helen-louise (baratron) wrote,

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chem geek moments of the week

I love being a geek.

The other day I was walking down the road when I saw a van belonging to a cryogenic company. It had Hazchem plates saying it was transporting a non-hazardous non-flammable gas, and on the front was a sign in simple English saying "Delivering liquid carbon dioxide". I instantly went into chemistry geek mode, saying "well, I know carbon dioxide usually sublimes, so what sort of pressure must that carbon dioxide be under to be liquid?". And it's been bothering me for days, until just now when I had time to Google it and find out that the triple point of CO2 is 5.2 atm. So, not actually all that much pressure.

Then lately I have been busy trying to solve a mystery relating to sulphuric acid. The Brønsted-Lowry theory of acids and bases refers to an acid as something which can donate an H+ ion (generally in chemistry referred to as "a proton" - although physicists will eeek at the thought of protons wandering around loose by themselves, which is further proof of why physicists and chemists can't talk to each other despite ~33% of their subjects being exactly the same material). Sulphuric acid is H2SO4, so it contains 2 hydrogens which can be lost as H+ (it's called a diprotic acid). So far, so good.

Now, the concentration of H+ in solution is useful for a lot of things, such as determining the strength of an acid or base, and measuring its pH. A strong acid is one which is fully split into ions in solution (compared to a weak acid where only a small proportion, e.g. 1/10000 of the molecules are split into ions). Therefore, the concentration of H+ (abbreviated as [H+]) for a strong acid is just the same as the concentration of the acid.

H2SO4 is a strong acid. Therefore, a lot of textbooks and web sites state that the concentration of H+ in sulphuric acid should be 2 x the concentration of the acid, because it's diprotic. I am wondering if this is actually true. Obviously, H2SO4 is a strong acid so the first dissociation is 100%:

H2SO4 (aq) --> H+ (aq) + HSO4- (aq)

but then HSO4- is a weak acid, and only partially dissociates:
HSO4- (aq) --> H+ (aq) + SO42- (aq) (reversible)

The acid dissociation constant, Ka, for HSO4- is, btw, 0.012.

So I think the [H+] for a 1 mol dm-3 solution of sulphuric acid should be 1.012 mol dm-3 : 1 mol dm-3 from the first dissociation, then 0.012 mol dm-3 from the second dissociation. This is a huge difference from the various books which claim that [H+] for a 1 mol dm-3 solution of sulphuric acid should be 2 mol dm-3. And I have no idea which of us are right.

I should write to the Royal Society of Chemistry about it for a definitive answer (says the person with MRSC after her name - if that doesn't qualify me to know what I'm talking about I don't know what does!).
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