Transition metals vary widely in their chemical reactivity. Explain this with examples or write about the reactivity of $3d$ series metals with acids.

(N/A) Transition metals exhibit a wide range of chemical reactivity. Many of them are sufficiently electropositive to dissolve in mineral acids,although some are noble (e.g.,$Ag$,$Au$,$Pt$).
For the $3d$ series,the reactivity is related to the standard electrode potential $(E^{\circ})$ values. Metals with more negative $E^{\circ}$ values are more reactive.
For example,$Cr^{2+}$ is a strong reducing agent and can liberate hydrogen from acids:
$2Cr^{2+}_{(aq)} + 2H^+_{(aq)} \rightarrow 2Cr^{3+}_{(aq)} + H_{2(g)}$
In this reaction,$Cr^{2+}$ is oxidized to $Cr^{3+}$ (oxidation state changes from $+2$ to $+3$),while $H^+$ is reduced to $H_2$ (oxidation state changes from $+1$ to $0$).