Types of Chemical Reactions: Combination, Displacement, Decomposition

The processes in which a substance or substances change to produce new substances with new properties are known as chemical reactions. There are different types of chemical reactions happening around us. 

Chemical reactions involve breaking old chemical bonds between the atoms of reacting substances and then making new chemical bonds between the rearranged atoms of new substances. The substances that take part in a chemical reaction are called reactants, and the new substances formed as a result of a chemical reaction are called products. In this article, let’s understand everything about chemical reactions, and their types.

Chemical Reactions

A rearrangement of atoms occurs between reactant molecules during a chemical reaction to form new products with entirely different properties. Therefore, it may be concluded that chemical reactions involve breaking bonds in reactant molecules and forming new bonds to give new products.

A chemical change is made through a chemical reaction in which reactants combine to form one or more products. As we have read, a chemical reaction involves the breaking of old bonds and forming new bonds.

Types of Chemical Reactions

The chemical reactions can broadly be classified into the following types:

1. Combination reaction,
2. Decomposition reaction,
3. Displacement reaction,
4. Double displacement reaction,
5. Oxidation and Reduction reaction

Combination Reaction or Synthesis Reaction

The chemical reaction involving combining two or more reactants (elements or compounds) to form a single new product is called a combination reaction. A reaction made to obtain a product using two or more elements is usually referred to as the synthesis of that compound.

Combination reactions, like other reactions, also involve the exchange of energy, i.e. heat, light, electricity, pressure or catalyst.

\({\text{A}} + {\text{B}} \to {\text{AB}}\)

Where \({\text{A}}\) and \({\text{B}}\) are reactants while \({\text{AB}}\) is the product.

(i) Combination of Hydrogen and Oxygen Forming Water

Hydrogen burns in an atmosphere of oxygen to form only water.

\(2{{\text{H}}_2} + {{\text{O}}_2} \to 2{{\text{H}}_2}{\text{O}}\)

(ii) Combination of Carbon and Oxygen to form Carbon Dioxide

Formation of \({\text{C}}{{\text{O}}_2}\) by burning carbon (coke or coal) in oxygen (of air).
\({\text{C}} + {{\text{O}}_2} \to {\text{C}}{{\text{O}}_2}\)

In all the above reactions, two or more reactants combine together to form a single product. Hence, these are combination reactions. In all combination reactions, heat is evolved, i.e., all combination reactions are exothermic.

(iii) Dissolution of Quick Lime in Water to form Lime Water

\({\text{CaO}} + {{\text{H}}_2}{\text{O}} \to {\text{Ca}}{\left({{\text{OH}}}\right)_2} + {\text{Heat}}\)

In this reaction, the two compounds \({\text{CaO}}\) and \({{\text{H}}_2}{\text{O}}\) combine to form a single product, \({\text{Ca}}{\left({{\text{OH}}} \right)_2}.\) Hence, it is a combination reaction.

A solution of calcium hydroxide produced in the above reaction is known as slaked lime and is used for whitewashing. Slaked lime is applied on the walls, forming a thin layer of calcium carbonate reacting with carbon dioxide present in the air. The formation of this layer gives a shiny appearance to the walls.

\({\text{Ca}}{\left({{\text{OH}}} \right)_2}\left({{\text{aq}}} \right) + {\text{C}}{{\text{O}}_2}\left( {\text{g}} \right) \to {\text{CaC}}{{\text{O}}_3}\left({\text{s}} \right) + {{\text{H}}_2}{\text{O}}\left({\text{l}} \right)\)

Decomposition Reactions or Analysis Reaction

The chemical reaction in which a compound is broken up into two or more -simpler substances (elements or compounds) is called a decomposition reaction.

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

Where reactant \({\text{A}}\) decomposes to give simpler products \({\text{B}}\) and \({\text{C}}.\)

Decomposition reactions involve absorption of energy, e.g. heat, light or electricity. Heat, light and electricity are different forms of energy and causes the breaking of bonds in the molecules of reactants to yield simpler products. The decomposition reactions involving absorption of heat are called thermal decomposition reactions. The decomposition reactions involving the absorption of light are called photochemical decomposition reactions. Those caused by involving the absorption of electricity are termed electrolytic decomposition reactions.

(i) Thermal decomposition of calcium carbonate: Calcium carbonate (limestone) on heating strongly decomposes to give calcium oxide (quick lime) and carbon dioxide.

