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Depending upon the nature of reaction, i.e., the type of chemical change taking place, the various chemical reactions have been broadly classified into the following types: Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah.
Double displacement reactions
Reduction-oxidation (Redox) reactions.
Let’s understand each of these reactions one by one in detail.
Type of chemical reactions-1 Combination Reactions
Those reactions in which two or more elements or compounds combine together to form a single compound is called combination reactions.
Combination between two elements.
Combination between two compounds.
Combination between an element and a compound.
Experiment 1. Burning of a magnesium ribbon. if a piece of clean magnesium ribbon is burnt, a white powder containing magnesium oxide is formed. The reaction taking place is
2 Mg (s) + O2 (g) 2 MgO (s)
Magnesium Oxygen Magnesium oxide
Here, two elements, magnesium and oxygen have combined to form a single compound, i.e., magnesium oxide.
Experiment 2. Reaction of water on quick lime. If we take a small amount of quick lime (calcium oxide) in a beaker and add water to it slowly, they combine vigorously to form slaked lime, i. e., calcium hydroxide.
The reaction may be represented as follows:
CaO (s) + H2O (1) Ca(OH)2 (aq)
Quick lime Water Slaked lime
In the reaction, two compounds, namely, calcium oxide and water have combined to form a single compound.
i.e. calcium hydroxide.
Those reactions in which a single compound breaks down to give two or more simpler substances are called decomposition reactions. Thus, these reactions are just opposite of combination reactions.
The decomposition reaction takes place only when the energy in the form of heat, electricity or light is supplied. Thus, there are three types of decomposition reactions possible. These are:
Uses of decomposition reactions: The decomposition reactions are used in the extraction of metals. The electrolytic decomposition reactions are directly used to obtain certain metals by the electrolysis of their molten salts (generally the naturally occurring chlorides or oxides), e.g., sodium from molten sodium chloride and aluminium from molten aluminium oxide (alumina). The thermal decomposition reaction forms
one of the steps in the extraction of metals, e.g., zinc carbonate, the naturally occurring ore of zinc, is first decomposed to give zinc oxide which is then reduced to obtain zinc metal.
Decomposition reactions occurring in our body: The digestion of food in our body takes place through a number of decomposition reactions. The food that we eat mainly contains starch (from wheat, rice, potatoes etc.) and proteins (from pulses, egg, meat etc.). In the presence of a special type of catalysts present in the body, i.e., biological catalysts called “enzymes”, starch decomposes into simple sugars called glucose. Similarly, proteins decompose to form simpler molecules, called amino acids.
Decomposition Reactions are Endothermic Reactions: We observe that all decomposition reactions require energy in the form of heat, light or electricity. Hence, all decomposition reactions are endothermic reactions.
Those reactions in which a more active element displaces a less active element from its compound are called displacement reactions.
These reactions are generally found to occur in the solution. The elements involved may be metals or non-metals, i.e., a more active metal may displace a less active metal or a more active non-metal may displace a less active non-metal from its compound.
Before we take up examples of displacement reactions, it is important to understand the relative reactivates of metals and the relative reactivates of non-metals.
Examples of Displacement Reactions:
Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s)
Zinc Copper sulphate Zinc sulphate Copper
(Blue) (Colourless) (Reddish brown)
Thus, when zinc pieces are added to copper sulphate solution, zinc, being more reactive than copper, displaces copper from copper sulphate solution. The reddish brown copper particles are deposited on the surface of zinc or fall at the bottom of the reaction vessel (beaker). Further, the blue colour copper sulphate solution fades due to the formation of the colourless zinc sulphate in the solution.
Some Useful Information About Displacement Reactions.
4 Double Displacement Reactions
Those reactions in which two different atoms or groups of atoms are exchanged are called double displacement reactions.
Those reactions in which two ionic compounds in the solution react by exchange of their ions to form new compounds are called double displacement reactions.
The ionic compounds taken as reactants are soluble in water. However, one of the products formed is insoluble and separates out as a solid, called precipitate (written in short as ppt.) or it is a gas.
Reaction between barium chloride solution and sodium sulphate solution.
In one test tube, shake a pinch of barium chloride with about 5 mL of water. In another test tube, shake a pinch of sodium sulphate with about 5 mL of water. Mix the two solutions with gentle shaking. We observe that a white solid (precipitate) is formed. This white precipitate is of barium sulphate which has been formed due to the following reaction:
BaC12 (aq) + Na2SO4 (aq) 4 BaSO4 (s) + 2 NaC1 (aq)
Barium chloride Sodium sulphate Barium sulphate Sodium chloride
4 Precipitation Reactions:
Those reactions in which aqueous solutions of two compounds on mixing react to form an a solid (called precipitate) or such an insoluble through the solution of a compound are called insoluble compound which separates out as compound is formed when a gas is passed precipitation reactions.
Experiment. Reaction between barium chloride solution and sodium sulphate solution forming a white precipitate of barium sulphate is not only a double displacement reaction (as already discussed) but is
also a precipitation reaction.
Neutralization Reactions: A reaction in which an acid reacts with a base to form salt and water is called neutralization reaction, i.e.,
Acid + Base Salt + Water
In fact, these reactions are also double displacement reactions.
(i) NaOH + HCl NaCl + H2O
Sodium hydroxide Hydrochloric acid Sodium Water
(Base) (Acid) chloride
(ii) 2 NaOH + H2SO4 Na2SO4 + 2 H2O
Sodium hydroxide Sulphuric acid Sodium sulphate Water
Oxidation - Reduction Reactions:
Let us first understand the terms “Oxidation” and “Reduction” Oxidation is defined as a process which involves
(i) gain of oxygen or (ii) loss of hydrogen. Reduction is defined as a process which involves
(i) gain of hydrogen or (ii) loss of oxygen.
From the above definitions, we observe that reduction is just the opposite of oxidation.
Oxidizing and Reducing agents. A substance which helps the other substance to undergo oxidation is called an oxidizing agent. Similarly, a substance which helps the other substance to undergo reduction is called a reducing agent.