Although +3 is the characteristic oxidation state for lanthanoids but cerium also shows +4 oxidation state because
Why does copper not replace hydrogen from acids?
Copper not replace hydrogen from acids because Cu has positive $E^{\circ}$ value, i.e., less reactive than hydrogen which has electrode potential 0.00 V .
Why $\mathrm{E}^{-}$values for $\mathrm{Mn}, \mathrm{Ni}$ and Zn are more negative than expected?
Negative values of $\mathrm{Mn}^{2+}$ and $\mathrm{Zn}^{2+}$ are related to stabilities of half-filled and completely filled configuration respectively. But for $\mathrm{Ni}^{2+}, E^{\circ}$ value is related to the highest negative enthalpy of hydration. Hence, $E^{\mathrm{S}}$ values for $\mathrm{Mn}, \mathrm{Ni}$ and Zn are more negative than expected.
Why first ionisation enthalpy of Cr is lower than that of Zn ?
Ionisation enthalpy of Cr is less than that of Zn because Cr has stable $d^5$ configuration. In case of zinc, electron comes out from completely filled $4 s$-orbital. So, removal of electron from zinc requires more energy as compared to the chromium.
Transition elements show high melting points. Why?
Transition elements show high melting point, due to involvement of greater number of electrons in the interatomic bonding from $(n-1) d$-orbitals in addition to $n s$ electrons in forming metallic bond. Thus, large number of electrons participate forming large number of metallic bond.