Common reactions between acids and bases are many and numerous, but they include;
As with all acid/base reactions a proton (hydrogen with no electrons) is being transferred from one molecule to another. Most of the time this will result in the decomposition of water from the molecule, but sometimes the new molecules will be stable - as shown in the second example. We can predict the likelyhood and speed of a reaction by using pka values.
Also note that every acid has a base that is itself with one proton less, this is konown as an acid's conjugate base. The same is true for bases, that is they have conjugate acids.
For reference a table of pka values can be found here
A more basic molecule will always react with a more acidic one. This means that a molecule with a high pka will always react with one of a lower pka. Taking the examples above we can see that hydrochloric acid has a pka of about -7 and sodium hydroxide has a pka of about 15 (due to the OH group) and so the reaction is allowed. Looking at the second example we see that a carboxylic acid (pka 5) can react with ammonia (pka 10) and also that sulphuric acid (pka -3 for loss of the first proton and 2 for the second proton) can react with sodium hydroxide (pka 15).
It is useful to note that when pka values are quoted they are approxiamate due to the exact value being dependant on concentration, temperature (all values quoted here are true at 25oC) and solvent.
Indicators are a useful way of 'seeing' the end of reactions withour having to do a titration, however some knowledge of the pH at the equivalence point is useful. Most indicators are weak acids of the general type Hln in which the conjugate base ln- has a different colour to the acid, Hln. Obviously they have a use in titartions for easily locating they equivalence points. A table of selected indicators is below.
| Indicator | pka | Colour of acid | Colour of base | pH range |
|---|---|---|---|---|
| Phenolphthalein | 9.50 | Colourless | Pink | 8.0-10.0 |
| Phenol red | 8.00 | Yellow | Red | 9.2-8.2 |
| Bromocresol purple | 6.40 | Yellow | Purple | 5.2-6.8 |
| Bromocresol green | 4.90 | Yellow | Blue | 3.8-5.4 |
| Methyl orange | 3.46 | Red | Yellow | 3.2-4.4 |
| Thymol blue | 1.65 | Red | Yellow | 1.2-2.8 |
| Thymol blue | 9.20 | Yellow | Blue | 8.0-9.8 |
Taken from Page 474, Chemistry (2nd ed.), Housecroft and Constable, Prentice Hall 2002
This is based on the general acid equilibrium dicussed earlier. For a discussion on how indicactors work please see page 159, Elements of Physical Chemistry (3rd ed.), Atkins, Oxford University Press 2001 or any other physical chemistry book that discusses Le Chatelier's principle
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