A chemical reaction takes place when 2 or more chemicals combine to form a new compound. Chemical reactions require certain conditions to be met . Any 2 elements can react differently in 2 different conditions. A simple example is the formation of curd is quick and easy during the hot season but the same process takes too long during winter. In our day-to-day life, we can experience many chemical reactions. Burning a piece of wood, fermentation of idly batter, bread preparation are some of the examples of chemical reactions. Depending upon the reactants involved and the factors that cause chemical reactions, there are number of methods to classify chemical reactions. Some of the common reaction types are explained below.
Redox is a short form of reduction-oxidation. It is a chemical reaction in which the atom’s oxidation states undergo changes. Redox involves the combination of the reduction process and a complementary oxidation process through which electrons transfer between the atoms. Oxidation is the process of loss of electrons or gain in the oxidation state by a molecule or atom, whereas, the reduction is the gain of electrons or a loss in oxidation state by a molecule, atom, or ion. In the following reaction, I2 is reduced to I– and S2O32- (thiosulfate anion) is oxidized to S4O62-. It is written as
It is the process of mixing of chemical species to form the more complex product. The simple way to represent this reaction is, X + Y → XY. For example, iron and sulphur undergoes direct combination to form iron sulphide. The reaction is expressed as 8 Fe + S8 → 8 FeS
3. Chemical Decomposition or Analysis Reaction
It is the process of destruction or breaking a compound into simpler chemical species( quite opposite to synthesis). It is expressed as AB → A + B. The electrolysis of water into oxygen and hydrogen gas is a simple decomposition reaction (2 H2O → 2 H2 + O2)
4. Endothermic Reactions
The reactions which require energy in the form of heat, light or electricity are called Endothermic Reactions. It is written as 2Ba (OH) 2 + NH4Cl → 2BaCl2 + NH4OH.
5. Single Displacement or Substitution Reaction
It is a chemical reaction during which a functional group in a chemical compound is replaced by another functional group. If you look at halogenation, where chlorine gas (Cl-Cl) is irradiated, some of the molecules are split into two separate chlorine radicals (Cl.) by forming a weak C-H covalent bond. The radicles will further grab the liberated protons to form the electrically neutral H-Cl. The other radical reforms a covalent bond with the CH3 to form CH3Cl (methyl chloride). One of the examples for substitution reaction is chlorination of methane.
6. Metathesis or Double Displacement Reaction
There is an exchange of bonds or ions between groups of compounds in order to form different compounds. It is simply written as AB + CD → AD + CB. When a double displacement reaction occurs between sodium chloride and silver nitrate to form sodium nitrate and silver chloride, nitrate comes along with sodium chlorine comes to silver; it is expressed as NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
7. Acid-Base Reaction
It is a very common and familiar reaction characterized by a double displacement between an acid and a base. The H+ ion in the acid reacts with the OH– ion in the base to form water and an ionic salt. It is written as HA + BOH → H2O + BA. For example, the reaction between a hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction (HBr + NaOH → NaBr + H2O)
It’s a simple redox reaction in which combustible reactants come into contact with an oxidizer to form oxidized products and finally they generate heat, this is called an exothermic reaction. Simple example is, C10H8 + 12 O2 → 10 CO2 + 4 H2O
When the structural arrangement of a compound gets changed but the total or net atomic composition remains constant, then it’s an isomerization (the same composition)
10. Hydrolysis Reaction
A hydrolysis reaction involves water. For example, X–(aq) + H2O (l) ↔ HX (aq) + OH–(aq).
It’s the formation of insoluble solid after combing 2 elements. If you mix a solution of potassium chloride and a solution of silver nitrate, a white insoluble solid is formed in the resulting solution and it is called silver chloride. Aqueous silver nitrate (AgNO3) is mixed with a solution containing potassium chloride (KCl), the precipitation formed is of a white solid called silver chloride (AgCl), is observed as shown below.
Concept of Reversibility and Equilibrium in chemical reactions
The majority of the chemical reactions travel in a single direction, which means you can’t reverse them by misplacing the reactants and products in the opposite direction. In other words, reactants remain reactants and products remain products and you can’t convert them into other way around. But, sometimes, reversible reactions are possible which can travel in both the forward and backward directions. This circle of moving back and forth continues until there is a creation of evenly balanced status between reactants and products and this is known as the equilibrium. Each reaction has a varied level of equilibrium point and the unique level to a given reaction is called equilibrium constant. A reversible reaction is expressed with paired arrows in both forward and backward directions (⇌). For instance, when it comes to human blood, excess hydrogen ions (H+) attach to bicarbonate ions (HCO3), eventually forming carbonic acid H2CO3. It is written as HCO3+ H ⇌ H2CO3.