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Z thermodynamics > Chapter 3 > Flashcards

Chapter 3 Flashcards

1
Q

A substance that has a fixed chemical composition throughout

A

Pure substance

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2
Q

Air

A

Pure substance even though it is a mixture of several gases

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3
Q

Nitrogen and gaseous air

A

Pure substances

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4
Q

Mixture of liquid and gaseous water

A

Pure substance

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5
Q

Mixture of liquid and gaseous air

A

Not a pure substance

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6
Q

Molecules are at relatively fixed positions in a ___

A

Solid

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7
Q

Groups of molecules move about each other in the ____

A

Liquid phase

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8
Q

Molecules move about at random in the ___

A

Gas phase

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9
Q

The molecules in a solid are kept at their positions by…

A

The large springlike inter-molecular forces

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10
Q

A substance that is not about to vaporize

A

Compressed liquid

Subcooled liquid

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11
Q

At 1 atm and 20 degrees C water exists in the ___

A

Liquid phase (compressed liquid)

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12
Q

A liquid that is about to vaporize

A

Saturated liquid

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13
Q

At 1 atm pressure and 100 degrees C, water exists as…

A

A liquid that is ready to vaporize (saturated liquid)

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14
Q

A vapor that is about to condense

A

Saturated vapor

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15
Q

The state at which the liquid and vapor phases coexist in equilibrium

A

Saturated liquid-vapor mixture

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16
Q

A vapor that is not about to condense (i.e., not a saturated vapor)

A

Superheated vapor

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17
Q

As more heat is transferred, part of the saturated liquid vaporizes

A

Saturated liquid-vapor mixture

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18
Q

At 1 atm pressure, the temperature remains constant at 100 degrees C until the last drop of liquid is vaporized

A

Saturated vapor

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19
Q

As more heat is transferred, the temperature of the vapor starts to rise

A

Superheated vapor

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20
Q

The temperature at which water starts boiling depends on the ____

A

Pressure

If the pressure is fixed, so is the boiling temperature

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21
Q

Water boils at

A

100 degrees C at 1 atm pressure

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22
Q

The temperature at which a pure substance changes phase at a given pressure

A

Saturation temperature (Tsat)

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23
Q

The pressure at which a pure substance changes phase at a given temperature

A

Saturation pressure (Psat)

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24
Q

The amount of energy absorbed or released during a phase-change process

A

Latent heat

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25
Q

The amount of energy absorbed during melting. It is equivalent to the amount of energy released during freezing

A

Latent heat of fusion

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26
Q

The amount of energy absorbed during vaporization and it is equivalent to the energy released during condensation

A

Latent heat of vaporization

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27
Q

The ____ of latent heats depend on the temperature or pressure at which the phase change occurs

A

Magnitudes

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28
Q

Latent heat of fusion of water

A

At 1 atm, 333.7kJ/kg

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29
Q

Latent heat of vaporization of water

A

At 1 atm, 2256.5kJ/kg

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30
Q

The atmospheric pressure, and thus the boiling temperature of water, ____ with elevation

A

Decreases

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31
Q

At supercritical pressures (P > Pcr)

A

There is no distinct phase-change (boiling) process

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32
Q

The point at which the saturated liquid and saturated vapor states are identical

A

Critical point

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33
Q

The pressure in a piston-cylinder device can be reduced by…

A

Reducing the weight of the piston

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34
Q

Triple point of water

A

Ttp = 0.01 degrees C

Ptp = 0.6117kPa

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35
Q

At ____ pressure and temperature, a substance exists in three phases in equilibrium

A

Triple-point

36
Q

Passing from the solid phase directly into the vapor phase

A

Sublimation

37
Q

At low pressures (below the triple-point value), solids…

A

Evaporate without melting first (sublimation)

38
Q

Deposition

A

Vapor to solid

39
Q

Enthalpy

A

A combination property

40
Q

Energy units

A

Pressure*volume

41
Q

The amount of energy needed to vaporize a unit mass of saturated liquid at a given temperature or pressure

A

Enthalpy of vaporization (latent heat of vaporization)

Hfg

42
Q

The ratio of the mass of vapor to the total mass of the mixture

A

Quality (x)

Between 0 and 1

43
Q

Quality x = 0

A

Sat. Liquid

44
Q

Quality x = 1

A

Sat. Vapor

45
Q

The properties of the ____ are the same whether it exists alone or in a mixture with saturated vapor

