Question 8.5: Calculating the Amount of Heat Released in a Reaction How mu......
Calculating the Amount of Heat Released in a Reaction
How much heat in kilojoules is evolved when 5.00 g of aluminum reacts with a stoichiometric amount of Fe_2O_3?
2 Al(s) + Fe_2O_3(s) → 2 Fe(s) + Al_2O_3(s) ΔH° = -852 kJ
STRATEGY
According to the balanced equation, 852 kJ of heat is evolved from the reaction of 2 mol of Al. To find out how much heat is evolved from the reaction of 5.00 g of Al, we have to find out how many moles of aluminum are in 5.00 g.
The molar mass of Al is 26.98 g/mol, so 5.00 g of Al equals 0.185 mol:
5.00 \mathrm{~g} \ \mathrm{Al} \times \frac{1 \mathrm{~mol} \mathrm{Al}}{26.98 \mathrm{~g} \mathrm{Al}}=0.185 \mathrm{~mol} \ \mathrm{Al}Because 2 mol of Al releases 852 kJ of heat, 0.185 mol of Al releases 78.8 kJ of heat:
0.185 \mathrm{~mol} \ \mathrm{Al} \times \frac{852 \mathrm{~kJ}}{2 \mathrm{~mol} \mathrm{Al}}=78.8 \mathrm{~kJ}BALLPARK CHECK
Since the molar mass of Al is about 27 g, 5 g of aluminum is roughly 0.2 mol, and the heat evolved is about (852 kJ/2 mol)(0.2 mol), or approximately 85 kJ.