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Combustion reactions are identified by the presence of oxygen molecule involved in burning a substance.

The combustion reaction of chromium (Cr) is as follows:

\({\bf{Cr + }}\frac{{\bf{3}}}{{\bf{4}}}{{\bf{O}}_{\bf{2}}} \to {\bf{C}}{{\bf{r}}_{\bf{2}}}{{\bf{O}}_{\bf{3}}}\); ∆H_rxn = -564.2 kJ/mol.

The above reaction means that the combustion of 52 g Cr releases an energy value of -564.2 kJ/mol.

The energy released for 1 g Cr is\(\frac{{\left( {{\rm{ - 564}}{\rm{.2 kJ/mol}}} \right)}}{{{\rm{52}}}}.\)

Hence, the energy released for 1.25 g Cr \(\begin{array}{l} = \frac{{\left( {{\rm{ - 564}}{\rm{.2 kJ/mol}}} \right)}}{{{\rm{52}}}}{\rm{ \times 1}}{\rm{.25\;\;}}\\{\rm{ = \; - 13}}{\rm{.56 kJ/mol}}{\rm{.}}\end{array}\)