A chemical reaction is any irreversible change in the initial composition of a substance to form an altogether different chemical product. The process of formation of these products by the reactants is an amazing and fascinating phenomenon. Here is some interesting information about the types of chemical reactions.
Chemical equations are used to describe a chemical transformation of elementary particles that takes place during a reaction, and are described using chemical symbols. A chemical reaction occurs under conducive conditions like suitable pressure, concentration and temperature. At any given moment, there may be thousands of chemical reactions taking place in and around us. Some reactions are rapid and spontaneous, and occur instantly. Several others are non-spontaneous and need a catalyst or an external source of energy (like heat, light, etc.) to proceed further.
Primarily, reactions can be exothermic (in which energy is released) or endothermic (in which energy is absorbed). Reactions can be studied under two different branches of chemistry, namely, inorganic chemistry and organic chemistry. This article enlists the types of reactions from both the fields.
Reactions in Inorganic Chemistry
All the occurrences and chemical processes that involve inorganic compounds and chemical elements fall under this category. The reactions of these elements, their combinations, breakdown of compounds, etc., is classified into 6 major types of chemical reactions.
Synthesis or Combination Reaction
2H2 + O2 ➜ 2H2O
P4 + 3O2 ➜ 2P2O3
In this reaction, two or more simple substances (reactants) combine together to yield a more complex substance. For example, hydrogen gas combines with oxygen to form a more complex product, water. Likewise, phosphorous and oxygen combine to form phosphorous trioxide.
2H2O ➜ 2H2+ O2
CaCO3 ➜ CaO + CO2
Here, a substance breaks down into simple substances and a single reactant produces two or more products. For example, a water molecule can be broken down into hydrogen and oxygen. Similarly, calcium carbonate breaks down into calcium oxide and carbon dioxide.
Single Displacement or Substitution Reaction
Zn + 2HCL ➜ ZnCl2 + H2
Pb + FeSO4 ➜ PbSO4 + Fe
A less active element is replaced by a more active element present in a compound. Two reactants produce two products. For example, when zinc reacts with hydrochloric acid, hydrogen molecule is replaced by zinc to form ZnCl2. Also lead replaces iron in ferrous sulfate (FeSO4) to form lead sulfate(PbSO4).
Metathesis or Double Displacement Reaction
AgNO3 + NaCl ➜ AgCl + NaNO3
BaCl2 + MgSO4 ➜ BaSO4 + MgCl2
The cations and anions of two different substances switch their places to yield two totally different compounds. For example, when silver nitrate reacts with sodium chloride, sodium and silver switch their places and there is a formation of sodium nitrate and silver chloride. Also, barium from barium chloride and magnesium in magnesium sulfate replace each other to form barium sulfate and magnesium chloride respectively.
C3H8 + 5O2 ➜ 3CO2 + 4H2O
CH3COOH + 2O2 ➜ 2CO2 + 2H2O
A hydrocarbon is burnt in the presence of oxygen to form carbon dioxide (in complete combustion), or carbon monoxide (in partial combustion due to a limited amount of oxygen). Combustion is an exothermic reaction that produces heat and can also produce light in the form of a flame or glow. Burning of propane and acetic acid produces water and releases carbon dioxide.
Neutralization or Acid-Base Reaction
HBr + NaOH ➜ NaBr + H2O
HNO3 + Mg(OH)2 ➜ 2H2O + Mg(NO3)2
It is a special type of double displacement reaction, which is characterized by the reaction between an acid and a base. H+ ion in the acid reacts with OH- ion present in the base, leading to formation of water. Generally, the end products of this reaction are water and some ionic salts. For example, hydrobromic acid reacts with sodium hydroxide to yield water and sodium bromide (NaBr). Nitric acid and magnesium hydroxide react to form water and magnesium nitrate.
