Cliffs AP Chemistry, 3rd Edition

Samples: Free-Response Questions

1. 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

1. (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