Question

An alloy of zinc and copper is dissolved in dilute hydrochloric acid. Hydrogen gas is evolved. In this evolution of gas

• A. only zinc reacts with dilute hydrochloric acid.
• B. only copper reacts with dilute hydrochloric acid.
• C. both zinc and copper react with dilute hydrochloric acid.
• D. only copper reacts with water.

Answer 1

The correct option is A

only zinc reacts with dilute hydrochloric acid.

The explanation for the correct option A:

• Arrangement of the metals on the basis of their reactivity towards other elements is termed as activity series. The activity series in decreasing order is:

$\mathrm{K}>\mathrm{Na}>\mathrm{Li}>\mathrm{Ba}>\mathrm{Sr}>\mathrm{Ca}>\mathrm{Mg}>\mathrm{Al}>\mathrm{Mn}>\mathrm{Zn}>\mathrm{Cr}>\mathrm{Fe}>\mathrm{Cd}>\mathrm{Co}>\mathrm{Ni}>\mathrm{Sn}>\mathrm{Pb}>\mathrm{H}>\mathrm{Sb}>\mathrm{Bi}>\mathrm{Cu}>\mathrm{Hg}>\mathrm{Ag}>\mathrm{Au}>\mathrm{Pt}$

• Elements at the starting of this series are comparatively more reactive than elements present after them in the series.
• In a single displacement reaction, an element from its compound is substituted by another element to form two product molecules. Elements that are more reactive according to the activity series can displace the elements less reactive than them in the series.
• In the reaction between metal and Hydrochloric acid or Hydrogen chloride $\left(\mathrm{HCl}\right)$, metal displaces Hydrogen $\left(\mathrm{H}\right)$ from Hydrochloric acid and forms metal chloride and Hydrogen molecule $\left({\mathrm{H}}_2\right)$.
• Since Zinc $\left(\mathrm{Zn}\right)$ is more reactive Hydrogen in the activity series, it can displace Hydrogen from Hydrochloric acid to form Zinc chloride $\left({\mathrm{ZnCl}}_2\right)$ and Hydrogen molecule $\left({\mathrm{H}}_2\right)$.

$\mathrm{Zn}\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{ZnCl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

• Since Copper$\left(\mathrm{Cu}\right)$ is less reactive than Hydrogen in the activity series, it cannot displace Hydrogen from Hydrochloric acid and Hydrogen molecule is not formed.

Final Answer: Hence, option A is correct. Only zinc reacts with dilute hydrochloric acid.