Question

The thermite reaction, used for welding iron, is the reaction of Fe3O4 with Al. 8 Al (s) + 3 Fe3O4 (s) \longrightarrow⟶ 4 Al2O3 (s) + 9 Fe (s) \DeltaΔH° = -3350. kJ/mol rxn Because this large amount of heat cannot be rapidly dissipated to the surroundings, the reacting mass may reach temperatures near 3000. °C. How much heat (in kJ) is released by the reaction of 12 g of Al with 75.4 g of Fe3O4? Enter a positive number since released already tells us it is a negative number (to 1 decimal place).

Answer 1

The balanced equation is

8Al +   3 Fe3O4 ----------------------> 4Al2O3 + 9Fe , Delta H = -3350 kJ/mol

12g/27g/mol 75.4g/231.53g/mol 0 0 3350 kJ

= 0.444 =0.326

To know the limiting reagent , we find the ratio of given moles/required moles

0.444/8=0.055 0.326/3=0.1085

As the ratio of Al is less, it is the limiting reagent.

Thus the heat produced is calculated from moles of AL.

8Al +   3 Fe3O4 ----------------------> 4Al2O3 + 9Fe , Delta H = -3350 kJ/mol

12g/27g/mol 75.4g/231.53g/mol 0 0 3350 kJ

= 0.444 =0.326

8 moles of Al gives 3350 kJ heat

0.444 moles of Al gives heat = 0.444 moles x 3350kJ /8 =185.925kJ