The choice of a reducing agent in a particular case depends onm thermodynamic factor.

The above figure is a plot of Gibbs energy  vs. T for formation of some oxides.

It can be observed from the above graph that a metal can reduce the oxide of other metals, if the standard free energy of formation  of the oxide of the former is more negative than the latter. For example, since  is more negative than , Al can reduce Cu2O to Cu, but Cu cannot reduce Al2O3. Similarly, Mg can reduce ZnO to Zn, but Zn cannot reduce MgO because  is more negative than .