Combination Reaction

In this type of reaction, two or more reactants combine to form a single larger product. 

A + B  →  AB

e.g. S + O₂  →  SO₂


Decomposition Reaction

In this type of reaction, a single reactant is broken down to form two or more products.

AB  →  A + B 

e.g. CaCO₃  →  CaO + CO₂


Single Displacement Reaction

A more reactive element displaces or substitutes a less reactive element from its compound. 

A + BC  →  B + AC

Here, A is more reactive than B and it substitutes B from its compound. 

e.g. Zn + 2HCl  →  H₂ + ZnCl₂


Double Displacement Reaction

You may have already guessed. This can be imagined as an exchange of partners. 

One element from one compound combines with an element from the other compound. 

The second element from the first compound combines with the second element from the second compound. 

For ionic compounds, the positive ion in the first compound combines with the negative ion in the second compound, and the positive ion in the second compound combines with the negative ion in the first compound. 

AB + CD  →  AD + BC

e.g. HCl + NaOH  →  NaCl + H₂O


REDOX Reaction
RedOx = Reduction and Oxidation. 

In terms of oxygen or hydrogen transfer

  • Oxidation is gain of oxygen or loss of hydrogen
  • Reduction is gain of hydrogen or loss of oxygen

e.g.
Fe₂O₃ + 3CO  →  2Fe + 3CO₂

‘Fe₂O₃’ loses its oxygen atom, forming ‘Fe’ and hence, it is reduced. 
On the other hand, ‘CO’ gains an oxygen atom, forming ‘CO₂’. Therefore, it is oxidised. 

e.g.
2NH₃ + 3Br₂  →  N₂ + 6HBr

‘NH₃’ loses its hydrogen, forming ‘N₂’. Therefore, it is oxidised. 
‘Br₂’ gains hydrogen, forming ‘HBr’. Therefore, it is reduced.

In terms of electron transfer

  • Oxidation is loss of electrons
  • Reduction is gain of electrons

Most popular way to remember this is by: 
OIL RIG

OIL:

  • Oxygen
  • Is
  • Loss of electrons

RIG:

  • Reduction
  • Is
  • Gain of electrons

Example: 
CuO + Mg  →  Cu + MgO

Reactants:
Oxidation state of Cu: +II
Oxidation state of O: -II
Oxidation state of Mg: 0
Products:
Oxidation state of Cu: 0
Oxidation state of O: -II
Oxidation state of Mg: +II

Writing out the ionic equation of this, we get: 
Cu²⁺ + Mg  →  Cu + Mg²⁺

In this reaction, Cu gains electrons because its oxidation number decreases from +II to 0. So, it is reduced as Reduction is gain of electrons. 

In this reaction, Mg loses electrons because its oxidation number increases  from 0 to +II. So, it is oxidised as oxidation is loss of electrons. 

Since both reduction and oxidation occurs in this reaction, it is a REDOX reaction. 


Oxidising and Reducing Agents

  • An oxidising agent is a substance which oxidises something else.
  • A reducing agent is a substance which reduces something else.

Let us take the example that we used in above.
CuO + Mg  →  Cu + MgO

As we discussed, 
Cu is being reduced in this reaction. However, the substance that is reducing it is the Mg. Therefore, Mg is the reducing agent. 

Similarly, Mg is being oxidised in the above reaction. The substance that is oxidising it is the Cu. Therefore, Cu is the oxidising agent.

Conclusion: 
An oxidising agent oxidises something else, but it itself is reduced.
A reducing agent reduces something else, but it itself is oxidised. 

This may seem confusing but it is crucial that you understand the concept. 


Neutralisation Reaction

Neutralisation occurs when an acid reacts with a base. `

You may wonder what the difference is between an alkali and a base. An alkali is basically a base which is soluble in water. Not all bases are soluble but the ones that are soluble are called alkali. 

Acid in solution produces H⁺ ions.
Alkali in solution produces OH⁻ ions. 

H⁺ + OH⁻  →  H₂O

A neutral pH substance (pure water) is formed when an alkali (soluble base) reacts with an acid. 
A salt is also formed.