Why does pKa affect equilibrium?

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Answer 1 · 2018-07-18T06:19:15

Because #pK_a# is a MEASUREMENT of the equilibrium?

For the generalized reaction.....

#A+BrightleftharpoonsC+D#

Now there is a rate forward....

#"Rate forward"=k_f[A][B]#, and a #"rate backward"=k_r[C][D]#.

And equilibrium does NOT specify cessation of chemical change; it does specify EQUALITY of FORWARD or REVERSE rates.

And so.......

#"Rate forward"=k_f[A][B]-="rate backward"=k_r[C][D]#.

And thus the quotient #([C][D])/([A][B])=k_r/k_f=K_"eq"#

And #K_"eq"# is otherwise known as the thermodynamic equilibrium constant.

#pK_a# or #pK_"eq"# is simply the logarithm of #K_a#, #K_"eq"#..

#pK_a=-log_10K_a#