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Properties of Thermodynamic System

A variety of physio-chemical changes occurring in nature involve energy changes. These changes may be in the form of heat, light, sound, chemical, mechanical, and electrical, etc.

During these physical and chemical changes one form of energy gets transformed into another form of energy. Chemical thermodynamics is a branch which deals with the energy changes during chemical reactions. It deals with feasibility of reactions, equilibrium point of a reaction, etc.

Thermodynamics basically refers to flow of heat and it deals with the quantitative relationship between heat and other forms of energy observed in physio-chemical transformations. Thermodynamics is based on three fundamental laws. These are applicable to the entire phenomenon that takes place in nature. These laws are not theoretical but are based on experimental outcomes.

Thermodynamics under a standard or given set of conditions can help us predict whether a process will occur spontaneously or not. The laws provide necessary criteria for predicting the feasibility of a process, however, it gives no information with regard to the rate at which a given change will proceed.

Thermodynamics deals only with state of the system and makes no mention of the mechanism of how the change is accomplished and it will provide answer to why a change occurs but not how it occurs.

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Thermodynamic System

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Thermodynamics deals with the changes that occur during a chemical reaction. Thermodynamics is not based on any theoretical hypothesis. It is based on practical observations and measurable properties.
Measurable properties of a system may be divided into two classes:
  1. Extensive property
  2. Intensive property

Extensive property

An extensive property of a system depends upon the total amount of material in the system. Mass, volume, internal energy, heat contents, free energy, entropy, and heat capacity are all extensive properties.
  • Mass: This gives the idea of how much of the initial matter was contained in the system and how much is left after the process is complete.
  • Volume: This gives an idea of the dimension of the matter contained in and what will be the final dimension after the process is over.
  • Internal energy: It is the total energy contained in to create the thermodynamic system but excludes the energy to displace the system’s surroundings. It has two major components of kinetic energy and potential energy due to the movement of particles and the static electric energy of the atoms contained in.
  • Heat contents: Under a given pressure, the heat content or Enthalpy is a measure of total energy of a thermodynamic system. It includes internal energy which is required to create a system and establish its volume and pressure.
  • Free energy: It is the energy in the physical system which can be converted into work.
  • Entropy: It is a thermodynamic property which is used to determine the energy available for useful work in a thermodynamic process.
  • Heat capacity: Heat capacity or thermal capacity is the measurable physical quantity that gives an idea of the amount of heat required to change a substance’s temperature by a given range.

Intensive property

An intensive property is defined as a property which is independent of the amount of material in the system. Density, molar property, surface tension, viscosity, specific heat, thermal conductivity, refractive index, pressure, temperature, boiling point, freezing point, and vapour pressure of a liquid are all intensive properties.
  • Density: Density of a material is defined as a ratio between its volume and the matter contained in or mass.
  • Molar property: Molar property mainly consists the detailing of molar volume, molar energy, molar entropy, molar heat capacity and all these are quantified from the point of moles of the substance involved in.
  • Surface tension: It is a property of a liquid surface which helps in resisting any kind of external force applied on it.
  • Viscosity: It is a measurable internal quantity of a fluid which resists its flow.
  • Specific heat: It is the amount of heat per unit mass required to raise the temperature by one degree Celsius.
  • Thermal conductivity: Thermal conductivity (λ) is the intrinsic property of a material which relates its ability to conduct heat.
  • Refractive index: The measure of the speed of light in a medium is referred as refractive index of that medium.
  • Pressure: It is the perpendicular force acting per unit area on the surface of an object.
  • Temperature: It is the property of the matter which quantitatively expresses the coldness or hotness of substance.
  • Boiling point: It is the temperature of the substance at which the vapour pressure of the liquid equals environmental pressure.
  • Freezing point: It is the temperature at which a liquid composition solidifies under a given pressure.
  • Vapour pressure of a liquid: It is defined as the equilibrium pressure above its liquid resulting due to the evaporation of liquid.
Thermodynamic system depending upon interactions between system and surroundings can be classified as open systems, closed systems and isolated systems.

