A 3.540-g sample of an unknown metal M is burned in the presence of excess oxygen,producing the oxide M₂O₃(s) and liberating 18.56 kJ of heat at constant pressure.What is the identity of the metal?
4M(s) + 3O₂(g) → 2M₂O₃(s)
Substance
ΔH°_f (kJ/mol)
Yb₂O₃(s)
-1814.6
Tb₂O₃(s)
-1865.2
Sm₂O₃(s)
-1823.0
Sc₂O₃(s)
-1908.8
Y₂O₃(s)
-1905.3

Question

Quizplus · Accepted Answer

The enthalpy change of formation of M₂O₃ for metal Yb has the highest magnitude (-1814.6 kJ/mol) among the given options. The negative sign indicates that the formation of the compound is exothermic, which agrees with the fact that heat is liberated in the reaction. Hence, the unknown metal M is likely Yb (ytterbium).

A 3540-G Sample of an Unknown Metal M Is Burned in in the Presence