The Foundations of Chemistry

Exercises 639

*040.The thermite reaction, used for welding iron, is the reac-
tion of Fe 3 O 4 with Al.

8Al(s)3Fe 3 O 4 (s)88n4Al 2 O 3 (s)9Fe(s)


H^0 3350 kJ/mol rxn

Because this large amount of heat cannot be rapidly dis-

sipated to the surroundings, the reacting mass may reach

temperatures near 3000°C. How much heat is released

by the reaction of 23.0 g of Al with 57.6 g of Fe 3 O 4?

*043.Natural gas is mainly methane, CH 4 (g). Assume that

gasoline is octane, C 8 H 18 (
), and that kerosene is

C 10 H 22 (
). (a) Write the balanced equations for the com-

bustion of each of these three hydrocarbons in excess O 2.

The products are CO 2 (g) and H 2 O(
). (b) Calculate


H^0 rxnat 25°C for each combustion reaction. 
H^0 ffor

C 10 H 22 is 300.9 kJ/mol. (c) When burned at standard

conditions, which of these three fuels would produce the

most heat per mole? (d) When burned at standard con-

ditions, which of the three would produce the most heat

per gram?

Bond Energies

*044.(a) How is the heat released or absorbed in a gas phase

reactionrelated to bond energies of products and reac-

tants? (b) Hess’s Law states that

H^0 rxnn
H^0 f productsn
H^0 f reactants

The relationship between 
H^0 rxnand bond energies for a

gas phase reactionis

H^0 rxn

bond energiesreactantsbond energiesproducts

It is nottrue, in general, that 
H^0 ffor a substance is equal

to the negative of the sum of the bond energies of the

substance. Why?

*045.(a) Suggest a reason for the fact that different amounts of

energy are required for the successive removal of the three

hydrogen atoms of an ammonia molecule, even though

all NXH bonds in ammonia are equivalent. (b) Suggest

why the NXH bonds in different compounds such as

ammonia, NH 3 ; methylamine, CH 3 NH 2 ; and ethyl-

amine, C 2 H 5 NH 2 , have slightly different bond energies.

*046.Use tabulated bond energies to estimate the enthalpy of

reaction for each of the following gas phase reactions.

(a) H 2 CUCH 2 Br 2 88nBrH 2 CXCH 2 Br

(b) H 2 O 2 88nH 2 O^12 O 2

*047.Use tabulated bond energies to estimate the enthalpy of

reaction for each of the following gas phase reactions.

(a) N 2 3H 2 88n2NH 3

(b) CH 4 Cl 2 88nCH 3 ClHCl

(c) COH 2 O88nCO 2 H 2

*048.Use the bond energies listed in Table 15-2 to estimate

the heat of reaction for

*049.Estimate 
Hfor the burning of one mole of butane, using

the bond energies listed in Tables 15-2 and 15-3.

C F(g)

F

Cl

Cl F F(g) C F(g)

F

F

F Cl Cl(g)

*041.When a welder uses an acetylene torch, the combustion
of acetylene liberates the intense heat needed for weld-
ing metals together. The equation for this combustion
reaction is

2C 2 H 2 (g)5O 2 (g)88n4CO 2 (g)2H 2 O(g)

The heat of combustion of acetylene is1255.5 kJ/mol
of C 2 H 2. How much heat is liberated when 3.462 kg of
C 2 H 2 is burned?
*042.Silicon carbide, or carborundum, SiC, is one of the hard-
est substances known and is used as an abrasive. It has the
structure of diamond with half of the carbons replaced by
silicon. It is prepared industrially by reduction of sand
(SiO 2 ) with carbon in an electric furnace.

SiO 2 (s)3C(graphite)88nSiC(s)2CO(g)

H^0 for this reaction is 624.6 kJ, and the 
H^0 ffor SiO 2 (s)

and CO(g) are 910.9 kJ/mol and 110.5 kJ/mol, respec-

tively. Calculate 
H^0 ffor silicon carbide.

The thermite reaction.