Cobalt is _______ reactive than nickel.

Reactivity series: Reactivity series is the arrangement of metal in a tabular form in the decreasing order of their reactivity.

Reactivity series:

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

• The metal present at a higher position can displace the element which is present below that element in the activity series.
• Since Cobalt(Co) is present above Nickel(Ni) in the reactivity series.
• So, Cobalt is more reactive than nickel.

Therefore, Cobalt is more reactive than nickel.