PKa: Difference between revisions
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<b> K<sub>eq</sub> = <math> \frac{[A^-][H3O^+]}{[HA][H2O]}</math></b> | <b> K<sub>eq</sub> = <math> \frac{[A^-][H3O^+]}{[HA][H2O]}</math></b> | ||
In the special case of dilute acids, the concentration of water is very large and essentially constant, allowing for the definition of the [[acidity constant]], [[acidity constant|K<sub>a</sub>]], as follows | In the special case of dilute acids, the [[concentration]] of water is very large and essentially constant, allowing for the definition of the [[acidity constant]], [[acidity constant|K<sub>a</sub>]], as follows | ||
<b> K<sub>a</sub> = <math> \frac{[A^-][H3O^+]}{[HA]}</math></b> | <b> K<sub>a</sub> = <math> \frac{[A^-][H3O^+]}{[HA]}</math></b> | ||
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Because the range of values of K<sub>a</sub> range from 10<sup>14</sup> to 10<sup>-50</sup>, it is more convenient to express such extreme values using a logrhythmic scale. Thus, just like pH is defined as the negative log of the hydronium concentration (pH = -log [H3O]+), one defined pKa in a similar fashion. | Because the range of values of K<sub>a</sub> range from 10<sup>14</sup> to 10<sup>-50</sup>, it is more convenient to express such extreme values using a logrhythmic scale. Thus, just like pH is defined as the negative log of the hydronium concentration (pH = -log [H3O]+), one defined pKa in a similar fashion. | ||
pK<sub>a</sub> = -log K<sub>a</sub> | pK<sub>a</sub> = -log K<sub>a</sub>[[Category:Suggestion Bot Tag]] | ||
Latest revision as of 16:00, 30 September 2024
pKa is a measure of acid strength, with smaller numbers indicating stronger acids. Very strong acids have negative values for pKa, while weaker organic acids have values of about 5. Extremely weak acids, like alkanes, have pKa values as high as 50.
Definition
When an acid, AH is added to water, the acid becomes depronated to become the acid's conjugate base, A-, and a hydronium ion, H3O+, is formed. The equilibrium constant, Keq, for this reaction can be expressed as
Keq =
![{\displaystyle {\frac {[A^{-}][H3O^{+}]}{[HA][H2O]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/855c566e14a7bc888ae13f8bf1652ec5f53a3702)
In the special case of dilute acids, the concentration of water is very large and essentially constant, allowing for the definition of the acidity constant, Ka, as follows
Ka =
![{\displaystyle {\frac {[A^{-}][H3O^{+}]}{[HA]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8038421a23099030ffacdc3c9044cfe2b14cd5c7)
Because the range of values of Ka range from 1014 to 10-50, it is more convenient to express such extreme values using a logrhythmic scale. Thus, just like pH is defined as the negative log of the hydronium concentration (pH = -log [H3O]+), one defined pKa in a similar fashion.
pKa = -log Ka