Chemistry - A Molecular Science

Figure 7.13 Definitions and energies of phase changes (a) The energy of a gas is much gr

eater than the energy of a liquid,

which is greater than the energy of a solid. Heat,

ΔH, must be

supplied to carry out a change from a lower energy state to a higher energy state.

ΔH

is the heat of fusion, fus

ΔH

vap

is the heat of

vaporization, and

ΔH

sub

is the heat of sublimation. (b) A molecular

view of the various changes in state.

evaporate

H

vapHD

subHD

fusHD

condense

sublime

deposit

gas liquid solid

(a)

(b)

freeze

melt

draw the liquid up the walls of the glass and produce the concave shape of the meniscus (Figure7.12a). Mercury atoms do not interact

well with glass, so the adhesive forces are

very weak. Consequently, mercury forms a c

onvex meniscus (Figure 7.12b) to maximize

Hg-Hg interactions and minimize interac

tions with the walls of the container.

7.6

CHANGES IN STATE

Molecules in a solid orient th

emselves to maximize their interactions, which lowers their

potential energy. Indeed, solids have the lowest

potential energy of the three states of

matter. Molecules in the liquid state are close to

one another, but they are not restricted to

positions that maximize their interactions, so their potential energy is slightly higher than that of a solid. Molecules in the gas phase are far apart, so th

ey interact only weakly, if at

all, and their potential energy is the highest of the three states of matter. Figure 7.13a summarizes the relative energies of the three states of matter.

When heat is added to a substance, the en

ergy increase of the substance can be in the

form of either kinetic or potential energy. In

creases in kinetic energy result in increases in

the thermal energy or temperature of the s

ubstance. However, once certain temperatures

are reached, the heat is used to increase the

potential energy of the substance, not the

kinetic energy. Increases in potential energy r

esult in changes in states; solids melt or

sublime to the higher energy liquid and gas stat

es and liquids evaporate to the gas state.

(Figure 7.13b). However, the state of matter is also a function of pressure; solids also melt and liquids boil as a result of pressure changes.

In this section, we examine how the phases

of a substance vary with both its temperature and its pressure. SOLID-LIQUID TRANSITION The particles in a solid occupy fixed positions that optimize their interactions with one another, and their kinetic energy is in the form of oscillations about these positions. As the temperature is increased, the speed and amplit

ude of the oscillations increase. Eventually,

the oscillations become so agitated and their amplitude so great that the separation between particles becomes sufficiently large

that the forces between them are no longer

strong enough to keep the particles in their fixed positions. At this point, the structure of the solid collapses into a liquid

as the substance melts. The forces that held the particles in

their fixed positions in the solid still exist in

the liquid, but they are weaker because the

particles are farther apart.

Chapter 7 States of Matter and Changes in State

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