Why is acetic acid written as hc2h3o2 rather than c2h4o2?

Acetic acid is under the family of carboxylic acids which has a carboxyl group –COOH bond attached to the alkyl group. Alkyl group can be methyl (CH3-), ethyl (CH3CH2-) and so on. The term acetic also means methyl. So combining the two gives CH3COOH or HCH3CO2. The H before the C in HCH3CO2 means that it is the H attached to the O in the carboxyl group –COOH. You cannot write it as C2H4O2 because it would represent a different compound.

In Endothermic reactions, energy is absorbed.