A

Saturated liquid

46
Q

Temperature and pressure are ___ properties for a mixture

A

Dependent

47
Q

The relative amounts of liquid and vapor phases in a saturated mixture are specified by the ____

A

Quality (x)

48
Q

In the region to the right of the saturated vapor line and at temperatures above the critical point temperature, a substance exists as _____

A

Superheated vapor

49
Q

In the superheated vapor region, temperature and pressure are ____ properties

A

Independent

50
Q

Superheated vapor is characterized by:

A

Lower pressures (P < Psat at a given T)

Higher temperatures (T > Tsat at a given P)

High specific volumes (v > vg at a given P or T)

Higher internal energies (u > ug at a given P or T)

Higher enthalpies (h > hg at a given P or T)

51
Q

Compressed liquid properties depend more on what?

A

Temperature than pressure

52
Q

Compressed liquid is characterized by:

A

Higher pressures (P > Psat at a given T)

Lower temperatures (T < Tsat at a given P)

Lower specific volumes (v < vf at a given P or T)

Lower internal energies (u < uf at a given P or T)

Lower enthalpies (h < hf at a given P or T)

53
Q

Any equation that relates the pressure, temperature, and specific volume of a substance

A

Equation of state

54
Q

The simplest and best-known equation of state for substances in the gas phase is the ____ of state

A

Ideal gas equation

55
Q

Ideal gas equation

A

Predicts the P-v-T behavior of a gas quite accurately within some properly selected region

56
Q

Real gases behave as an ideal gas at ____

A

Low densities (low pressure, high temperature)

57
Q

When can water vapor be treated as an ideal gas?

A

At pressures below 10kPa

58
Q

Ideal gas applications

A

Air-conditioning applications

59
Q

When can ideal gas not be used?

A

At higher pressures (unacceptable errors in the vicinity of the critical point and the saturated vapor line)

Steam power plant applications

60
Q

A factor that accounts for the deviation of real gases from ideal-gas behavior at a given temperature and pressure

A

Compressibility factor Z

61
Q

The farther away Z is from unity,

A

The more the gas deviates from ideal-gas behavior

62
Q

What is the criteria for low pressure and high temperature?

A

The pressure or temperature of a gas is high or low relative to its critical temperature or pressure

63
Q

Ideal gas Z

A

Z=1

64
Q

Real gases Z

A

Z>1
Z=1
Z<1

65
Q

As P goes to 0,

A

Real gas to ideal gas

66
Q

Van Der Waals Equation

A

Intermolecular attraction forces, volume occupied by the molecules themselves

2 constants, accurate over a limited range

67
Q

Beattie-Bridgeman

A

5 constants

Accurate for density<=0.8Pcr

68
Q

Benedict-Webb-Rubin

A

8 constants

Accurate for density<=2.5Pcr

69
Q

Strobridge

A

16 constants

More suitable for computer calculations

70
Q

Virial

A

May vary.

Accuracy depends on the number of terms used

71
Q

The same of the individual pressures each component exerts on the system

A

Pressure of a gas mixture

72
Q

Atmospheric pressure can be seen as…

A

The sum of “dry” air pressure (Pa) and vapor pressure (Pv) from water in the air

73
Q

Air can only hold so much ____

A

Water vapor

74
Q

The ratio of the actual amount of water vapor in the air to the maximum amount is called ___

A

Relative humidity (phi)

75
Q

The relative humidity range

A

0 (dry air) to 1 (saturated air)

76
Q

The desirable range for “thermal comfort” is ____

A

40-60%

77
Q

Amount of water air can hold

A

Directly related to saturation pressure which increases with temperature

78
Q

Air can hold more water at ____ temperatures

A

Higher

79
Q

At low temperatures, water in the air ____

A

Condenses

80
Q

What happens in the early morning hours when it is still cool?

A

Fog and dew

81
Q

Example of phase equilibrium

A

At 100%, the air cannot hold any more water, so drying will not take place

82
Q

Large differences between _____ causes air to evaporate

A

Vapor pressure and saturated pressure

83
Q

Boiling vs. Evaporation

A

Liquid to vapor

Boiling - vapor bubbles

84
Q

Evaporation occurs…

A

At a liquid-vapor interface when the vapor pressure is less than the saturation pressure

85
Q

Evaporation examples

A

Evaporation of water from a pool or lake

Sweating

86
Q

Boiling occurs…

A

At a solid-liquid interface when the solid surface temperature is higher than Tsat for the liquid

87
Q

Boiling examples

A

Boiling water on a stove top

Z thermodynamics (11 decks)

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