Step 1
The reaction:
3 Fe + 2 O2 => Fe3O4 ΔH = -1,300 kJ (completed and balanced)
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Step 2
Information provided:
ΔH = -1,300 kJ is the reaction heat. According to the negative sign, the reaction is exothermic (the heat is released).
12 g of iron (Fe), the limiting reactant.
O2 is the excess reactant.
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Information needed:
The molar mass per mole of iron = 56 g/mol approx.
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Step 3
By stoichiometry,
1 mole Fe = 56 g
Procedure:
3 Fe + 2 O2 => Fe3O4 ΔH = -1,300 kJ
3 x 56 g Fe ------------ 1,300 kJ of heat released
12 g Fe ------------ X
X = 12 g Fe x 1,300 kJ of heat/3 x 56 g Fe = 92.9 kJ of heat released (remember that the sign of this amount of heat is negative)
Answer: 92.9 kJ will be released