₂S(g) → CuS ↓ + H₂SO₄

CuS will form a black-coloured precipitate and settle in the test tube.

5. Oxidation-Reduction Reactions:

Those reactions which involve the loss and gain of electrons are called oxidation and reduction reactions. The reaction in which an ion (or atom) loses electrons is called an oxidation reaction. While the reaction in which an ion (or atom) gains electrons is called a reduction reaction.

Some chemical reaction examples are:

6. Acid-Base Reactions:

Those reactions involving acid and base reactions are called acid and base reactions. These reactions are also known as neutralisation reactions. Generally, the products that form in these reactions are salt and water. The general chemical equation for this reaction is

Acid + Base → Salt + Water

Some chemical reaction examples are:

HCl + NaOH → NaCl + H₂O

Here, HCl is acid, and NaOH is a base. While NaCl is sodium salt and H₂O is water.

2HCl + Ca(OH)₂ → CaCl₂+ 2H₂O

CH₃COOH + NaOH → CH₃COONa + H₂O

Conclusion

To conclude, the chemical reactions involve a dance of atoms. This process either breaks or makes chemical bonds but normally does both. It leads to various types of chemical reactions, including typical chemical reactions such as addition, elimination, displacement, rearrangement and redox reactions.

Frequently Asked Questions

Q1. What happens to chemical bonds when chemical reactions occur?

Answer: When a chemical reaction occurs, the bonds between the atoms of the reactants start to break down for a certain amount of time. Simultaneously, the atoms or pieces of molecules start to reassemble those reactant atoms and form products by new bonds.

During the breaking of the bonds, energy is absorbed by the atoms. While, during the formation of new bonds, atoms or molecules release energy. If the energy of breaking bonds is higher than the energy released during forming bonds, energy absorption takes place. As a result, different types of chemical reactions occur.

Q2. How can we make a chemical reaction more informative?

Answer: The chemical equation which gives more information about the chemical reaction is called a more informative or information-giving equation. There are three ways to make a chemical equation more informative:

1. By indicating the physical state of the reactants and products:

There are four physical states of reactants and products in the chemical equation, i.e., solid, liquid, aqueous solution, or gas. These are indicated by (s), (l), (aq), and (g) symbols. Symbols indicate the gas is released (↑), and an insoluble product or precipitate forms (↓).

2. By indicating the ‘heat change’ taking place in the reaction:

If heat is released during the reaction, ‘+ Heat (or Δ)’ is used with products. While, if heat is absorbing during the reaction, ‘+ Heat (or Δ)’ is used with reactants.

3. By indicating the ‘conditions’ under which the reaction takes place:

If heat is required for a reaction to occur, the heat sign delta (Δ) is placed over the arrow of the equation. Also, suppose the reaction takes place in the presence of a catalyst. In that case, the symbol or formula of the catalyst is also written above or below the arrow sign in the equation.

Q3. What is the rate of a chemical reaction?

Answer: During a chemical reaction, reactants are converted into products. It indicates how much of a reactant is consumed during a reaction or how much of a product is formed in a given time.

Rate of a chemical reaction = (Change in the concentration of a reactant or product) / Time Taken.

Therefore, you can define the rate of chemical reaction as the change of concentration of a particular reactant (or product).

The unit of rate of the reaction is M/sec or (mol L^-1 sec^-1).