Question

Zinc liberates Hydrogen from dilute HCl, but copper doesn't whereas copper reacts with concentrated nitric acid to release water. Explain the changes taking place.

Answer 1

Metals which presents above hydrogen in reactivity series can replace hydrogen from acids. Zinc presents above hydrogen in reactivity series but copper is located below. So zinc is able to replace hydrogen from acid but copper does not.
Nitric acid does not release hydrogen gas upon reaction with copper. But it get oxidised on reaction with nitric acid as shown below:
Cu(s) → Cu²⁺(aq) + 2e^-

Copper reacts in two different ways with nitric acid.

In concentrated condition, it forms copper nitrated along with brown fumes of nitrogen dioxide and water.

Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

However, it reacts with dilute nitric acid in order to produce copper nitrate along with the a colourless gas called nitrogen monoxide and water.

3Cu + 8 HNO₃(dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O