Question

Explain the activity series accounting for each of the following phenomena:

(a) Occurrence of metals (b) Tendency to corrosion

(c) Reaction with water (d) Reaction with acid

Answer 1

Activity series is the arrangement of metals in decreasing order of their reactivity.

$\mathrm{K}>\mathrm{Na}>\mathrm{Ca}>\mathrm{Mg}>\mathrm{Al}>\mathrm{Zn}>\mathrm{Fe}>\mathrm{Ni}>\mathrm{Sn}>\mathrm{Pb}>\mathrm{H}>\mathrm{Cu}>\mathrm{Hg}>\mathrm{Ag}>\mathrm{Au}>\mathrm{Pt}$

(a) Occurrence of metals: Metals at the bottom of the series are present in their native state and becomes least reactive.

For example, gold $\left(\mathrm{Au}\right)$, silver $\left(\mathrm{Ag}\right)$, and platinum $\left(\mathrm{Pt}\right)$ occurs in a free state. Other metals occur in a combined state as their oxides, carbonates, etc.

(b) Tendency to corrosion: Metals at the top of the series are most reactive and corrode easily, metals at the bottom of the series are least reactive, and do not undergo corrosion.

(c) Reaction with water: Metals in the high position are most reactive, and react even with cold water such as, $\mathrm{K},\mathrm{Na},$ etc. to produce hydrogen.

$2\mathrm{K}\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{KOH}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

• Metals like $\mathrm{Al},\mathrm{Fe},\mathrm{Mg},\mathrm{etc}.$ in the middle of the activity series react with hot or steam to produce hydrogen.

$3\mathrm{Fe}\left(\mathrm{s}\right)+4\mathrm{HO}\left(\mathrm{g}\right)\to {\mathrm{Fe}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)+4{\mathrm{H}}_2\left(\mathrm{g}\right)$

• Metals like $\mathrm{Ag},\mathrm{Au},\mathrm{Pt}$ etc, at the bottom, do not react with water.

(d) Reaction with acids: Metals at the top and middle position react with dilute acids to give hydrogen.

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

• Metals at the bottom do not react with dilute acids.