Bài giảng Hóa lý - Chương 5: Pure substance

THERMODYNAMICS

Dr. Ngo Thanh An

Chapter 5 – Pure substance

• Pure substance: A pure substance has a homogenous and fixed chemical

composition throughout, and may exist in more than one phase

• Air is a mixture of several gases, but it is considered to be a pure

substance.

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Chapter 5 – Pure substance

The molecules in a

solid are kept at

their positions by

the large springlike

inter-molecular

forces.

In a solid, the

attractive and

repulsive forces

between the

molecules tend

to maintain them

at

relatively

constant

distances from

each other.

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Chapter 5 – Pure substance

Solid:

• The molecules in solid are arranged in a three-dimensional pattern (lattice)

• The molecular spacing is close each other

• The molecules cannot move, but they continually oscillate about their

equilibrium position

• Their velocity depends on the temperature => Increasing temperature leads to

group molecules breaking away => melting process

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Chapter 5 – Pure substance

Liquid:

• The molecular spacing is not much different from that of solid phase, but it

slightly increases

• The molecules can translate and rotate freely

Gas:

• The molecules are far apart each other and a molecular order is nonexistent,

moving about at random and continually colliding

• Having a considerably higher energy level than they are in the liquid and solid

phases => it must potentially release a large amount of its energy prior to

condensing or freezing

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Chapter 5 – Pure substance

• Compressed liquid (subcooled liquid): A substance that it is not about

to vaporize.

• Saturated liquid: A liquid that is about to vaporize.

At 1 atm and 20°C, water

exists in the liquid phase

(compressed liquid).

At 1 atm pressure and

100°C, water exists as

a liquid that is ready to

vaporize (saturated

liquid).

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Chapter 5 – Pure substance

• Saturated vapor: A vapor that is about to condense.

• Saturated liquid–vapor mixture: The state at which the liquid and vapor phases

coexist in equilibrium.

• Superheated vapor: A vapor that is not about to condense (i.e., not a saturated

vapor).

As more heat is transferred,

the temperature of the

vapor starts to rise

(superheated vapor).

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As more heat is transferred, part of

the saturated liquid vaporizes

(saturated liquid–vapor mixture).

At 1 atm, the temp. remains

constant at 100°C until the last

drop of liquid is vaporized

(saturated vapor).

Chapter 5 – Pure substance

If the entire process between state 1 and 5 described in the figure is

reversed by cooling the water while maintaining the pressure at the

same value, the water will go back to state 1, retracing the same path,

and in so doing, the amount of heat released will exactly match the

amount of heat added during the heating process.

T-v diagram for the heating

process of

water at

constant pressure.

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Chapter 5 – Pure substance

Saturation Temperature and Saturation Pressure

• The temperature at which water starts boiling depends on the pressure;

therefore, if the pressure is fixed, so is the boiling temperature.

• Water boils at 100C at 1 atm pressure.

• Saturation temperature T_sat: The temperature at which a pure substance

changes phase at a given pressure.

• Saturation pressure P_sat: The pressure at which a pure substance changes

phase at a given temperature.

The liquid–vapor saturation

curve of a pure substance

(numerical values are for

water).

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Chapter 5 – Pure substance

• Latent heat: The amount of energy absorbed

or released during a phase-change process.

• Latent heat of fusion: The amount of energy

absorbed during melting. It is equivalent to the

amount of energy released during freezing.

• Latent heat of vaporization: The amount of

energy absorbed during vaporization and it is

equivalent to the energy released during

condensation.

• The magnitudes of the latent heats depend on

the temperature or pressure at which the

phase change occurs.

• At 1 atm pressure, the latent heat of fusion of

water is 333.7 kJ/kg and the latent heat of

vaporization is 2256.5 kJ/kg.

• The atmospheric pressure, and thus the boiling

temperature of water, decreases with

elevation.

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Chapter 5 – Pure substance

The variations of properties during phase-change processes are best studied and

understood with the help of property diagrams such as the T-v, P-v, and P-T diagrams

for pure substances.

T-v diagram of constant-pressure

phase-change processes of a pure

substance at various pressures

(numerical values are for water).

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Chapter 5 – Pure substance

• saturated liquid line

• saturated vapor line

• compressed liquid region

• superheated vapor region

• saturated liquid–vapor mixture

region (wet region)

At supercritical pressures

(P > P_cr), there is no

Critical point: The point at

which the saturated liquid

and saturated vapor states

are identical.

distinct

phase-change

(boiling) process.

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Chapter 5 – Pure substance

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Chapter 5 – Pure substance

The P-v-T surfaces present a great deal of information at once, but in a

thermodynamic analysis it is more convenient to work with two-dimensional

diagrams, such as the P-v and T-v diagrams.

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Chapter 5 – Pure substance

Sublimation: Passing

Phase Diagram

from the solid phase

directly into the vapor

phase.

At low pressures (below the

triple-point

evaporate without melting first

value),

solids

P-T diagram of pure substances.

(sublimation).

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Chapter 5 – Pure substance

• For most substances, the relationships among thermodynamic properties are too

complex to be expressed by simple equations.

• Therefore, properties are frequently presented in the form of tables.

• Some thermodynamic properties can be measured easily, but others cannot and are

calculated by using the relations between them and measurable properties.

• The results of these measurements and calculations are presented in tables in a

convenient format.

Enthalpy—A Combination Property

The combination u +

Pv

is

frequently

encountered in the

analysis of control

volumes.

The product pressure  volume

has energy units.

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Chapter 5 – Pure substance

Enthalpy of vaporization, h_fg(Latent heat of

vaporization): The amount of energy needed

to vaporize a unit mass of saturated liquid at

a given temperature or pressure.

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Chapter 5 – Pure substance

Examples:

Saturated liquid and

saturated vapor states

of water on T-v and P-

v diagrams.

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Chapter 5 – Pure substance

Quality, x : The ratio of the mass of vapor to the total mass of the mixture. Quality is

between 0 and 1 → 0: sat. liquid, 1: sat. vapor.

The properties of the saturated liquid are the same whether it exists alone or in a mixture

with saturated vapor.

Temperature and pressure are

dependent properties for a mixture.

The relative

amounts

liquid

of

and

vapor phases

in a saturated

mixture

are

specified by

the quality x.

A two-phase system can be treated

as a homogeneous mixture for

convenience.

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Chapter 5 – Pure substance

y

v, u, or h.

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