Acid-base equilibria
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Is calcium hydroxide a strong or a weak base?
What is the definition of a weak base? Is calcium hydroxide a weak base because it is sparingly soluble?
When it came into the site this question led to an extended discussion within the tutor team.
One of our tutor team replied:
Think of weak acids: only a very small proportion of their molecules dissociate to give H+ ions ( or, more correctly, donate protons to water molecules to produce H3O+ ions). Weak bases mirror this behaviour: their molecules have only a very slight tendency to accept protons from water molecules and produce OH– ions. Ammonia, NH3, is a typical weak base.
Calcium hydroxide is an interesting example. Yes, I think I would regard it as a weak base because its low solubility means it produces a low concentration of OH– ions in solution.
We then got this follow-up question
I can see that your definition of a weak base would apply well to ammonia i.e. ''their molecules have only a very slight tendency to accept protons from water molecules and produce OH- ions''. But this wouldn't apply to calcium hydroxide would it? Is there an 'all-inclusive' definition of a weak base?
Our tutor responded:
Part of the difficulty lies in which definition of base you use. The Arrhenius theory defines bases as substances which produce OH- ions in aqueous solution; the greater the OH- concentration, the stronger the base. Since calcium hydroxide can only give a fairly low OH- concentration, by virtue of its low solubility, it would be classed as a weak base according to this theory.
The Bronsted-Lowry theory defines bases as proton acceptors. Calcium hydroxide is not itself a proton acceptor; the proton acceptor is the actual hydroxide ion, present in both solid and aqueous solution, which is the strongest base of all: it will accept protons from any species capable of transferring them. This may illustrate a problem with definitions in general: they may be useful statements, summarising a lot of information, but they cannot always be applied to every situation. So, no, there isn't an all-inclusive definition of a weak base.
We discussed this in the team and gained views on this question:
(1) I think I might have gone about it rather differently by homing in on the B-L theory a bit earlier which would have by-passed the need for the supplementary question. Even solid calcium hydroxide contains hydroxide ions which are strongly basic whether they are in solution or not so, personally, I'd have stressed the strong base line and I would say that, if we ignore Arrhenius altogether (as I think we should), there is a universal definition - it is the one which the B-L theory gives us.
(2) Calcium hydroxide is an awkward base to classify, since with this compound there is an obvious contradiction between the Arrhenius and Bronsted-Lowry definitions. Since the latter superseded the former, I would have (conveniently!) by passed the relatively inadequate Arrhenius theory, and used the Bronsted-Lowry theory to classify calcium hydroxide as a strong base on the basis that it already possesses hydroxide ion content, whether in the solid or aqueous form. When calcium hydroxide reacts as a base it doesn't have to create any fresh hydroxide ions (as ammonia needs to) - it simply has to release them either from solution or to dislodge them from the solid state
In conclusion:
When considering ionic salts it is better to think of them being made up of two distinct species – the cation and the anion. So it is not calcium hydroxide that is a base but the hydroxide in the calcium hydroxide. The hydroxide ion is certainly a strong base. In a saturated solution of calcium hydroxide (lime water) you have a dilute solution of a strong base.
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updated: 22 August 2003
