What units are used to express energy transfer?

The balanced equation is

4Fe(s) +  3O2(g)  ---> 2Fe2O3(s)

here four moles of Fe will react with O2 to give 2 moles of Fe2O3

mass of Fe reacted = 33.4 grams

moles of Fe reacted = mass of Fe / atomic mass of Fe

moles of Fe reacted = 33.4 / 55.8 = 0.599 moles

moles of Fe2O3 formed = 0.5 X 0.599 = 0.2995 moles

so mass of Fe2O3 formed = moles X molar mass = 0.2995 X 159.69 = 47.83 g