Chemical Reactions: Definition, Equation, Examples

Chemical Reactions: Chemical Reaction happens all around us and we do not even notice when this happens. It is a process in which the bonds are broken within reactants molecules and new bonds are created within the product molecules. We are surrounded by chemical reactions, and they could be anything, such as sugar dissolution, water evaporation, etc. There are different types of chemical reactions, such as synthesis reactions, decomposition reactions, single-replacement reactions, combustion reactions, reduction-oxidation reactions, double displacement reactions, and acid-Base reactions.

Chemical reactions have been occurring since the beginning of the world. However, the chemists came to know about them first in the 18th century; that is when they started to study and get their head around them. Processes like fermentation—wherein the sugar is converted into alcohol—have been there for centuries; however, the chemical basis of reaction was not figured out at that time. In this comprehensive article, we have explained everything about chemical reactions. Continue reading to find out more.

What is Chemical Reaction?

The processes in which a substance or substances change to produce new substances with new properties are known as chemical reactions.

For example, when calcium carbonate is heated, calcium oxide (lime) and carbon dioxide are formed. Likewise, the breaking up of calcium carbonate into calcium oxide and carbon dioxide is a chemical reaction because calcium carbonate changes into new substances, calcium oxide, and carbon dioxide.

\({\rm{CaC}}{{\rm{O}}_3} \to {\rm{CaO}} + {\rm{C}}{{\rm{O}}_2}\)

In a chemical reaction, the only rearrangement of atoms occurs, and no new atoms are formed.

The substances that take part in a chemical reaction are called reactants, and the new substances formed are called products. The products formed have entirely different properties than those of the reactants.

Characteristics of Chemical Reactions

There are some characteristics by which we can identify the occurrence of a chemical reaction and they are discussed below:

Evolution of Gas

Some chemical reactions occur with the evolution of a gas or a mixture of gases. For example,

When lead nitrate is heated, it decomposes to give a solid residue of lead monoxide with the evolution of nitrogen dioxide and oxygen gases.

\(2\;{\rm{Pb}}{\left( {{\rm{N}}{{\rm{O}}_3}} \right)_2} \to 2{\rm{PbO}} + 4{\rm{N}}{{\rm{O}}_2} + {{\rm{O}}_2}\)

Formation of a Precipitate

A precipitate is a solid product that separates from the solution during a chemical reaction.

The formation of a precipitate characterizes some reactions. A precipitate is formed when one of the products is insoluble in the solvent. For example,

When the aqueous solutions of lead acetate and potassium iodide are mixed, a yellow precipitate of lead iodide produces.

\({\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COO}}} \right)_2}\;{\rm{Pb}} + 2{\rm{KI}} \to {\rm{Pb}}{{\rm{I}}_2} + 2{\rm{C}}{{\rm{H}}_3}{\rm{COOK}}\)

Change in Temperature

Several reactions occur with a temperature change. For example, when quicklime (calcium oxide) is treated with water in a beaker, a large quantity of heat is produced. As a result, the beaker becomes very hot. Such reactions in which heat is produced are called exothermic reactions.

\({\rm{CaO}} + {{\rm{H}}_2}{\rm{O}} \to {\rm{Ca}}{({\rm{OH}})_2} + {\rm{heat}}\)

When ammonium chloride and barium hydroxide are mixed in a test tube and the mixture is stirred with a glass rod, the bottom of the test tube becomes cold. In this reaction, heat is absorbed. Such a reaction is called an endothermic reaction.

\(2{\rm{N}}{{\rm{H}}_4}{\rm{Cl}} + {\rm{Ba}}{({\rm{OH}})_2} \to 2{\rm{N}}{{\rm{H}}_4}{\rm{OH}} + {\rm{BaC}}{{\rm{l}}_2}\)

Change in Color

In some chemical reactions, a change in color observed,

A brown solution was obtained when potassium iodide solution(colorless) was added to chlorine water (yellowish).

\({\rm{C}}{{\rm{l}}_2} + 2{\rm{KI}} \to 2{\rm{KCl}} + {{\rm{I}}_2}\)

Change in State

Some chemical reactions are wax characterized by a change in their physical state.

For example, the combustion reaction of the wax candle is characterized by a difference in the physical condition. When a wax candle burnt, liquid wax and carbon dioxide formed. Here, the wax is a solid, wax after heating is in liquid form, and carbon dioxide is a gas. Thus, it shows that during the burning of wax candles, the physical state changes from solid (wax) to liquid (wax) and gas (carbon dioxide).

