For standard conditions, is the activity of #"1 M"# arbitrary?

1 Answer
Truong-Son N.
Jan 9, 2018

The activity does not have to be #"1 M"#. For dissolved components, it could be any concentration (within reason). However, we as chemists have chosen for standard conditions to have activities of #"1 M"#.

This may have been introduced to you back in general chemistry, when you talked about standard conditions in thermodynamics, or even in electrochemistry.

For example, recall:

#DeltaG = DeltaG^@ + RTlnQ#

where #DeltaG# is the change in Gibbs' free energy, #@# indicates some standard state (usually #25^@ "C"# and #"1 bar"#), #R# is the universal gas constant, #T# is temperature in #"K"#, and #Q# is the reaction quotient.

For a nonequilibrium process, (that is, #Q ne K#), if the concentrations are all #"1 M"#, then #Q = 1#, and #RTlnQ = 0#.

Then it follows that

#DeltaG = DeltaG^@#,

i.e. we are at concentrations that correspond to a change in Gibbs' free energy at standard conditions.

(If we ARE at equilibrium, then #DeltaG = 0 = DeltaG^@#, and we would then be in equilibrium at standard conditions.)

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