Question

Write an equation to show autoprotolysis of water.

Answer 1

Hint: A chemical reaction which experiences a transfer of protons within a molecule is termed a protolysis. This reaction is majorly shown by the compounds or molecules that consist of a proton or the reaction consisting of weak protonic acids.

Complete answer: When the transfer of a proton takes place in self-ionization of a molecule, then it is termed as an autoprotolysis reaction. In this reaction a molecule consisting of hydrogen ions reacts with the other atom of the same molecule which results in the formation of respective ions.
Autoprotolysis of water:
When a mole of water reacts with another mole of water, then the molecule gets dissociated and hydrogen ions are released which are attracted by another molecule of water and formation of hydronium ion and hydroxide ion takes place. The reaction for autoprotolysis of water is as follows:
Step-1: Water molecule gets dissociated into hydrogen ion and hydroxide ion. The reaction is as follows:
${H_2}O \to {H^ + } + O{H^ - }$
Step-2: Hydrogen ion formed in step 1 reacts with another molecule of water to form a hydronium ion. The reaction is as follows:
${H^ + } + {H_2}O \to {H_3}{O^ + }$
Step-3: Combining the individual reactions:
${H_2}O + {H_2}O + {H^ + } \to {H^ + } + {H_3}{O^ + } + O{H^ - }$
Step-4: Cancelling out the common terms i.e., hydrogen ion is common in reactant and product, so it will be cancelled out. The reaction is as follows:
$2{H_2}O \to {H_3}{O^ + } + O{H^ - }$
Hence, the overall equation for autoprotolysis of water is $2{H_2}O \to {H_3}{O^ + } + O{H^ - }$.

Note:
It is important to note that to undergo autoprotolysis reaction, the molecule should not necessarily be an acid. Any solvent which consists of acidic hydrogen to donate and a lone pair of electrons to accept protons can undergo autoprotolysis reaction.