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Thermodynamic Terms - Functions and Relations

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  • Chemical energy - is related to the relationships between molecules in chemical compounds. When chemicals react with each other, they may give off heat (exothermic reaction) or require heat (endothermic reaction)
  • Electric energy - is related to electrons moving through a conductor
  • Energy - can be reduced to the concepts of heat and work and can be found in various forms: potential energy, kinetic energy, thermal or internal energy, chemical energy, and nuclear energy
  • Enthalpy - is a term with energy units that combines internal energy with a pressure/volume or flow work term
  • Entropy - is a property of matter that measures the degree of randomization or disorder. The natural state is for entropy to be produced by all processes
  • Heat - is energy in motion from one region to an other as a result of temperature difference
  • Internal energy - has to do with activity within the molecular structure and is typically observed with temperature measurement
  • Kinetic energy - is the energy of motion and is proportional to the square of the velocity as well to the mass of the moving body
  • Nuclear energy - is related to the energy of atomic relationships between the fundamental particles. Nuclear fission and fusion are reactions which release nuclear energy
  • Potential energy - is the energy of location or position of a mass in a force field
  • Property - is a measurable characteristic of a system or substance. Temperature, density, pressure etc
  • Specific Heat - The specific heat is the amount of heat required to change a unit mass (or unit quantity, such as mole) of a substance by one degree in temperature
  • Temperature - is a term used to quantify the difference between warm and cold level of internal energy of a substance
  • Work - is an energy form which can be equated to the rising of a weight as moving a mass in a force field or moving a liquid against a resisting force

See also Symbols Used to Denote a Chemical Reaction, Process or Condition

Thermodynamic Terms - Functions and Relations
Term Function
Activity coefficient γi = fi/(xifiθ)
Chemical potential μi = (∂G/∂ni)T,p,nj≠i
Energy U
Enthalpy H = U + pV
Entropy S
Fugasity fi = (xi)exp{(μi - μiÞg)/RT}
Gibbs (free) energy G = U + pV - TS
Gibbs-Duhem relation 0 = SdT - Vdp + Σinidμi
Gibbs-Helmholtz equation H = G - T(∂G/∂T)p
Helmholtz energy A = U - TS
Isentropic (constant heat and mass) compressibility κS = - (∂V/∂p)S/V
Isothermal (constant temperature) compressibility κT = - (∂V/∂p)T/V
κT - κS  = T αV2V/Cp
Isobaric (constant pressure) expansivity αV= (∂V/∂T)p/V
Isobaric heat capacity Cp = (∂H/∂T)p
Isochoric (constant volume) heat capacity CV = (∂U/∂T)v
Cp - CV = Tα2V/κT
Joule-Thompson expansion μJT = (∂T/∂p)H = - {V - (∂V/∂T)p}/Cp
ΦJT = (∂H/∂p)T = V - T(∂V/∂T)p
Maxwell relations (∂S/∂p)T = - (∂V/∂p)p
(∂S/∂V)T = - (∂p/∂T)V
Partial molar quantity Xi = (∂X/∂ni)T,p,nj≠i
Perfect (ideal) gas [symbol Þg] pV = (Σini)RT
μiÞg = μiθ + RTln(xip/pθ)


p = pressure
V = Volume
T = temperature
ni = amount of substance i
xi = nijnj = mole fraction of substance i
= gas constant

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Related Topics


Work, heat and energy systems.

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