  1. Open systems: A system which can exchange both matter and energy with the surroundings.
  2. Closed system: A System in which the exchanges of energy with the surroundings is possible, and the transfer of matter to and from the surroundings does not take place.
  3. Isolated system: It is a system that prevents any interactions between the system and the surroundings.

Thermodynamic Properties of Air

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Air is a mixture of gases. Air shows different thermodynamic properties at different concentrations, pressures and temperatures. Its thermodynamic properties also vary with the change in state of air and its composition. The following table gives an idea of the properties.

Properties in the saturation state:

P(boil) (bar)
P(con) (bar)
ρ (liq) (g/cc)
ρ (gas) (g/L)
65 0.1468 0.0861 0.939 0.464
70 0.3234 0.2052 0.917 1.033
80 1.146 0.8245 0.871 3.709
90 3.036 2.397 0.819 9.980
100 6.621 5.599 0.763 22.39
130 34.16 33.32 0.487 184.33
132.55 37.69 37.79
0.313 312.89

Properties of liquid air:

P (bar) T (K)
ρ (g/cc) H (J/g)
S (J/g) Cp (J/Kg.K)
1 75
0.8935 –131.7 2.918 1.843
5 80 0.8718 –122.3 3.031 1.868
10 75 0.8952 –131.1 2.913 1.836
10 80 0.8729 –122.0 3.028 1.863
50 75 0.9025 –128.2 2.892 1.806
50 100 0.7859 –81.8 3.415 1.939
100 75 0.9111 –124.5 2.867 1.774
100 100 0.8033 –79.4 3.376 1.852

Properties of air in the gaseous state:

ρ H S
1 100 3.5300 98.3 5.759 1.032
1 200 1.746 199.7 6.463 1.007
1 300 1.161 300.3 6.871 1.007
10 200 17.835 195.2 5.766 1.049
10 300 11.643 298.3 6.204 1.021
100 200 213.950 148.8 4.949 1.650

Thermodynamic Properties of Water

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Water has the second highest value of specific heat which is attributed to its hydrogen bonding. Water exists in three states at a reasonably convenient temperature range. When it changes from solid to liquid states it takes heat from the surroundings and when heat is supplied it changes from liquid to vapour. Ice to water (liquid) has different specific heat from liquid to gas. Latent heat of fusion is the enthalpy that gets used to break the crystal bonding. Latent heat of vaporisation is the enthalpy used to break the hydrogen bonding.

Thermodynamic Tables

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Tables of ΔfHo (kJ/Mol) at 298K
Aqueous ions:

Aqueous Ions ΔfHo (kJ/mol)
H+ 0
H3O+ -285.8
Ca2+ -542.8
Cu2+ +64.77
HSO4- -887.3
NH4+ -132.5
SO42- -909.3
OH- -230.0
Na+ -240.1
NO3- -205.0

Inorganic compounds Tables of ΔfHo (kJ/Mol) at 298K

ΔfHo (kJ/mol)
Al(OH)3 (s) -1675.7
AlCl3 (s) -705.6
H2S (g) -20.6
H2SO4 (aq) -909.3
NH4Cl(s) -314.6
NO2(g) +33.2
NaCl (s) -411.54
Mg(OH)2(s) -924.7
FeCl2 (s) -399.4
H2CO3(aq) -699.65

Organic Compounds Tables of ΔfHo (kJ/Mol) at 298K

Organic compounds ΔfHo (kJ/mol)
C2H6 -83.85
C2H2 227.48
C3H8 -104.68
C4H10(n-but) -125.65
C4H8(iso-but) 134.18
C4H8(iso butene) -0.54
CH3OH(l) -238.66
2H5OH(g) -235.310
CH3COOH -484.5
C4H8(1-butene) -7.4
C4H8(trans2-butene) -11.0
C6H6(g) 82.93
C2H4 (g) +52.26
More topics in Properties of Thermodynamic System
Enthalpy Heat Capacity
Gibbs Free Energy
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