Question

What do you understand by the term ‘auto-protolysis’ of water? What is its significance?

Answer 1

We know that protolysis means a reaction involving the transfer of a proton. The positively charged hydrogen ion is called proton because it contains only one proton and no electrons. Try to figure out what auto word means in this context.

Complete step by step answer:
We know that protolysis is the transfer of proton reaction which is the same as hydrolysis. Auto-protolysis is the transfer of protons between the same molecules. Water undergoes an auto-protolysis reaction and forms hydronium ion and hydroxide ion. Water molecules undergo ionization producing hydrogen ion and hydroxide ion. As soon as hydrogen ions are produced it will be attracted by other water molecules producing hydronium ions. A hydrogen ion is positively charged so it attracts negatively charged particles. In a water molecule, the oxygen atom has 2 lone pair electrons so it attracts hydrogen ions. A coordinate covalent bond is formed between oxygen and hydrogen ions resulting in hydronium ions.
Self-ionization of water:
${ H }_{ 2 }O\quad \rightleftharpoons \quad { H }^{ + }\quad +\quad { OH }^{ - }$
Proton reacting with another water molecule to hydronium ion. This reaction is very fast.
${ { H }^{ + }\quad +\quad H }_{ 2 }O\quad \longrightarrow \quad { H_{ 3 } }O^{ + }$
Combining these two reactions we get the auto-protolysis reaction of water which produces hydronium ion and hydroxide ion.
${ 2H }_{ 2 }O\quad \rightleftharpoons \quad { H_{ 3 } }O^{ + }\quad +\quad O{ H }^{ - }$
Here proton is transferred between 2 water molecules.

Note: Ionization of water molecules produces one hydrogen ion and one hydroxide ion so if n molecules of water are ionized then we get n hydrogen ions and n hydroxide ions are produced. The concentration of hydrogen ion and hydroxide ion will be the same resulting in a neutral solution. So, water is a neutral solution.