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In biochemistry, when do rate constant units matter in calculating free energy changes?

up vote 14 down vote favorite

In my experience, it is always been the ratio of products to reactants that mattered, so as long as your units are consistent; what they are doesn't matter. However, an in-class problem today required that the units be changed to molar (M) from millimolar (mM). Can you summarize when/why this matters in some cases, but not in others?

2 Answers

up vote 4 down vote accept

$K$ should be a unitless value. Otherwise, you'd get a nonsensical unit when you try to take the log of the unit. Each component of the product $K$ is usually an activity, which is frequently similar in value to the molar concentration, or a fugacity, which is frequently similar in value to the partial pressure of a gas.

up vote 3 down vote

If the equilibrium constant is defined as a ratio of quantities having units (concentrations, partial pressures, molalities), then unless the sum of the exponents in the numerator happens to equal the sum of the exponents in the denominator, the units matter.


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In biochemistry, when do rate constant units matter in calculating free energy changes?

up vote 14 down vote

In my experience, it is always been the ratio of products to reactants that mattered, so as long as your units are consistent; what they are doesn't matter. However, an in-class problem today required that the units be changed to molar (M) from millimolar (mM). Can you summarize when/why this matters in some cases, but not in others?


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up vote 9 down vote

$K$ should be a unitless value. Otherwise, you'd get a nonsensical unit when you try to take the log of the unit. Each component of the product $K$ is usually an activity, which is frequently similar in value to the molar concentration, or a fugacity, which is frequently similar in value to the partial pressure of a gas.

edit

IUPAC permits the "equilibrium constant" to have units goldbook.iupac.org/html/E/E02177.html , but not the "standard equilibrium constant" goldbook.iupac.org/html/S/S05915.html - DavePhD Jan 17 at 14:39

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