Types of Chemical Reactions

Chemical reactions are of different types. Some of them are described here.

Combination Reactions

The reaction in which two or more reactants combine to form a single product is called a combination reaction.

\({\rm{A + B}} \to {\rm{C}}\)

where \({\rm{A}}\) and \({\rm{B}}\) are the reactants that combine to form a single product, \({\rm{C}}\).

Let us now illustrate the combination reaction between two elements with an example.

Example- Formation of hydrogen chloride.

When hydrogen reacts with chlorine, it forms hydrogen chloride.

\({{\rm{H}}_{\rm{2}}}\left( {\rm{g}} \right){\rm{ + C}}{{\rm{l}}_{\rm{2}}}\left( {\rm{g}} \right) \to {\rm{2HCl}}\left( {\rm{g}} \right)\)

In this reaction, hydrogen and chlorine combine to form a single compound, hydrogen chloride. In addition, this reaction is used in industry for the manufacture of hydrogen chloride.

Let us now illustrate the combination reaction between two compounds with an example.

Example– Reaction between ammonia and hydrogen chloride.

When ammonia reacts with hydrochloric acid, it forms ammonium chloride.

\({\rm{N}}{{\rm{H}}_3}(\;{\rm{g}}) + {\rm{HCl}}({\rm{g}}) \to {\rm{N}}{{\rm{H}}_4}{\rm{Cl}}({\rm{s}})\)

In this reaction, ammonia and hydrogen chloride combine to produce a new compound, ammonium chloride.

Let us now illustrate the combination reaction between an element and a compound with an example.

Example- Reaction between oxygen and carbon monoxide.

When carbon monoxide reacts with oxygen, it forms carbon dioxide.

\(2{\rm{CO}}({\rm{g}}) + {{\rm{O}}_2}(\;{\rm{g}}) \to 2{\rm{C}}{{\rm{O}}_2}(\;{\rm{g}})\)

In this reaction, carbon monoxide is a compound that reacts with an element, oxygen, to form a new compound, carbon dioxide.

Decomposition Reactions

The reactions in which a compound is broken down into two or simpler substances are known as decomposition reactions. We may represent these reactions as 

\({\rm{XY}} \to {\rm{X + Y}}\)

These reactions are just the opposite of a combination reaction. The decomposition reaction occurs only when the energy is taken in the form of heat, light, or electricity. 
Depending upon the form of energy supplier, decomposition reactions are categorized into three different types.

1. Thermal Decomposition
2. Photolytic Decomposition
3. Electrolytic Decomposition

Thermal Decomposition Reaction (Thermolysis)

When a decomposition reaction takes place by absorbing heat, it is known as a thermal decomposition reaction.

When sodium hydrogen carbonate (baking soda) is heated strongly, baking soda gets decomposed into sodium carbonate, carbon dioxide, and water.

\(2{\rm{NaHC}}{{\rm{O}}_3} \to {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} + {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}}\)

Photolytic Decomposition Reaction (Photolysis)

Some reactions take place in the presence of light. These are called photochemical reactions. For example,

When silver bromide is exposed to light, it decomposes to form silver metal and bromine vapours.

In this reaction, the pale-yellow colour of silver bromide changes to greyish white due to the formation of silver metal -this reaction is also used in black and white photography.

Electrolytic Decomposition

Some compounds like oxides and chlorides of metals are decomposed by passing electricity through them in their molten state. Water decomposed when an electric current passed through it. For example,

Electrolytic decomposition of acidified water

On passing an electric current through acidified water, it decomposes to form hydrogen and oxygen gases.

We can represent the reaction as: 

\(2{{\rm{H}}_2}{\rm{O}}({\rm{l}}) \to 2{{\rm{H}}_2}(\;{\rm{g}}) + {{\rm{O}}_2}(\;{\rm{g}})\)

In this reaction, the acidified water is decomposed in an electric current, so it is called an electrolytic decomposition reaction.

This decomposition reaction is carried out with the help of electricity. Therefore, it is an electrolytic decomposition reaction.

Electrolysis of water gives two volumes of hydrogen gas and one volume of oxygen gas. Therefore, it shows that water is made up of two parts of hydrogen and one part of oxygen. Hence, its formula \({{\rm{H}}_2}{\rm{O}}\) is justified.

Decomposition Reaction Occurring in our Body

Digestion of food in our body is an example of a decomposition reaction. The food we eat mainly contains starch and proteins, which decompose into simpler substances such as simple sugars and amino acids, respectively, in the presence of enzymes. 