\({\text{CaC}}{{\text{O}}_3}\overset{\Delta }{\mathop \to } {\text{CaO}} + {\text{C}}{{\text{O}}_2}\)

(ii) Electrolytic Decomposition Reactions: A chemical reaction involving decomposition of a compound by the passage of electric current (direct current, d. c.) is called electrolytic decomposition. This phenomenon is also known as electrolysis.

Electrolytic decomposition of water: On passing electric current (d. c.) through acidified water, decomposition of water yields hydrogen gas and oxygen gas.

Electrolytic decomposition of water to hydrogen and oxygen has also termed the electrolysis of water.

Experiment: As shown in Figure, take a wide-mouthed bottomless glass vessel and clamp it in an inverted position. Fit the rubber corks in the hole of the container. Now insert carbon (graphite) electrodes in the holes of the corks as shown in Figure. The container is filled with water to the height of the electrodes immersed. Now add a few drops of sulphuric acid to the glass vessel to acidify the water. The two identical test tubes are inverted over the two graphite electrodes.

The lower ends of graphite electrodes are connected to a 6-volt battery through a switch. The electrode connected to the ‘+’ terminal of the battery is called an anode. On passing the current for some time, the formation of bubbles inside the test tubes is noticed.

These bubbles are the result of the formation of gases at the two electrodes, which slowly cause the downward displacement of water in the two test tubes. The gases collect in the test tubes, and water levels go on falling as long as the current is passed. When sufficient gases are collected, the passage of current is stopped. The test tubes are taken out carefully.

The volume of the gas collected over the cathode (negative electrode) is double that of collected over anode (positive electrode). If a burning candle is brought near the mouth of the cathode test tube, the gas burns violently with a popping sound.

This gas is hydrogen. When the burning candle is brought near the mouth of the anode test tube, the candle starts burning more brightly, which indicates that the gas in the anode test tube is oxygen. Thus, the electrolysis of water gives hydrogen and oxygen in the ratio of \(2:1\) by volume.

(iii) Photochemical Decomposition Reactions A chemical reaction involving decomposition by the action of light is called a photochemical decomposition reaction.

Photochemical decomposition of silver chloride: Silver chloride, on exposure to sunlight, undergoes photochemical decomposition forming silver metal and chlorine gas. In the below reaction, the white colour of silver chloride changes to greyish white due to the formation of silver metal.

Displacement Reactions

The chemical reactions in which an element displaces some other elements from a compound are called displacement reactions.

\({\text{A}} + {\text{BC}} \to {\text{B}} + {\text{AC}}\)

Displacement of copper by iron from copper sulphate solution

\({\text{Fe}}\left({\text{s}} \right) + {\text{CuS}}{{\text{O}}_4}\left({{\text{aq}}} \right) \to {\text{FeS}}{{\text{O}}_4}\left({{\text{aq}}} \right) + {\text{Cu}}\)

In this reaction, iron displaces copper from copper sulphate solution. The deep blue colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate. A red-brown coating of copper metal is formed on the surface of iron metal. This is a displacement reaction because iron is more reactive than copper.

Experiment: An iron nail is cleaned thoroughly with the help of sandpaper. Place the iron nail in the aqueous copper sulphate solution taken in a tube. And keep the setup undisturbed for about \(30\) minutes.

After \(30\) minutes, take out the nail from the test tube. The iron nail is covered with a red-brown layer of copper metal, and the blue colour of the solution in the test tube is faded and turned to light green.

The displacement of an element by some other element is related to the activity of metal expressed in terms of activity series or electrochemical series. A metal placed higher in the activity series can displace any other metal placed below it in the series from its salt solution. The metal placed higher in the series is more reactive than the one which is placed lower in the series, i.e. a more reactive metal displaces a less reactive metal from its salt solution.

Double Displacement Reaction

The chemical reaction involving two compounds reacting together and forming two new compounds by exchanging one of their ions is called a double displacement reaction.

\({\text{AB}} + {\text{CD}} \to {\text{AD}} + {\text{CB}}\)

Two compounds \({\text{AB}}\) and \({\text{CD,}}\) react together to form two new compounds, \({\text{AD}}\) and \({\text{CB,}}\) due to the mutual exchange of ions. The ion \({\text{A}}\) of \({\text{AB}}\) combines with ion \({\text{D}}\) of \({\text{CD}}\) to form a new compound \({\text{AD.}}\) Similarly, the ion \({\text{C}}\) of \({\text{CD}}\) combines with \({\text{B}}\) of \({\text{AB}}\) to form another new compound \({\text{CB.}}\)

(a) The double displacement reactions involving the formation of a precipitate are also called precipitation reactions.

