Ideal Gas Constant (R) – Universal Gas Constant

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Gas Constant (R)
R is the ideal gas constant, universal gas constant, or molar gas constant.

The gas constant (R) is a proportionality constant used in the ideal gas law and Nernst equation. It’s also called the ideal gas constant, universal gas constant, or molar gas constant. Basically, the gas constant is the same as the Boltzmann constant (k), except the gas constant includes Avogadro’s number (NA):

R = NAk

The gas constant is in units of energy per temperature per mole, while the Boltzmann constant has units of energy per temperature per particle.

Ideal Gas Equation and Nernst Equation

The ideal gas equation relates the pressure and volume of an ideal gas to the number of moles and temperature:

PV = nRT

Here, P is pressure, V is volume, n is number of moles of an ideal gas, R is the gas constant, and T is temperature.

The Nernst equation relates the reduction potential of a half-cell to the standard electrode potential, temperature, moles of electrons, Faraday’s constant, and reaction quotient:

E = E0 – (RT/nF)lnQ

Here, E is the cell potential, E0 is the standard cell potential, R is the gas constant, T is the temperature, n is the number of mole of electrons exchanged, F is Faraday’s constant, and Q is the reaction quotient.

Gas Constant Value in Different Units

In 2019, the redefinition of several SI base units included the gas constant. The gas constant is now defined as precisely 8.31446261815324 J⋅K−1⋅mol−1. However, many different gas constant values exist, depending on the desired units.

Value of RUnits
8.31446261815324J⋅K−1⋅mol−1
8.31446261815324m3⋅Pa⋅K−1⋅mol−1
8.31446261815324kg⋅m2·K−1⋅mol−1s−2
8.31446261815324L⋅Pa⋅K−1⋅mol−1
8.31446261815324L⋅bar⋅K−1⋅mol−1
0.082057366080960L⋅atm⋅K−1⋅mol−1
0.730240507295273atm⋅ft3⋅lb⋅mol−1°R−1
10.731557089016psi⋅ft3⋅⋅lb⋅mol−1°R−1
0.082057366080960L⋅atm⋅K−1⋅mol−1

Why R Is the Gas Constant Symbol

It’s easy to assume the symbol for the gas constant is R to honor French chemist Henri Victor Regnault. After all, Regnault performed the experiments used to determine constant. However, the origins of the symbol are unknown.

Specific Gas Constant

The specific gas constant or individual gas constant also goes by the symbol R, but it depends on the particular gas and its molecular weight. This constant is independent of temperature. In engineering, R is the specific gas constant, while Ru is the universal gas constant:

R = Ru/Mgas

Tables list the values for common gases. The SI unit for the specific gas constant is J/kg K.

References

  • Jensen, William B. (July 2003). “The Universal Gas Constant R“. J. Chem. Educ. 80 (7): 731. doi:10.1021/ed080p731
  • Moran, M; Shapiro, H. N., et al. (2014). Fundamentals of Engineering Thermodynamics (8th ed.). Wiley. ISBN 978-1118412930