C6H12O6 + 6 O2 ➜ 6 CO2 + 6 H2O
4Fe (s) + 3O2(g) ➜ 2Fe2O3
These reactions involve the change in the oxidation states of the reactants. Redox is the short-form of reduction-oxidation reactions. For a given atom, ion or molecule, depending upon the reaction oxidation and reduction implies any of the following transfers:
Oxidation:- is the loss of electrons or hydrogen, or gain of oxygen or increase in oxidation state
Reduction:- is the gain of electrons or hydrogen, or loss of oxygen or decrease in the oxidation state
Redox reactions occur in both inorganic and organic chemistry. The most common and widely seen example of a redox reaction is cellular respiration. It involves oxidation of glucose (C6H12O6) to CO2 And the reduction of oxygen to water (H2O). Weathering of minerals in nature is also an example of a redox reaction. Iron is oxidized and oxygen reduced to form ferric oxide.
Reactions in Organic Chemistry
Organic chemistry is the study of the characteristics, mechanisms, reactions and structures of organic materials, i.e. compounds that contain carbon atoms in various forms. Organic compounds form the basic structure of life on Earth and have varied structures. They are very diverse, hence the reactions of these compounds are broadly classified into 4 categories.
An atom or a group of atoms is added to a molecule. These reactions mostly involve the unsaturated compounds (compounds that contain double bond or a triple bond between atoms) like alkenes, alkynes or ketones. Addition reactions are also called saturation reactions as they saturate carbon atoms with the maximum number of attached groups. This is done by breaking down the double or triple bonds between the atoms in order to accommodate the additional atoms or group of atoms in the molecule.
For example, bromine adds to ethylene to form 1,2-dibromoethane. Similarly, hydrogen cyanide adds to ethanal to form 2-hydroxypropanenitrile.
Elimination reactions involve the removal of an atom or group of atoms from a molecule. It is the process through which saturated compounds are converted to unsaturated compounds. This is done usually through the action of acids, bases, metals or heat. Elimination reactions are the opposite of addition reactions. They are known by the atom or group of atoms leaving the molecule.
Dehydrohalogenation:- removal of a hydrogen and halogen
Dehydration:- leaving of a water molecule
Dehydrogenation:- elimination of hydrogen
The first is an example of dehydration wherein water is eliminated from cyclohexanol to form cyclohexene in presence of a strong acid, H2SO4. The other reaction is an example of dehydrohalogenation of bromoethane to form ethylene.
It is the class of chemical reaction in which an atom, ion or a group of atoms/ions is substituted by another group of ion, atom or functional group. For instance, amino group (-NH2) substitutes chlorine from acetyl chloride to form acetamide. Chlorine substitutes hydrogen in methane to form chloromethane.
Isomerization or Rearrangement Reaction
It is the chemical process in which a compound rearranges itself into its isomeric form. Isomers are compounds with the same molecular weight and composition but differ in their structure and configuration.
Here, cyclopropane rearranges to propene. 2-butene is an alkene with four C-atoms that exists as two geometrical isomers trans-2-butene and cis-2-butene respectively. Due to this difference, the chemical and physical properties change. Pericyclic reactions are also a type of rearrangement reaction.
3O2 + sunlight ➜ 2O3
6 CO2 + 6 H2O + Light Energy ➜ C6H12O6 + 6 O2
A photochemical reaction is initiated when the atoms and molecules get excited by the absorption of energy, in the form of light, and release energy by breaking chemical bonds. Photochemistry is involved in many important life processes like photosynthesis, formation of vitamin D in the skin, conversion of oxygen to ozone in the Earth’s atmosphere, bio-luminescence in fireflies and even our vision. The above mentioned reactions take place during formation of ozone from oxygen and production of glucose and oxygen in plants during photosynthesis in the presence of sunlight.
The biochemical reactions that govern and regulate our metabolic processes are also a type of chemical reactions. There is a long, never-ending list of chemical changes taking place every second. They are present everywhere and hence, it is important to know the types of chemical reactions. It is fun to study these and will provide a better understanding of chemistry as a science.