Question

why is hot concentrated Sulphuric acid not used in the preparation of hydrogen ?

Answer 1

HI FRIEND HERE'S YOUR ANSWER pure zinc and concentrated sulphuric acid not used in the preparation of hydrogen completely pure zinc metal is not used for the preparation of hydrogen because it is slowly attacked by the acid the presence of impurities increases the rate of the reaction by forming electrochemical couples
HOPE IT'S HELPFUL PLEASE MARK IT AS BRAINLIST

Answer 2

Dilute sulphuric acid will react to produce hydrogen gas.

Increasing the concentration will increase the rate of reaction.

Since the product is in another phase (hydrogen is a gas) too rapid a reaction brings a risk of overpressurization and explosion. (This was once an issue with car batteries).

Also rapid generation of hydrogen bubbles could splash concentrated acid onto onlookers or sensitive equipment.