chem geek moments of the week
Oct. 19th, 2004 01:34 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
baratronI 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!).
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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Re: AFAIR...
Date: 2004-10-21 05:42 pm (UTC)Because that's what molarity means: it's a measure for the amount of molecules or atoms in volume. It contains 2 mol/dm-3 of H+ in both associated -and- disassociated state.
Isn't PH a measure for the concentration of only the disassociated H+?
Anyway, 2 mol/dm-3 H2SO4- (aq) is '2 mol/dm-3 sulfuric acid' which -has- 1.012 mol/dm-3 H+ (aq)
I guess you are right and the books are wrong when they say [H+] is 2 mol/dm-3
Re: AFAIR...
Date: 2004-10-22 05:53 pm (UTC)I think you're getting confused because you're half-remembering those horrible calculations where you have already mixed together an acid and a base and you're being asked to determine the concentration of each species and/or the pH of the mixture, but I'm not talking about that. I'm talking about the concentration of acid in a bottle of sulphuric acid that contains only sulphuric acid (and water) :)
Also, saying
It contains 2 mol/dm-3 of H+ in both associated -and- disassociated state.
is, er, rubbish (sorry), because the H+ only exists when the acid dissociates. Until then, all you can be certain of is that you have 2 mol dm^-3 (remember x^-y = x/y - mol per dm^-3 would be moles X dm^+3! (argh, I really need superscripts)) of HYDROGEN. 2 moles of H is not the same as 2 moles of H+ (aq)/H3O+ ion. I mean, you could have 2 moles of H existing as part of OH-! Why not?
Re: AFAIR...
Date: 2004-10-25 07:56 am (UTC)This come from me remembering that a '1 mol/dm3' afair means '1 molecular equivalent per cubic decimeter'. You do agree with me, that if you have a liter of sulfuric acid solution containing 1 mol of sulfuric acid, you need 2 liters of NaOH solution 1 mol/dm3 or 1 liter of NaOH solution 2 mol/dm3 to neutralize it?
It contains 2 mol/dm3 of H+ in both associated -and- disassociated state.
Maybe I should have said 'It contains acid in both associated -and- disassociated state so you need of the same amount of volume of 2 mol/dm3 NaOH solution to neutralize it'
Let's take it from this way: you have an acid solution, you have no idea which acid it is, but you want to determine it's concentration and all you have is 1 molar NaOH solution. So, you try a titration. After some fiddling to find out the correct indicator you realize you need one that changes color around PH 7. So you do the titration with this, and measure: for 10 ml of acid, I need 20 ml of this NaOH solution to neutralize the acid. Ergo, the original acid contains 2 mol/dm3. So, how much H2SO4 do you need to put into 1 dm3 to make this acid? 1 mol, right?
Please note I already agree with you that books claiming [H+/SUP>] for 1 molar sulfuric acid is 2 mol/dm3 are wrong. We are currently quibbling over my wording of my first reply ;-)