What is Chemical Reaction?
o Chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products.
Type of Chemical Reactions
There are three types of reaction that occurs in solution;
I. Precipitation reaction
II. Acid-base reaction
III. Oxidation-reduction reaction
I. Precipitation Reaction
o This reaction takes place when two aqueous reactants, one solid and one liquid, react to form an insoluble product. This insoluble substance is termed as a precipitate.
o For instance; lead nitrate is mixed with potassium to produce the following compounds as shown in the following chemical reaction:
Pb (NO3)2 + 2KI → PbI2 (ppt) + 2KNO3
o Lead iodide formed here is an insoluble product and hence is titled as a precipitate. Another example for the same include;
AgNO3(aq) + K2Cr2O7(aq) → Ag2Cr2O7(ppt) + KNO3(aq)
o A reddish precipitate of silver dichromate is formed in the above reaction. These reactions shown above are also termed as Double-displacement reactions.
II. Acid-Base reaction
o Arrhenius stated that an acid is defined as a substance that dissolves in water to produce H+ ions, for instance, HCl whereas a base is a substance that dissolves in water to produce hydroxide (OH−) ions.
o When an acid reacts with base in their solution form, they produce salt and water.
o These reactions are also named as Neutralization reactions.
The chemical reaction below shows some of the examples of acid-base reactions;
H2SO4 + Ca(OH)2 → CaSO4 + H2O
H2SO4 + CaO → CaSO4 + H2O
HF + H2O → F– + H3O+
HBr + NH3 → Br– + NH4+
III. Oxidation Reduction Reaction
o Oxidation is defined as a process in which a chemical substance loses electrons during a reaction whereas Reduction is the totally opposite process in which a substance gains electrons or gain hydrogen during a reaction.
o If in a reaction the total number of electrons lost is equal to the total number of electrons gained then these types of reactions are known as Redox reactions.
o The figure given below describes the redox reaction with an example;
Reduction: Removal of Oxygen from CuO
Oxidation: Addition of Oxygen in H2
CuO + H2 → Cu + H2O
Reduction: Removal of Oxygen from Fe3O4
Oxidation: Addition of Oxygen in H2
Fe3O4 + 4H2 → 3Fe + 4H2O
Types of Redox Reactions
o There are a few important types of redox reactions that are mentioned below;
o Synthesis Reactions: The formation of any compound straight from the elements in a redox reaction is known as synthesis reaction, for instance, the formation of water from hydrogen and oxygen as given below:
2H2(g)+O2(g) → 2H2O(g)
o Decomposition Reactions: The decomposition of a compound into its elements is also defined as a type of as redox reaction for instance; the electrolysis of water:
2H2O(l) → 2H2(g)+O2(g)
o Combustion Reactions: It is a kind of redox reaction which occurs between molecular oxygen and compound (reactants) to produce oxygen-containing products. For example;
CH4(g)+2O2(g) → CO2(g)+2H2O
o A solution is a type of homogeneous mixture in which substances that are present in lesser amounts are called solutes dispersed uniformly throughout the substance present in the greater amount, the solvent.
Types of Solution
o The solution can be of two types as mentioned below;
o Aqueous Solution
o A Non-aqueous Solution
o An aqueous solution is commonly defined as a solution in which the solvent is water, whereas, in a non-aqueous solution, the solvent is not water.
Examples of non-aqueous solvents include ethyl acetate, turpentine, etc.
Why Most Reaction Occur in Aqueous Solution
o For a chemical reaction to occur, individual atoms, molecules, or ions must collide thus, collisions between solids, do not occur at a visible rate.
o It is easy to control the amount of heat used or produced in a reaction when it occurs in an aqueous solution.
o The nature of the reaction can also be controlled when the proper solvent is chosen.
o All the chemical reaction that occurs in our body are possible only in aqueous solution.
o For instance; reaction involved in the digestion of food.
o When an element dissolves in water, it is represented by writing (aq) after its chemical name.
o Hydrophilic or Water-loving substances dissolve in water.
o For instance, when sodium chloride dissolves in water, dissociates into Na+(aq) and Cl– (aq).
o Hydrophobic or water-fearing substances normally do not dissolve in water or form aqueous solutions.
o Example for the same include mixing oil and water which will not result in any dissociation.
Solubility in Water
o Water has some unique properties and is available on Earth in a huge amount.
o Water is also called as a Universal solvent.
o The water molecule comprises of two hydrogen atoms bonded covalently to an oxygen atom in a V-shaped structure.
o The oxygen atom attracts electrons more strongly as compared to the hydrogen atom. It means that the oxygen and hydrogen nuclei do not share equal electrons.
o Hydrogen atoms are generally electron-poor and attain a partial positive charge, which is shown by δ+ whereas the oxygen atom, is more electron-rich, so it attains a partial negative charge.
o This above-mentioned charge is twice as large as the partial positive charge on each hydrogen as the molecule of water has a net zero charge. So, its charge is indicated by 2δ−.
o This unequal distribution of charge forms a polarity in the individual water molecules. Because of this arrangement of a water molecule, it is termed as a polar substance.
o The unequal distribution of charge in polar liquids such as water makes them good solvents for different ionic compounds.
o When ionic solid dissolves in water, the ions present dissociates into cation and anion. The partially negatively charged oxygen atoms of the H2O molecules surround the cations and the partially positively charged hydrogen atoms in H2O surround the anions.
o The arrangement in which cations and anions are surrounded by polar water molecules is termed as hydrated ions.
o Example for the same include; common salt in water.
Chemical Reaction Citations
- Understanding chemical reaction mechanisms in ionic liquids: successes and challenges. Chem Soc Rev . 2011 Jan;40(1):272-90.
- Chemical reactions which affect the biological activity of human gonadotrophins. Biochem J. 1961 Apr; 79(1): 64–70.
- Seeding chemical reactions by focusing. J Chem Phys . 2013 Apr 14;138(14):144502.
- Application of chemical reaction engineering principles to ‘body-on-a-chip’ systems. AIChE J . 2018 Dec;64(12):4351-4360.
- Chemical reactions. Am J Med Technol . 1948 May;14(3):142-6.
- Classification of chemical reactions and chemoinformatic processing of enzymatic transformations. Methods Mol Biol . 2011;672:325-40.