Why conjugate base of strong acid is weak

Unless you have been given a list of acids to memorise as weak, very weak etc, these terms are quite arbitrary, and should only be used to compare the strength of one acid with another. The strength of a weak acid is best described using the value of its acid dissociation constant acid ionisation constant , K a or pK a. It is far more important that you understand that the strength of an acid or base and its conjugate base or acid is based on the equilibrium position for the dissociation ionisation reaction.

IUPAC prefers the term "hydron" rather than "proton" for the positively charged hydrogen ion. We use the term proton because most of the naturally occurring hydrogen is hydrogen-1, 1 H. When this isotope of hydrogen loses its electron, what is left is just a proton.

That makes it a weak Lewis base. A strong Bronsted-Lowry acid would strongly want to donate a proton to something else. A bond that is easily broken is hard to form again. So, the conjugate Bronsted-Lowry base that forms weakly wants to accept a proton. That makes it a weak Bronsted-Lowry base. Why does a stronger acid have a weaker conjugate base? This table enables us to see how readily a given acid will react with a given base.

The reactions with most tendency to occur are between the strong acids in the top left-hand comer of the table and the strong bases in the bottom right-hand comer. If a line is drawn from acid to base for such a reaction, it will have a downhill slope.

By contrast, reactions with little or no tendency to occur between the weak acids at the bottom left and the weak bases at the top right correspond to a line from acid to base with an uphill slope. When the slope of the line is not far from horizontal, the conjugate pairs are not very different in strength, and the reaction goes only part way to completion.

Thus, for example, if the acid HF is compared with the base CH 3 COO — , we expect the reaction to go part way to completion since the line is barely downhill. A strong acid like HCl donates its proton so readily that there is essentially no tendency for the conjugate base Cl — to reaccept a proton.

Consequently, Cl — is a very weak base. A strong base like the H — ion accepts a proton and holds it so firmly that there is no tendency for the conjugate acid H 2 to donate a proton.

That is the reaction of hydrochloric acid with water. Note the one-way arrow; the reaction is a one way street. In other words, this is the generalized reaction:. Therefore, we conclude that if one has a strong acid, then its conjugate base must be weak. The converse is also true; if an acid's conjugate base is weak, then the acid must be strong. Strong acids readily give up their protons, because the resulting conjugate base becomes more stable with respect to its new electron distribution.

An example of this would be the acid halides with the exception of HF. The conjugate bases of these acids achieve their octets and diffuse polarizability, rendering them electronically stable. Let us solve the mystery of relative 'strength' and 'weakness' of the acids and bases logically by understanding what makes it strong or weak. This energy requirement is large because the molecule attract the hydrogen atom more efficiently.

When Cl is a conjugate base in the product's side, the Cl has the same low efficiency of attracting H atom. Then it will become a strong base because of its efficiency of attracting H atoms. Sign up to join this community. The best answers are voted up and rise to the top.