Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: AussieKenDoll on December 19, 2018, 12:03:38 PM
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what are the units of each lnK on △G0 = - RTlnK formula?
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The equilibrium constant is unitless (dimensionless). In many practical applications, equilibrium constants are expressed in terms of molar concentrations, and so it often seems like the equilibrium constant should be some kind of unit related to concentration, and should depend on the reaction. In actuality, equilibrium constants are strictly formulated as a function of thermodynamic activities, which are dimensionless by definition. We use molar concentrations because in many situations it is simple and accurate to do so. I.e., for dilute solutions, the standard state is usually defined as a solution with concentration = 1 M, and the activity is proportional to the actual concentration (in M) divided by the concentration of the standard state. This works well in practice for ideal solutions, but in cases where solutions are non-ideal (say, high ionic strength solutions), the relative activity deviates significantly from the true molar concentration. Note in these instances the activity is still unitless, and therefore so are the equilibrium constants.
On inspection this should be evident because RT has a unit of energy (per mol), as does ΔG, so ln K must be unitless to satisfy a dimensional analysis.
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The formula depend on a special condition. When the system is balanced, the formula can be true.
ΔrGm=ΔrGmΘ+RTlnQ
when balanced ΔrGm=0 and Q=KΘso you can draw a conclusion like that.