Question

Lead cannot be used in preparation of hydrogen using dilute acids. Why?

Answer 1

Lead is above hydrogen in the metal activity series ( $H_2$ is included for comparison of reactivity with other metals). So it can displace H from compounds.

In the reactions with acids, insoluble salts of lead are produced which forms a coating on pure lead and prevents the reaction from proceeding further. Thus, Pb is not used to prepare $H_2$.