Chemistry · Redox Equilibrium · NEET
Yes. Zinc goes from 0 to +2, so it loses electrons and is oxidised. The hydrogen in HCl goes from +1 to 0, so it gains electrons and is reduced. Chlorine stays at -1 and does not change. Because oxidation numbers change, it is a redox reaction, not a simple acid reaction.
The metal is a stronger reducing agent than hydrogen, which means it gives away electrons more easily. Those electrons are taken by H+ ions, turning them into neutral H2 gas. A less reactive metal like copper, silver or gold cannot do this with dilute HCl, so no hydrogen gas comes out.
In metal displacement a more reactive metal pushes out a less reactive metal from its compound, for example Cr2O3 + 2Al -> Al2O3 + 2Cr (thermite). In hydrogen displacement a metal pushes out hydrogen (as H2 gas) from water or an acid, for example Zn + 2HCl -> ZnCl2 + H2. NCERT groups metal displacement under metal displacement and hydrogen displacement under non-metal displacement.
Very reactive metals like Na, K, Ca, Sr and Ba displace hydrogen from cold water (2Na + 2H2O -> 2NaOH + H2). Less reactive metals like Mg, Zn and Fe react slowly or not at all with cold water but do displace hydrogen from acids like dilute HCl. Even Cd and Sn, which do not react with steam, can give H2 with acids.
No. Copper, silver and gold are very unreactive and are weaker reducing agents than hydrogen, so they cannot displace H2 from dilute HCl. NCERT states that very less active metals such as Ag and Au do not react even with hydrochloric acid. This is a common NEET trap.
Which of the following reactions is a metal displacement reaction? Choose the right option.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a reaction where a metal takes the place of hydrogen in water or an acid, so hydrogen escapes as H2 gas. The metal is oxidised and the hydrogen is reduced, so it is a redox reaction.
It is always a redox reaction. The metal loses electrons (oxidation) and H+ gains electrons to form H2 (reduction). Both happen together.
When H+ ions gain electrons they become neutral hydrogen atoms that pair up as H2 molecules. H2 is a gas at room temperature and has very low solubility, so it bubbles out.
Very unreactive metals such as copper, silver and gold cannot displace hydrogen from dilute HCl because they are weaker reducing agents than hydrogen.
The higher a metal is in the reactivity (activity) series, the faster and more strongly it releases H2. Mg reacts faster than Zn, which reacts faster than Fe with the same acid.
