Let us first take a look at chemical reactions definition:
A chemical reaction is a process by which one or more substances (chemical elements or their compounds) are converted into one or more different substances.

Participants in a chemical reaction
The substances involved initially in the reaction are called reactants or reagents. Chemical reactions usually lead to a chemical change that results in one or more products.

A chemical reaction involves rearrangement of the atoms of the reactants. As a result of this chemical changes, the resulting products have different properties than the original reactants.

Reaction rate
Chemical reactions happen at a reaction rate at a given temperature and chemical concentration. Reaction rate increases with increasing temperature. This is because there is more thermal energy available to attain the activation energy levels required for the reaction to occur. Reactions can move in forward or reverse direction until they reach equilibrium.

Reactions that move in the forward direction do not need input of free energy and are called spontaneous reactions. Non-spontaneous reactions require input of free energy to move forward. It might be noted that many chemical reactions are non-reversible that is once the reactants have undergone the change to the products, we cannot reconvert the products back to the original reactant form.

Chemical reactions equations

Chemical reactions can be represented by chemical equations. A chemical equation consists of chemical formulae. The reactants are written on the left side of the equation and the products are on the right side. They are separated by an arrow that indicates the direction and the type of reaction. The arrow means ‘yields’. Equations must be balanced i.e. the number of atoms for each substance should be the same on both sides of the equation.

Did you know: A double arrow pointing in opposite directions is used for equilibrium reactions.

A + B → C + D
Where A and B are the reactants and C and D are the products.

Types of chemical reactions

There are different types of chemical reactions. The main categories are listed here:

Synthesis reaction / Combination reaction: In this reaction two or more reactants combine to form a complex product. It can be represented by the following equation:
A + B → AB
Example: Iron and sulphide combine to form iron sulphide.
8Fe + S8 → 8FeS

Decomposition reaction: This is the reverse of synthesis reaction. In this reaction a complex reactant breaks down into simpler products. Most decomposition reactions are endothermic. It can be represented by the following equation:
AB → A + B
Examples:  
Electrolysis of water to make oxygen and hydrogen.
2H2O → 2H2 +O2
Decomposition of molten sodium chloride into molten sodium and chlorine gas
2NaCl(l)→2Na(l)+Cl2(g)

Single replacement reaction: In this reaction one uncombined reactant replaces another reactant in a compound or exchanges places with it. It can be represented by the following equation:
A + BC → AC +B
Example: Magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas.
Mg + 2H2O → Mg(OH)2 + H2

Double replacement reaction: In this  reaction the anions and the cations exchange places to form two new compounds. It can be represented by the following equation:
AB + CD → AD + BC
Example: Lead nitrate with potassium iodide form lead iodide and potassium nitrate.
Pb(NO3)2 + 2KI → PbI2 + 2KNO3

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