A chemical reaction is a process of transformation of one set of substances (the reactants) into another (the products). Chemical reactions occur when bonds between broken or formed. The substances on which chemical reaction takes place are reactants and the substances that are formed at the end of a chemical reaction are called the products.
Types of Chemical Reactions
Chemical reactions are of four basic types. These types of chemical reactions have been explained with examples below:
** 1. Combination Reactions**
Combination reaction which is also known as a synthesis reaction, is a reaction in which two or more substances combine to form a single new substance.* Combination reactions can also be called synthesis reactions .The general form of a combination reaction is:
A+B→AB
For example two elements combining to form a compound e.g sodium metal reacts with chlorine gas to product solid sodium chloride
2. Decomposition reaction
A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general form of a decomposition reaction is:
AB→A+B
For example calcium carbonate decomposes into calcium oxide and carbon dioxide.
CaCO3(s)→CaO(s)+CO2(g)
3. Displacement reaction
Displacement reaction is a reaction in which one element replaces a similar element in a compound. The general form of a displacement (also called single-displacement) reaction is:
A+BC→AC+B
In this general reaction, element A is a metal and replaces element B also a metal, in the compound. Similarly a nonmetal can also replace another nonmetal in a compound, and the general equation becomes:
Y+XZ→XY+Z
in this reaction Y is a nonmetal that replaces the nonmetal Z in the compound with X.
Example of displacement reaction: Magnesium metal reacts with copper nitrate solution to give magnesium nitrate and copper metal.
Mg(s) + Cu(NO3)2 (aq) → Mg(NO3)2 + Cu(s)
Similarly metals react with acids toproduce hydrogen gas. This is also an example of displacement reaction. For example zinc reacts with hydrocholric acid to produce aqueous zinc chloide and hydrochloric acid.
Zn(s) + 2HCl (aq) → ZnCl2(s) + H2(g)
4. Double Displacement Reactions
A double-replacement reaction is a reaction in which exchange take place between the positive and negative ions of two ionic compounds to form two new compounds. The general form of a double-displacement reaction is:
AB+CD→AD+CB
In this reaction, A and C are positively-charged cations, while B and D are negatively-charged anions. Double-replacement reactions generally occur between substances in aqueous solution. In order for a reaction to occur, one of the products is usually a solid precipitate, a gas, or a molecular compound such as water.
A precipitate is formed in a double displacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound for example when aqueous solutions of potassium iodide and lead (II) nitrate are mixed, the yellow precipitates of lead iodide are formed by following reaction.
2KI(aq)+Pb(NO3)2(aq)→2KNO3(aq)+PbI2(s)
Other common types of chemical reactions are:
5. Combustion reactions
- Acid-base reactions or neutralization reactions
- Oxidation reduction reactions
Reactions are also classified on the basis of energy changes that occur during chemical reaction into two types: exothermic reactions and endothermic reactions