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Acid !

We all know that a puddle of water will slowly evaporate, the spill of water on the floor will become smaller and then disappear. I was working recently with some concentrated sulphuric acid and I recalled that sulphuric acid has a very high affinity for water. I have always thought that the classic mineral acids can be defined in the following table. 

Acid name Acid formula True character
     
Nitric acid HNO3 Oxidant
Sulphuric acid H2SO4 Dehydration agent
Phosphoric acid H3PO4 Dehydration agent
Hydrochloric acid HCl Source of chloride anions
Perchloric acid HClO4 Kinetically inert oxidant (The sleeping dragon)

Note that the addition of a little nitrous acid to nitric acid will convert the oxidant to the rampant oxidant. I know that nitric acid without a catalyst can be slow to react at first. For instance the oxidation of cyclohexanones with nitric acid always needs a little V2O5 to get it going. Once it gets going it is a very vigorous reaction.

But lets get back to sulphuric acid, now I know that you all like to see those dramatic chemical reactions. The jets of flame, reactions which belch sparks and thunderous explosions etc etc etc. Here is one which is rather less dramatic, but it still teaches an important point.

Take a beaker, weigh it and pour in a little concentrated sulphuric acid. Weight it again, leave it to stand and keep weighing it now and then. What you will soon discover is that the mass (weight) of the sulphuric acid will increase with time. Here is a graph of the results of the experiment. I will add the graph soon

This proves that sulphuric acid absorbs something from the air. We now need to think at length about sulphuric acid. It is a strong dehydration agent which is able to react with water, it also happens to be an acid. It is important that we understand that chemicals do not just react in one way.

Nitric acid for example can act as a base. When nitric and sulphuric acids are mixed the sulphuric acid protonates the nitric acid.

H2SO4 + HNO3 goes to  HSO4 + H2NO3+

The protonated nitric acid then loses water, the concentrated sulphuric acid helps with this by combining with the water to give us the NO2 cation which is responsible for most nitration reactions.

H2NO3+ goes to  H2O + NO2+

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