Question

Why is iron not used in the lab preparation of hydrogen?

Answer 1

Metals that are more electropositive than hydrogen, i.e. which lie above hydrogen in the reactivity series such as Zn, Fe, Mg can be used with dilute mineral acids to liberate hydrogen gas. Metals such as Cu, Hg, Ag etc. are less electropositive than hydrogen, as they lie below hydrogen in the reactivity series. Hence, they cannot liberate hydrogen gas from acids.

Complete answer:
Hydrogen gas (${H_2}$) is a colorless gas which does not have any distinct odor. This gas is sparingly soluble in water and t solubility of hydrogen gas in water is not affected very much by any changes in temperature.
The laboratory preparation of the hydrogen gas usually involves the action of dilute sulphuric acid (${H_2}S{O_4}$) or dilute hydrochloric acid (HCl) on zinc granules. In the chemical laboratories, the granulated zinc is ideal for the preparation of hydrogen gas.
Iron (Fe) is not used in the lab preparation of hydrogen because iron needs to be heated due to which impurities like ${H_2}S$ and $S{O_2}$ gases are produced.
Iron and Aluminium (Al) react with dilute acids to form hydrogen and their respective salts.
Therefore, iron (Fe) is not used in the lab preparation of hydrogen

Note:
Some metals other than iron are also not used in the lab preparation of hydrogen gas such as calcium (Ca) has a very uncontrollable reaction with dilute sulphuric acid (${H_2}S{O_4}$) or dilute hydrochloric acid (HCl). Another reason is also as calcium and magnesium are cost effective. Potassium (K) and sodium (Na) have a very uncontrollable reaction with dilute acids and hence can’t be used as the exothermic reaction sets the hydrogen burning. Lead can’t be used because it reacts with dilute sulphuric acid or hydrochloric acid to form salts.
Metals such as copper, mercury etc. cannot be used as they come below the hydrogen in the reactivity series. As, the metals which are more reactive than hydrogen itself can displace hydrogen from acids.
Example: $Cu + HCl \to no\,reaction$