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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
The amount of energy absorbed during melting. It is equivalent to the amount of energy released during freezing
Latent heat of fusion
26
The amount of energy absorbed during vaporization and it is equivalent to the energy released during condensation
Latent heat of vaporization
27
The ____ of latent heats depend on the temperature or pressure at which the phase change occurs
Magnitudes
28
Latent heat of fusion of water
At 1 atm, 333.7kJ/kg
29
Latent heat of vaporization of water
At 1 atm, 2256.5kJ/kg
30
The atmospheric pressure, and thus the boiling temperature of water, ____ with elevation
Decreases
31
At supercritical pressures (P > Pcr)
There is no distinct phase-change (boiling) process
32
The point at which the saturated liquid and saturated vapor states are identical
Critical point
33
The pressure in a piston-cylinder device can be reduced by...
Reducing the weight of the piston
34
Triple point of water
Ttp = 0.01 degrees C Ptp = 0.6117kPa
35
At ____ pressure and temperature, a substance exists in three phases in equilibrium
Triple-point
36
Passing from the solid phase directly into the vapor phase
Sublimation
37
At low pressures (below the triple-point value), solids...
Evaporate without melting first (sublimation)
38
Deposition
Vapor to solid
39
Enthalpy
A combination property
40
Energy units
Pressure*volume
41
The amount of energy needed to vaporize a unit mass of saturated liquid at a given temperature or pressure
Enthalpy of vaporization (latent heat of vaporization) Hfg
42
The ratio of the mass of vapor to the total mass of the mixture
Quality (x) Between 0 and 1
43
Quality x = 0
Sat. Liquid
44
Quality x = 1
Sat. Vapor
45
The properties of the ____ are the same whether it exists alone or in a mixture with saturated vapor
Saturated liquid
46
Temperature and pressure are ___ properties for a mixture
Dependent
47
The relative amounts of liquid and vapor phases in a saturated mixture are specified by the ____
Quality (x)
48
In the region to the right of the saturated vapor line and at temperatures above the critical point temperature, a substance exists as _____
Superheated vapor
49
In the superheated vapor region, temperature and pressure are ____ properties
Independent
50
Superheated vapor is characterized by:
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
Compressed liquid properties depend more on what?
Temperature than pressure
52
Compressed liquid is characterized by:
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
Any equation that relates the pressure, temperature, and specific volume of a substance
Equation of state
54
The simplest and best-known equation of state for substances in the gas phase is the ____ of state
Ideal gas equation
55
Ideal gas equation
Predicts the P-v-T behavior of a gas quite accurately within some properly selected region
56
Real gases behave as an ideal gas at ____
Low densities (low pressure, high temperature)
57
When can water vapor be treated as an ideal gas?
At pressures below 10kPa
58
Ideal gas applications
Air-conditioning applications
59
When can ideal gas not be used?
At higher pressures (unacceptable errors in the vicinity of the critical point and the saturated vapor line) Steam power plant applications
60
A factor that accounts for the deviation of real gases from ideal-gas behavior at a given temperature and pressure
Compressibility factor Z
61
The farther away Z is from unity,
The more the gas deviates from ideal-gas behavior
62
What is the criteria for low pressure and high temperature?
The pressure or temperature of a gas is high or low relative to its critical temperature or pressure
63
Ideal gas Z
Z=1
64
Real gases Z
Z>1 Z=1 Z<1
65
As P goes to 0,
Real gas to ideal gas
66
Van Der Waals Equation
Intermolecular attraction forces, volume occupied by the molecules themselves 2 constants, accurate over a limited range
67
Beattie-Bridgeman
5 constants Accurate for density<=0.8Pcr
68
Benedict-Webb-Rubin
8 constants Accurate for density<=2.5Pcr
69
Strobridge
16 constants More suitable for computer calculations
70
Virial
May vary. Accuracy depends on the number of terms used
71
The same of the individual pressures each component exerts on the system
Pressure of a gas mixture
72
Atmospheric pressure can be seen as...
The sum of “dry” air pressure (Pa) and vapor pressure (Pv) from water in the air
73
Air can only hold so much ____
Water vapor
74
The ratio of the actual amount of water vapor in the air to the maximum amount is called ___
Relative humidity (phi)
75
The relative humidity range
0 (dry air) to 1 (saturated air)
76
The desirable range for “thermal comfort” is ____
40-60%
77
Amount of water air can hold
Directly related to saturation pressure which increases with temperature
78
Air can hold more water at ____ temperatures
Higher
79
At low temperatures, water in the air ____
Condenses
80
What happens in the early morning hours when it is still cool?
Fog and dew
81
Example of phase equilibrium
At 100%, the air cannot hold any more water, so drying will not take place
82
Large differences between _____ causes air to evaporate
Vapor pressure and saturated pressure
83
Boiling vs. Evaporation
Liquid to vapor Boiling - vapor bubbles
84
Evaporation occurs...
At a liquid-vapor interface when the vapor pressure is less than the saturation pressure
85
Evaporation examples
Evaporation of water from a pool or lake Sweating
86
Boiling occurs...
At a solid-liquid interface when the solid surface temperature is higher than Tsat for the liquid
87
Boiling examples
Boiling water on a stove top

Thermodynamics (11 decks)

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