Uses of Decomposition Reaction

1. The decomposition reactions are generally used in the extraction of metals.
2. The electrolytic decomposition reactions are directly used to obtain certain metals by electrolysis of their molten salts. Example: sodium from molten sodium chloride and aluminum from molten alumina.
3. The thermal decomposition reactions also help in the extraction of metals. Example: zinc carbonate, ore of zinc is first decomposed to zinc oxide and then reduced to obtain zinc metal.

Single Displacement Reactions or Substitution Reactions

A reaction in which another atom displaces an atom or a group of atoms present in a molecule is known as a displacement reaction.

(a) Displacement of less active metal by a more active metal

Example- When a piece of zinc is added to a solution of copper sulfate, zinc sulfate, and copper are formed.

\({\rm{Zn}} + {\rm{CuS}}{{\rm{O}}_4} \to {\rm{ZnS}}{{\rm{O}}_4} + {\rm{Cu}}\)

In this reaction, copper is displaced from copper sulfate by zinc because zinc is more reactive than copper. As a result, a reddish deposit of copper is observed on the zinc strip. The blue color of copper sulfate disappears due to the formation of colorless zinc sulfate.

(b) Displacement of less active non-metal from its compounds

Example- When \({\rm{C}}{{\rm{l}}_2}\) gas passed through a solution of sodium bromide, sodium chloride, and bromine formed. As a result, the solution becomes brown due to the liberation of bromine in the solution.

\(2{\rm{NaBr}} + {\rm{C}}{{\rm{l}}_2} \to 2{\rm{NaCl}} + {\rm{B}}{{\rm{r}}_2}\)

(c) Displacement of hydrogen from acids by active metals

Example- When a piece of zinc was added to a dilute solution of sulphuric acid, hydrogen gas evolved, and zinc sulfate formed.

\({\rm{Zn}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{ZnS}}{{\rm{O}}_4} + {{\rm{H}}_2} \uparrow \)

Zinc can displace hydrogen from other acids also, such as hydrochloric acid \(\left( {{\rm{HCl}}} \right)\), phosphoric acid \(\left( {{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}} \right)\), and acetic acid \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COOH}}} \right)\).

Double Displacement Reaction

The reaction in which two compounds react by exchanging ions to form two new compounds is called a double displacement reaction.

\({\rm{AB + CD}} \to {\rm{BC + AD}}\)

A from reactant \({\rm{AB}}\) is exchanged by \({\rm{C}}\) from the reactant \({\rm{CD}}\) to form two new compounds \({\rm{BC}}\) and \({\rm{AD}}\).

Such reactions are also called double decomposition reactions or metathesis reactions and occur in ionic compounds.

When a solution of silver nitrate and sodium chloride is added, a white precipitate of silver chloride forms along with sodium nitrate.

\({\rm{NaCl}} + {\rm{AgN}}{{\rm{O}}_3} \to {\rm{AgCl}} \downarrow + {\rm{NaN}}{{\rm{O}}_3}\)

Precipitation Reactions

There are some chemical processes in which a solid gets separated from the solution. These processes are called precipitation reactions. The separated solid is called a precipitate.

When copper sulfate reacts with ammonium hydroxide, a bluish-white precipitate of copper hydroxide is formed along with ammonium sulfate.

\({\rm{CuS}}{{\rm{O}}_4}(aq) + 2{\rm{N}}{{\rm{H}}_4}{\rm{OH}}({\rm{aq}}) \to {\rm{Cu}}{({\rm{OH}})_2}(\;{\rm{s}}) \downarrow + {\left( {{\rm{N}}{{\rm{H}}_4}} \right)_2}{\rm{S}}{{\rm{O}}_4}(aq)\)

Neutralization Reactions

When an acid reacts with a base to form salt and water is known as a neutralization reaction.

\({\rm{Acid + Base}} \to {\rm{Salt + Water}}\)

\({\rm{HCl}} + {\rm{NaOH}} \to {\rm{NaCl}} + {{\rm{H}}_2}{\rm{O}}\)

In these reactions, the acid and the base neutralize the properties of each other.

Oxidation and Reduction Reactions (Redox Reactions)

Oxidation and reduction are either defined in terms of addition/removal of oxygen and hydrogen or terms of gain/loss of electrons.

When the copper powder is heated, its appearance becomes black. It is due to its oxidation and formation of black-colored copper (II) oxide.

