Romain - 16-1-2014 at 22:53
Hi,
I have a chemistry exam next week, on Tuesday and I'm having trouble finding the answer one of the exercises our teacher gave us. I asked him and he
told me to "reflect" and "find the answer"... He's not exactly the nicest teacher I've ever had.
Anyway here's the exercise, I tried to translate it because it was in french originally:
By carefully looking at the number and the type of chemical bonds to break, determine which of the following reactions should happen the fastest. (at
ambiant temperature).
1) C(s) + O2(g) -> CO2(g)
2)Fe(s) + 2H+(aq) -> Fe2+(aq) + H2(g)
3)Ca2+(aq) + SO42+(aq) -> CaSO4(s)
4)C8H18(l) + 25 1/2O2(g) -> 8CO2(g) + 9H2O(g)
Here are my guesses though I'd like yours too!
1) It's a synthesis from carbon and oxygen, I should not be very fast but not too slow either.
2) It's the oxydation of iron to iron ion in the 2+ oxydation state. I have no idea how fast it goes. I need your help for this one.
3)It's a crystallization from calcium and sulfate ions into calcium sulfate crystals. Moderatly fast.
4) It's a combustion of an alkane, very fast reaction. I'd say this reaction is the fastest.
Thanks in advance for your help,
R.
[Edited on 17-1-2014 by Romain]
DraconicAcid - 16-1-2014 at 23:01
Notice the phrase "at ambient temperature". Have you ever seen a combustion occur without being heated to an ignition point well above room
temperature? (Yes, I know certain pyrophoric materials do, but the ones in these examples aren't them.)
As for the other two, have you ever done a precipitation reaction, or dissolved a metal in acid?
[Edited on 17-1-2014 by DraconicAcid]
Romain - 17-1-2014 at 08:05
I guess he meant implicitly that the activation energy is provided to start the combustion, otherwise nothing happens...
I've seen iron dissolved in HCl and now that I think of it, it takes a long time...
I precipitated some PbI2 once, it goes quite quickly...
So the order from fastest to slowest would be 4, 1, 3, 2 but he only asks for the fastest reaction -> the fourth one.
Is it correct?
DraconicAcid - 17-1-2014 at 08:26
I wouldn't have assumed that the activation energy is provided, or it won't be at ambient temperature anymore.
ScienceSquirrel - 17-1-2014 at 09:12
No 3 will be the fastest by far at ambient temperatures, assuming that no initiation is provided.
It is a simple ionic reaction in aqueous solution to give a precipitate.
Romain - 17-1-2014 at 14:53
Ok then If no further indication is given about activation energy being provided, I'll answer the precipitation reaction. Else the combustion.