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What is a Chemical Reaction?

Chemical reaction is the transformation of one or more substances, known reactants, into new substances, known as products. This transition typically entails atom rearrangement as well as the breaking and building of chemical bonds. Energy is either received from or discharged into the environment during a chemical process.
Some examples of chemical reactions are:

Photosynthesis: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2
Combustion of Methane: CH4 + 2O2 → CO2 + 2H2O
Burning of Coal: C + O2 → CO2
Rusting of Iron: Fe + O2 + H2O → Fe2O3 + H2
Neutralization of an Acid and a Base: HCl + NaOH → NaCl + H2O

The Law of Conservation of Mass

The conservation of mass principle, which is fundamental in physics and chemistry, asserts that the mass of a closed system remains unchanged over the course of time. This indicates that mass can only be modified or restructured, not created or destroyed, by physical or chemical events.

Way back 1700s, Antoine Lavoisier conducted experiments in a sealed jar to estimate the mass of reactants and products in chemical processes. He discovered that the entire mass of the jar and its contents remained constant before and after the reaction, proving that matter cannot be created or destroyed by chemical reactions. This fundamental concept has significant importance in chemistry, as it allows for the balancing of chemical equations and the prediction of the amount of product produced from a given quantity of reactants.

1. Synthesis Reaction/Direct Combination Reaction

Synthesis reaction or direct combination reaction is a type of chemical reaction that involves the integration of two or more simple molecules (reactants) to create a more complex compound (product). It is characterized by the fact that the product has a higher molecular weight than the reactants. The general equation for a synthesis reaction is:

A + B → AB

In this reaction, A and B are the reactants, and AB is the product. The reactants may be elements, compounds, or both.
Examples:

The reaction of hydrogen and nitrogen to form ammonia:
- 3 H2(g) + N2(g) → 2 NH3(g)
The reaction of sodium and chlorine to form sodium chloride:
- 2Na + Cl2 → 2NaCl
The reaction of potassium and chlorine to form potassium chloride:
- 2 K(s) + Cl2(g) → 2 KCl(s)

2. Chemical Decomposition/Analysis Reaction

Chemical decomposition reaction or analysis reaction is a widely observed type of chemical reaction that involves the breakdown of a compound into smaller chemical components, forming two or more products. This type of reaction occurs when a compound is heated, exposed to light, or treated with an electric current, causing the bonds between the atoms to break down. This process can be represented by the equation AB → A + B, where the compound AB is broken down into its constituent parts, A and B.

Examples:

Electrolysis of water: Water decomposes into hydrogen gas and oxygen gas when an electric current passed through it.
- The chemical equation for this reaction is 2 H2O(l) → 2 H2(g) + O2(g)
Thermal decomposition of calcium carbonate: Calcium carbonate decomposes into calcium oxide and carbon dioxide gas when heated.
- The chemical equation for this reaction is CaCO3 (s) → CaO(s) + CO2(g).
Photodecomposition of silver chloride: Silver chloride decomposes into silver metal and chlorine gas when it is exposed to sunlight.
- The chemical equation for this reaction is 2AgCl(s) → 2Ag(s) + Cl2(g).

3. Single Replacement or Substitution Reaction

A single replacement reaction or substitution reaction is a type of chemical reaction that happens when one element in a compound is replaced or displaced by another. It is frequently observed in redox processes, which involve the transfer of electrons between species. This type of reaction can be described by the chemical equation A + BC → AC + B, where the element A displaces B from the compound BC to generate AC and B as distinct products.
Examples:

Zinc metal reacts with hydrochloric acid to produce zinc chloride and hydrogen gas:
- Zn(s) + 2HCl(aq)→ ZnCl2(aq)+ H2(g)
Copper metal reacts with silver nitrate solution to produce copper(II) nitrate solution and silver metal:
- Cu(s)+2AgNO3(aq)⟶Cu(NO3)2(aq)+2Ag(s).
Iron metal reacts with copper(II) sulfate solution to produce iron(II) sulfate solution and copper metal:
- Fe(s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu(s)

4. Double-Replacement Reaction or Salt Metathesis Reaction

Double-replacement reaction or salt metathesis reaction is a type of chemical reaction in which two compounds switch ions or bonds with each other to generate novel compounds. They are commonly observed in aqueous solutions and can result in the formation of a precipitate, gas or water as a product. This can be represented using the chemical equation:

AB + CD → AD + CB.