Question 8.3: 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_{2}O_{3}?
2 Al(s) + Fe_{2}O_{3}(s) → 2 Fe(s) + Al_{2}O_{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 g Al × \frac{1 mol Al}{26.98 g Al} = 0.185 mol Al
Because 2 mol of Al releases 852 kJ of heat, 0.185 mol of Al releases 78.8 kJ of heat:
0.185 mol Al × \frac{852 kJ}{2 mol Al} = 78.8 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.