\({\text{BaC}}{{\text{l}}_2} + {\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4} \to {\text{BaS}}{{\text{O}}_4} + 2{\text{NaCl}}\)

Double displacement reactions involving acid bases are called a neutralisation reaction.

(b) Reaction of hydrochloric acid with sodium hydroxide: Aqueous hydrochloric acid mixed with an aqueous solution of sodium hydroxide. The two compounds exchange their ions to form sodium chloride and water. Therefore, it is a double displacement reaction.

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

The reaction involves the neutralisation of an acid \(\left({{\text{HCl}}} \right)\) by a base \(\left( {{\text{NaOH}}} \right)\); thus, it is also called neutralisation reactions.

Oxidation and Reduction Reactions

(a) Classical Views Regarding Oxidation and Reduction

According to the earlier views, the process in which a substance adds oxygen or removes hydrogen was named oxidation, whereas the process in which a substance adds hydrogen or removes oxygen was called reduction.

Oxidation: Addition of oxygen

\({\text{C}} + {{\text{O}}_2} \to {\text{C}}{{\text{O}}_2}\)

Removal of hydrogen:

\({{\text{H}}_2}~{\text{S}} + {\text{C}}{{\text{l}}_2} \to 2{\text{HCl}} + {\text{S}}\)

The reactant which provides oxygen or removes hydrogen is called an oxidising agent. In the reaction \({\text{C}} + {{\text{O}}_2} \to {\text{C}}{{\text{O}}_2},{{\text{O}}_2}\) is an oxidising agent because it provides oxygen. Similarly, in the reaction \({{\text{H}}_2}~{\text{S}} + {\text{C}}{{\text{l}}_2} \to 2{\text{HCl}} + {\text{S}},{\text{C}}{{\text{l}}_2}\) is an oxidising agent because it removes hydrogen from hydrogen sulphide.

Reduction:

1. Addition of hydrogen: \({\text{C}}{{\text{l}}_2} + {{\text{H}}_2} \to 2{\text{HCl}}\)
2. Removal of oxygen: \({\text{CuO}} + {\text{C}} \to {\text{Cu}} + {\text{CO}},\)

(b) Oxidation and reduction are complementary processes and occur simultaneously:

A reactant will lose electrons only when some other substance gains them. Therefore, a substance can get oxidised only when some other substance gets reduced or vice versa. Thus, oxidation-reduction processes occur simultaneously and are complementary to each other. Such reactions are called redox reactions.

Summary

In a chemical reaction, one or more substances react to form one or more than one different substance. A substance that changes is called a reactant, and the new substance formed is called a product. During a chemical reaction, the atoms of reactants do not undergo any change. Reactants change to form products. This article learned about different chemical reactions like the combination, decomposition, displacement, double displacement, and their examples.

FAQs

Q.1. What are the types of chemical reactions?
Ans: The different types of chemical reactions are combination, double displacement, decomposition, displacement, oxidation and reduction reactions.

Q.2. What are the four major types of chemical reactions?
Ans: The different types of chemical reactions are combination, double displacement, decomposition, and displacement reaction.

Q.3. How do you classify reaction types?
Ans: Most chemical reactions can be classified into one or more of five basic types: acid–base reactions, exchange reactions, condensation reactions (and the reverse, cleavage reactions), and oxidation–reduction reactions. 

Q.4. How does a catalyst make a chemical reaction faster?
Ans: A catalyst lowers the activation energy of a reaction. These are the substances that influence the rate of a chemical reaction and remains unchanged in amount and chemical composition at the end of the reaction. However, their physical state may change. There are several very slow chemical reactions that require certain catalysts to show an appreciable extent of reaction.

Q.5. What is a combination reaction? Give an example of a combination reaction that is also exothermic.
Ans: The chemical reaction involving a combination of two or more reactants (elements or compounds) to form a single new product is called a combination reaction. A reaction made to obtain a product using two or more elements is usually referred to as the synthesis of that compound.\({\text{C}} + {{\text{O}}_2} \to {\text{C}}{{\text{O}}_2} + {\text{Heat}}\)

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