Samples: Free-Response Questions
- To produce molten iron aboard ships, the Navy uses the thermite reaction, which consists
of mixing iron(III) oxide (rust) with powdered aluminum, igniting it, and producing
aluminum oxide as a by-product. ∆H°f for aluminum oxide is −1669.8 kJ/mole, and that
for iron(III) oxide is −822.2 kJ/mole.
(a) Write a balanced equation for the reaction.
(b) Calculate ∆H for the reaction.
(c) The specific heat of aluminum oxide is 0.79 J/g ⋅°C, and that of iron is 0.48 J/g ⋅°C.
If one were to start the reaction at room temperature, how hot would the aluminum
oxide and the iron become?
(d) If the temperature needed to melt iron is 1535°C, and the heat of fusion for iron is
270 J/g, confirm through calculations that the reaction will indeed produce molten
iron.
Answer
- (a) Restatement: Balanced equation.
22 Al^^^^ss sshhhh++Fe O 23 "Al O 23 Fe
(b) Restatement: Calculation of ∆H.
∆H =Σ∆H°f products−Σ∆H°f reactants
∆H =[∆HfAl 2 O 3 (s) + 2 ∆HfFe(s)] −[2 ∆HfAl(s) + ∆HfFe 2 O 3 (s)]
∆H = [−1669.8 + 2(0)] −[2(0) + (−822.2)]
Remember that ∆Hf for elements is 0.
∆H =−847.6 kJ
(c) Restatement: How hot will the products become?
Given: Specific heat of aluminum oxide = 0.79 J/g ⋅°C
Given: Specific heat of iron = 0.48 J/g ⋅°C
Going back to the balanced reaction (answer a), you note that 1 mole of Al 2 O 3 is being
produced for every 2 moles of iron — they are not being produced in a ratio of 1 gram to
1 gram. For each of the two products, determine how much energy (in joules) is required
for a 1°C rise in temperature.
: C
:
. .J
1
1
1
101 96 079
For Al O 81
mole Al O
mole Al O
gAlO
g
23 J/ C
23
23
23
##% = %
gC:
:
..
1 /
2
1
55 85 048
For Fe 54
moles Fe
mole Fe
gFe J
##% = JC%
Thermochemistry