Strong Acids

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Why does a stronger acid have a lower pka value as opposed to a weaker acid?

[Myka G 1l]

A stronger acid has a lower pka value opposed to a weaker acid because a lower pKa equals a higher Ka value. A higher Ka value means the acid dissociates more readily because it has a larger concentration of Hydronium ions.

[Rory Simpson 2F]

If you look at the equation for chemical equilibrium, a larger Ka value would result if there is a greater amount of product than reactant at equilibrium (and so there would be a lower pKa value because pKa is -log Ka). Since strong acids almost entirely dissociate, there would be a much greater concentration of products than reactants.

[Shail Avasthi 2C]

The Ka is equal to the product of the concentrations of the products divided by the product of the concentrations of the reactants.

e.g.: HA ---> H+ + A- Ka = [H+][A-]/[HA]

Stronger acids more readily dissociate into their products. Therefore, the stronger the acid, the higher the Ka because the concentrations of the products (i.e. the numerator of the Ka) are much greater.

The pKa is lower for stronger acids because when you take the -log of a bigger number, you get a smaller number.

(e.g. if x > y, -log(x) < -log(y))