Chemistry - A Molecular Science

calculations in which energy is involved, the value used is R = 8.31451 J·mol

-1·K

-1. Room

temperature is ~300 K, so thermal energy at room temperature is approximately

ΔE

thermal

~ RT = (8.3x10

-3 kJ·mol

-1·K

-1^ )

(300 K) = 2.5 kJ·mol

-1^

Thus, processes requiring hundreds of kJ·mol

-1 occur only rarely at room temperature,

while those requiring less than ~10 kJ·mol

-1 occur frequently. RT is only an approximation

for thermal energy, but it does demonstrate that

the thermal energy of a system depends

only upon its temperature

.

Example 7.5 a) What is the approximate thermal energy of molecules in boiling water?

Water boils at 100

oC, so T = 100

oC + 273 = 373 K

We approximate the thermal energy

of the liquid as RT and write:

RT = (8.3 J

.K

-1.mol

-1)(373 K) = 3.1 x 10

3 J

.mol

-1^

Thus, the thermal energy is about 3 kJ

.mol

-1^

b) To what temperature should a sample be heated so that its thermal energy is ~100

kJ

.mol

-1?

E = (100 kJ

.mol

-1)(10

3 J

.kJ

-1)= 1.0 x 10

5 J

.mol

-1^

×⋅

×

5-⋅⋅

1

4

-1

-1

E1.010 Jmol

T =

=

= 1.2 10 K

R

8.314 J mol K

= 12,000 K

7.3

INTERMOLECULAR FORCES

Particles in the gas phase are relatively far apar

t, so their attractive forces are negligible

compared to their thermal energy. However, the attractive forces between particles in the condensed states are strong enough to hold the

particles in contact with one another. Ionic

compounds are always solids at room temperature because their cations and anions are held in the solid network by strong electros

tatic attractions called

ionic bonds. However,

molecular substances can occur as gases (O

), liquids (H 2

O), or solids (sugar) at room 2

conditions. This diversity in states arises from

a diversity in the strengths of the forces

between the molecules. Whereas the strength of ionic bonds determines the physical properties of a salt crystal, the physical properties of molecular substances are not related to the strength of their covalent bonds. In

molecular substances, we must distinguish

between two different kinds of forces:

Chapter 7 States of Matter and Changes in State

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North

Carolina

State

University