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Name: Isabelle
Status: other
Grade: other
Country: United Kingdom
Date: Summer 2011


Question:
The revision guide says that acid strength is "the proportion of the acid molecules of the acid molecules that ionize in water", which I took to mean "a measure of how many of the acid molecules will actually ionize and form H+ ions"; but on the next page in the revision guide it says that ALL of the molecules (in strong AND weak acids) will release hydrogen atoms that will form H+ ions and "since the total number of H+ ions available is the same, the amount of product will be the same" when the same volume and same concentration of strong and weak acids are used. So I was wondering whether acid strength does actually mean 'the proportion of molecules which will ionize; or whether it means that the proportion of molecules that ionize is the same in strong AND weak acids, and it just takes longer for weak acid molecules to ionize.

Replies:
Isabelle,

I am not sure of the context of the second statement in your revision guide. Perhaps it is in the context of an acid-base reaction? Let me try to clarify:

1) All strong acids and bases completely dissociate in water. As such strong acids and bases are defined by the fact that if it is dissolved in water, than it must be completely ionized.

2) Weak acids and bases do not completely dissociate in water. The molecules of acids and bases form an equilibrium with their ions. Thus, only some molecules will form ions, the rest will be in molecular form. Thus, the distinction between strong versus weak acids and bases is whether they are fully dissociated (strong) or form an equilibrium between the molecular and ionic forms (weak).

3) Since weak acids/bases form equilibria with their ions, any substance reacting with those ions will drive the equilibrium to form more ions (Le Chatelier Principle). Thus, it is possible for a base to react with all the H(+) of a weak acid because as the H(+) become consumed by the base, more of the H(+) will be formed - until all the weak base is consumed. In this sense, all the H(+) of a weak acid is available for reaction - however, for any given weak acid aqueous solution, the H will be in an equilibrium of HA and H(+) and A(-).

4) Do not confuse acid strength with acidity. A very poorly soluble HA compound may not be readily soluble in water, but if whatever is dissolved is completely ionized - then by definition it is a strong acid. Same with bases. For example Mg(OH)2 is only slightly soluble in water, but whatever portion is dissolved is completely ionized/dissociated, so it is classified as a strong base even though the pH (basicity) of the solution formed by Mg(OH)2 may be very close to 7. So, again, the difference between strong and weak is not how strongly acidic or basic the solution is, but rather whether the compound is fully dissociated or in equilibrium with its ions.

Greg (Roberto Gregorius)
Canisius College


Isabelle,

The definition of strong or weak acid as "the proportion of the acid molecules that ionise in water" is essentially correct and this deals with acid molecules dissolved in water, with no other base present (the water acts as a base by accepting H+ from the acid molecule when it ionises - forming H3O+). If, however, you have a stronger base than water present (such as hydroxide ions from NaOH etc.) this will react with the H3O+ ions in a neutralization reaction.

In the case of a strong acid, pretty much every acid molecule will have already formed an H3O+ ion and these will directly react with the base (giving a fast reaction). For a weak acid the undissociated acid molecules are in equilibrium with a few H3O+ ions, these react with the base and are replaced by the equilibrium. As new H3O+ ions are produced by the equilibrium they also react with the base and the store of undissociated acid is used up. This takes longer than the reaction of the strong acid so weak acids tend to react less vigorously with bases than strong.

The key point is, though, that a mole of monoprotic strong acid will react with the same amount of base as a mole of monoprotic weak acid, as both produce a mole of H3O+ ions. The difference is that the strong acid produces the H3O+ all at one time, but the weak acid has to have its equilibrium progressively drawn over to produce all of its H3O+ ions.

Best wishes,
Tom Collins


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