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Topic: Carbonic Acid and Salts Lab  (Read 30851 times)

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Offline Borek

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Re:Carbonic Acid and Salts Lab
« Reply #15 on: August 30, 2005, 12:51:47 PM »
Anyone have any other comments???

Me.

I have no idea what Alka Seltzer water is, but I still doubt in the presence of just carbonic acid.

AFAIK carbonic acid is reasonably stable ONLY in pure form. Once there are traces of water present it quickly decomoposes. Note that as one of the products of decomposition is water, the reaction goes faster and faster.

Thus, any solution saturated with CO2 contains only minute amounts of carbonic acid (think soda-water). As long as it is pressurized it can contain some more CO2, but it will loose it once the pressure goes normal (atmospheric).

I don't get it. Either I don't know something, or the experiment - as described - is impossible.
« Last Edit: August 30, 2005, 12:55:19 PM by Borek »
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MilchstrabeStern

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Re:Carbonic Acid and Salts Lab
« Reply #16 on: August 30, 2005, 02:35:55 PM »
Well I do know that the carbonic acid was somewhat pressurized.

Blueshawk, so how do the Ca+2 and Na+ effect the rate of decomposition (right?) of the H2CO3 ?

I wasn't clear wheather or not you explained that, sorry.


Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #17 on: August 30, 2005, 03:34:16 PM »
Ca2+ has higher polarising power than Na+, so Ca2+ is able to distort the electron distribution of carbonic acid to a greater extent than Na+. This means Ca2+ catalyses the breakdown of carbonic acid to carbon dioxide to a greater degree than Na+.

MilchstrabeStern.....geodone post explains it better than I can at the moment.

and alka selzter has aspirin, citric acid, and sodium bicarbonate----> NaHCO3

ADDED:  NaHCO3 is where the CO2 gas comes from.

What I dont understand is that the fastest rate happened in pure dH20.  When it seems from geodome that Ca+2 should have the fastest rate.

But now   i see why NaCl may slow the process.  When the salt is dissolved i water, it adds Na+ to the system.  So when the alka seltzer is added, the Na that dissociates may happen at a slower rate since there is already a concentration of Na in the system.

i will try to find out more for you.
« Last Edit: August 30, 2005, 03:35:48 PM by Blueshawk »

Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #18 on: August 30, 2005, 03:45:24 PM »
I just found out the actual reaction in the lab for the CO2.
 Well almost anyways.

Cirtic Acid + Bicarbonate = CO2 +  H20 + Na(citrate).

So the water dissolves the tablet, since the material do not react in dry form.  Once dissolved the material in the tablet react with each other. :)

ADDED:

HCO3-1 + H+ ------------> H2O(l) + CO2(g)

and CO3-2 + 2 H+ -----------> H2O(l) + CO2(g)

So having excess Na+ in the sysem from the salt should SLOW the reaction.

But how the Ca+2 affects it is unknown to me at this moment.

ADDED:   The salts act as catalysts to the sytems, so in your data

NaCl + H20 should have a faster rate than just H2O
CaCl2 + H2O should have a faster rate than just H2O

Though I haven't figured out exaclty how.....Geodome is probably right w.r.t. the catalysis.

HOPE THIS HELPS
« Last Edit: August 30, 2005, 04:06:47 PM by Blueshawk »

Offline Borek

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Re:Carbonic Acid and Salts Lab
« Reply #19 on: August 30, 2005, 04:14:42 PM »
So the water dissolves the tablet

So is it Alka Seltzer tablet, or Alka Seltzer water? Original procedure called for 23.00 mL of AS. I doubt it was 23.00 mL of tablet :)
« Last Edit: August 30, 2005, 04:16:07 PM by Borek »
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MilchstrabeStern

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Re:Carbonic Acid and Salts Lab
« Reply #20 on: August 30, 2005, 04:33:42 PM »
Nope it wasn't a tablet, based on the lab sheet, the chemical we are dealing with in the Alka Seltzer WATER is Carbonic Acid. No other chemicals are mentioned.

Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #21 on: August 31, 2005, 08:33:31 AM »
When you drop a AS tablet into water it does create what is Carbonic Acid...or at least its conjugate base HCO3-.  If you look back to the reactions I posted...you will notice the HCO3- anion is one of the reacting species for forming CO2.

Also you can create AS water by using water, vineger, and baking soda  :)

But I am still not satisfied with why Na+ and Ca+2 catalysize the reaction...I am still looking for a more complete answer on that.


Offline Borek

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Re:Carbonic Acid and Salts Lab
« Reply #22 on: August 31, 2005, 09:50:59 AM »
But I am still not satisfied with why Na+ and Ca+2 catalysize the reaction...I am still looking for a more complete answer on that.

Ionic strength, solution density and viscosity. These are my bets, although it is still not clear to me what is going on. To create CO2 from HCO3- you have to acidify the solution. It was not acidified (remember, 23.00 mL was added just to NaCl or CaCl2 solution) so there is no reason for CO2 to evolve.
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Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #23 on: August 31, 2005, 12:52:39 PM »
if using AS water,   water that has had a AS dissolved in previous to the start of the lab, then citric acid would be present.

Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #24 on: August 31, 2005, 12:54:33 PM »
Nope it wasn't a tablet, based on the lab sheet, the chemical we are dealing with in the Alka Seltzer WATER is Carbonic Acid. No other chemicals are mentioned.


So how exaclty was your AS water prepared?  Borek and I are still trying to figure out the roles of Ca and Na.

MilchstrabeStern

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Re:Carbonic Acid and Salts Lab
« Reply #25 on: August 31, 2005, 02:03:22 PM »
I believe it was store bought in small glass bottles. Not sure what brand though.

Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #26 on: August 31, 2005, 02:05:39 PM »
then it is most likely carbonated water.  So all the lab manual said was carbonic acid as the main chemical, huh.....are sure there wasn't anything else??

MilchstrabeStern

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Re:Carbonic Acid and Salts Lab
« Reply #27 on: August 31, 2005, 02:27:28 PM »
As far as I am aware, no, I did look over the lab sheet several times and the only chemical mentioned is Carbonic Acid. I can check again but I believe we're only dealing with the Ions interacting with the carbonic acid.

Blueshawk

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Re:Carbonic Acid and Salts Lab
« Reply #28 on: August 31, 2005, 03:15:52 PM »
I was just reading something..

Are you sure that NaCl and CaCl2 are supposed to act as catalysts???

because

CaCl2 +  H2CO3 ->  CaCO3(ppt) + 2HCl(aq)    Eq 1
and CaCO3 is a precipitate...which would reduce the amount of CO3-2 that is available for decomposition into CO2

Or   Ca+2 + CO3-2 -> CaCO3 (s).    Eq 2

So   CO3-2(aq) +  H2O(l) ->  CO2(g) + 2OH-(aq).   Eq 3

Therefore if CO3-2 reacts with the Ca+2 in Eq 2, there will be less available CO3-2 for Eq 3.

What I am not sure of are the reactioin conditions, ie is the solution for Eq 1 in water..or (aq).?

So from here I conclude as follows...though this may not be right..that is why i like forums, someone will correct me if I am wrong

Reaction rates with solutions

Fastest  dH2O
fast      NaCl
slow    CaCl2


Borek,  any ideas ?
« Last Edit: August 31, 2005, 03:47:20 PM by Blueshawk »

MilchstrabeStern

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Re:Carbonic Acid and Salts Lab
« Reply #29 on: August 31, 2005, 08:59:27 PM »
Well that makes sense to me blueshawk. Unless the reaction is different based on certain circumstances. But you might be right. Her data shows what you just said.

H20
NaCl
CaC2

that's the order from fastest rate of decomp. to slowest.

That is just from their data recordings tho, they did 3 trials on each tho so it should be fairly accurate.

I'll just wait for someone to confirum that lol.

Thanks guys!

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