\(2{\rm{Cu}}({\rm{s}}) + {{\rm{O}}_2}(\;{\rm{g}}) \to 2{\rm{CuO}}({\rm{s}})\)

In this reaction,

Substance oxidized \({\rm{ = Cu}}\); Substance reduced \({\rm{ = }}{{\rm{O}}_{\rm{2}}}\)

Reducing agent \({\rm{ = Cu}}\) ; Oxidizing agent \({\rm{ = }}{{\rm{O}}_{\rm{2}}}\)

When hydrogen gas passes over this heated material, its appearance again turns reddish-brown. Furthermore, it is due to the reduction of copper (II) oxide to copper.

\({\rm{CuO}} + {{\rm{H}}_2} \to {\rm{Cu}} + {{\rm{H}}_2}{\rm{O}}\)

Substance oxidized \({\rm{ = }}{{\rm{H}}_{\rm{2}}}\); Substance reduced \({\rm{CuO}}\)

Reducing agent \({\rm{ = }}{{\rm{H}}_{\rm{2}}}\); Oxidizing agent \({\rm{CuO}}\)

The above reactions involve both oxidation and reduction. Therefore, these are redox reactions.

Summary

The chemical reactions can be carried out in the science laboratory, but the chemical reactions also keep on occurring in our daily lives. Chemical reactions are the processes in which new substances with new products are formed. These reactions involve chemical changes. During these reactions, a rearrangement of atoms occurs between the reacting substances to form new substances having entirely different properties.

FAQs on Chemical Reactions

Here are some of the FAQs regarding Chemical Reactions:

Q.1. What type of chemical reaction takes place when electricity passes through water?
Ans: In this decomposition reaction, a single compound, water, split up to form two simpler substances, hydrogen and oxygen. This decomposition reaction takes place by the action of electricity. It is called the electrolysis of water.

Q.2. What is the chemical reaction of photosynthesis?
Ans: During photosynthesis, sunlight energy is absorbed by green plants. So, this process is an endothermic reaction. It can be represented in a chemical equation as shown alongside:
\({\rm{2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( {{\rm{aq}}} \right){\rm{ + 6C}}{{\rm{O}}_{\rm{2}}}{\rm{ + Sun}}\,{\rm{light}}\,{\rm{Energy}} \to {\rm{6}}{{\rm{O}}_{\rm{2}}}{\rm{ + }}{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}\left( {{\rm{aq}}} \right)\)

Q.3. What is meant by a chemical reaction?
Ans: The processes in which substances or substances undergo change to produce new substances with new properties are known as chemical reactions.

Q.4. What is the role of catalyst in a chemical reaction?
Ans: A catalyst is a substance that alters the rate of a reaction without being consumed in the reaction. The catalyst remains quantitatively and qualitatively unchanged after the reaction.
This reaction is commonly used in the hydrogenation of unsaturated vegetable oils to form saturated vegetable ghee using a nickel catalyst

Q.5. How to balance a chemical reaction?
Ans: A chemical equation in which the number of atoms of each element on both sides of the equation is equal; it is a balanced chemical equation.
To understand this statement more clearly, consider the following equation:
\({\rm{Mg}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{MgS}}{{\rm{O}}_4} + {{\rm{H}}_2}\)
In this reaction, magnesium reacts with sulphuric acid to produce magnesium sulfate and hydrogen gas. Now, we need to count the number of atoms of each element involved in the reaction and tabulate to find out whether the chemical equation is balanced or not.
For the above equation,

Thus, it is clear that both the reactants and the products contain \(1\) magnesium atom, \(2\) hydrogen atoms, \(1\) sulfur, atom, and \(4\) oxygen atoms, respectively. Therefore, it shows that there is an equal number of atoms of different elements in the reactants and products. Hence, the above chemical equation is said to be a balanced equation. In other words, we can say that in a balanced chemical equation, the masses of the elements in the reactants side and product side are equal.

Q.6. What are the \(4\) types of chemical reactions?
Ans: The four chemical reactions are combination reaction, decomposition reaction, displacement reactions and double replacement reactions.

Q.7. Is an explosion a chemical reaction?
Ans: Yes, an explosion is a combustion reaction. In an explosion reaction, a lot of heat is released during the reaction.

Q.8. What triggers a chemical reaction?
Ans: In a chemical reaction, the only rearrangement of atoms occurs, and no new atoms are formed. The substances that take part in a chemical reaction are called reactants, and the new substances formed are called products. The products formed have entirely different properties than those of the reactants.

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