Chemistry - A Molecular Science

heat required to raise the te

mperature of one gram of H

O by 1 2

oC. 1 cal = 4.18 J.

However, a dietary calorie, which appears on packaging of common food items, is actually a kcal (1000 cal). To convert from kJ to kcal,

we use the fact that 4.18 kJ = 1 kcal:

(79 kJ)(1 kcal/4.18 kJ) = 19 kcal or 19 dietary calories

9.4

BOND ENERGIES

The

bond

or

dissociation energy

(D

) is the energy required to break one mole of bonds

in

the gaseous state

. It is always positive.

H^2

(g)

→

2 H(g)

ΔH = +436 kJ = D(H-H)

HCl(g)

→ H(g) + Cl(g)

ΔH = +431 kJ = D(H-Cl)

Similarly, energy is always released when bonds are formed.

Table 9.2

Some common bond energies (kJ/mol)

C-H

H(g) + F(g)

→

HF(g)

ΔH

= -565 kJ = -D(H-F)

The enthalpy of a gas phase reaction is si

mply the difference between the amount of

energy required to break the reactant bonds and the amount of energy released when the product bonds are formed,

413 N-H

391

O-H

463 H-H

436

C-F

485 N-F

272

O-F

190 H-F

565

C-Cl

328 N-Cl

200

O-Cl

203 H-Cl

431

C-Br

276 N-Br

243

O-Br

235 H-Br

366

C-I

234

O-I

234 H-I

299

C-C

ΔH

∼

D(broken bonds) – Σ

D(formed bonds) Σ

Eq. 9.4

347 N-N

163

C-N

305 C-O

358

C=C

Enthalpies derived from Equation 9.4 are only approximate values when tabulated bond energies are used because the bond energy depe

nds on the environmen

t of the bond, while

the values presented in tables like Table 9.2 are average values of many environments. For example, the O-H bond in F

COH is slightly weaker than in (CH 3

) 33

COH, but we

would use the tabulated, average value of 463 kJ/mol for each.

612 N=N

418

C=N

615 C=O

799

C≡

C

820

N≡

N

941

C≡

N

891

C≡

O

1072

F-F 159

O-O

146

Cl-Cl 243

O=O

495

Br-Br 193

I-I 151

* Carbon-carbon bonds can be single, doubl

e, or triple bonds, but carbon

always has four bonds to it.

† The tabulated value is -92 kJ, so this answer is off by about 5%, which

is the approximation error in this example.

Example 9.3

Use the data in Table 9.2 to estimate

ΔH for the following reactions.

a) N

(g) + 3H 2

(g) 2

→

2NH

(g). 3

Bond energies depend upon both the number and types of bonds. Thus, the first thing we must do is to draw the Lewis structures of the molecules.*

NN

HHHHHH

HN

H

HHN

H

H

+

+

The N

N bond and three H-H bonds must be broken,≡

and six N-H bonds must be formed.

ΔH ~ D(N

≡N) + 3D(H-H) - 6D(N-H) = 941 kJ + 3(436) kJ - 6(391) kJ = -97 kJ.†

Chapter 9 Reaction Energetics

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North

Carolina

State

University