Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: MilchstrabeStern on August 29, 2005, 02:24:28 PM
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Hello,
I believe this is a common lab in a general chemistry course.
I am trying to help a friend with her lab, and although I am pretty good with chemistry, I am only in my first year of college, and all my memory is from my High School AP class. I am a Chemical Engineering major though.
The lab procedure is to take 1 Molar solutions(so the salts are first dissolved in distilled water) of CaCl2 and NaCl.
You then take 2 ml of ONE of these and place it in a graduated cylinder.
The next step is to add 23 ml of Carbonic Acid (also known as Alka Seltzer or H2CO3).
Then you place a stopper in, and flip the graduated cylinder over and measure the rate at which the gas is created.
You do this for both 1 molar solutions of the salts, and once with distilled water, so you have a total of 3 test.
My question is, what is going on here? What is the Carbonic Acid reacting with to release the gas (CO2?). How do the salts effect the rate of this decomposition, or release and gas, and why?
Any help would be Greatly appreciated!
Thanks Guys!
-Bryan
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It may be that you are creating Cl gas. both salts of Cl, so it may be that that is what is going on.
I am not 100% sure, I am not familiar with this lab.
2NaCl(aq) + H2CO3(aq) -> 2Na+(aq) + HCO3-(aq) + Cl2(g) + H3O+ ??
CaCl2(aq) + H2CO3(aq) -> Ca+2(aq) + HCO3-(aq) + Cl2(g) + H3O+ ??
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The next step is to add 23 ml of Carbonic Acid (also known as Alka Seltzer or H2CO3).
Something is wrong here. Alka Seltzer is not a carbonic acid. Also it will be very difficult to add a carbonic acid solution, as carbonic acid is unstable and decomposes by itself. You may add water saturated with CO2, but then it will be pretty difficult to know what was the CO2 concentration.
Are you sure about the procedure and reagents used?
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I am not 100% sure, I am not familiar with this lab.
You are completely, awfully wrong.
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It may be that you are creating Cl gas. both salts of Cl, so it may be that that is what is going on.
I am not 100% sure, I am not familiar with this lab.
2NaCl(aq) + H2CO3(aq) -> 2Na+(aq) + HCO3-(aq) + Cl2(g) + H3O+ ??
CaCl2(aq) + H2CO3(aq) -> Ca+2(aq) + HCO3-(aq) + Cl2(g) + H3O+ ??
it's just a replacement reaction resulting into Na2CO3 and HCl(aq) in my opinion.
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You are completely, awfully wrong.
Thanks for letting me know the truth..I like honesty....And I figured I was wrong...But it was fun typing it :)
From what was originally there..I wasnt sure how CO2 would be produced..so i guessed Cl gas
But thanks again...HATs off to Borek !
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Well I thought it was Carbonic Acid,but maybe not. I do know that it is Alka Seltzer water though.
I do know that a gas is being created (it's certianly not chlorine gas, that might be a little dangerous lol). I thought it was CO2 guys, but if I am wrong I'm sorry. I don't have the lab sheet, it's my friends.
I do know that the salts effect the rate that the gas is produced tho.
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Citric acid (C6H8O7) and sodium bicarbonate (NaHCO3) is what i found to be the main components of alka seltzer TABLETS, not sure if this is the same for alka seltzer water tho.
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What education level is this lab for...?
I did not think of it at first..but Cl gas is a little dangerous for beginners.
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I dont think Cl gas is being produced, and this is only a second semester college general chemistry course.
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No Cl is most likely not produced..i did a little browsing to see how easliy it can be made.
Most likely CO2...looking at what you have for the components of alka seltzer,
though I am not sure on the exact mechanism.
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Okay guys i've confirmed what we're dealing with with the Alka Seltzer WATER is Carbonic Acid (H2CO3) which decomposes into H20 and C02.
How does the Salt affect this decomposition?
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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+.
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Okay so you have Na+ Cl-
and you have Ca +2 and 2 Cl -
So NaCl and CaCl2 are polar molecules?
Also their data said that adding Carbonic Acid to distilled water had the fastest reaction rate, how does that work? Bad data?
**Edit**
or wait, when you make the NaCl and CaCl2 solutions, are the chlorines being sepearted and what is the Na and Ca bonding to?
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Well when NaCl is placed in water it dissociates to Na+ and Cl-.
What happens then is the oxygen in the water molecules are attracted to, but does not bond to, the Na+.
Since the O in water is partially negative.
With Cl-, the H's in water surround the Cl-, since the H's have a partial positive.
So if it is indeed the effects that the Na+ and Ca+2 that affect the rate, then it could be seen that the Na+ and Ca+2 have a harder time effecting the rate since they are hindered my the attraction of the water molecules.
And water does not affect the rate as much with the salt in solution since it's partially positive H's are hindered by the attraction to Cl-.
So for distilled water, it may be that the partial positives on the H's have a similar affect as the Na+ and Ca+2, but more effective ???
Anyone have any other comments???
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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.
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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.
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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.
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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
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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 :)
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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.
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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.
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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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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.
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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.
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I believe it was store bought in small glass bottles. Not sure what brand though.
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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??
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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.
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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 ?
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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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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+.
oops. i forgot to consider CaCO3 is insoluble. the precipitation of carbonate means there is a lower aq carbonate concentration consequently, so reaction rate is slowest for CaCl2.
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The reason that NaCl may slow the reaction compared to dH2O
is that as you add NaCl to water it adds molecules to the solution.
Therefore there are more "particles" in the solution for the CO3-2 anion to collide with.
Thus if it collides with the Na and Cl, though not resulting in anything...it will reduce the number or frequency of collision with water molecules, therefore resulting in a decrease in the rate of the reaction.
Though the effect on the rate should not have a substantial difference compare
to the rate of dH20, it should still slow the reaction to be noticable enough for measurement.
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The next step is to add 23 ml of Carbonic Acid (also known as Alka Seltzer or H2CO3).
Is the bottle of AS pressurized?
What were the volumes of CO2 evolved?
CO2 solubility is 1.8 g/L (about 0.041M) at room temperature (AWK posted this number (http://www.chemicalforums.com/index.php?board=4;action=display;threadid=3035;start=msg12713#msg12713) some time ago).
22 mL of saturated water contains about the same volume of dissolved CO2 (and there is no reason for it to evolve).
CaCO3 Kso = 4*10-9.
Ca2+ concentration is 0.08M.
Thus CO32- concentration needed for precipitation of CaCO3 is 5*10-8.
Assuming concentration of carbonic acid is the same as CO2 concentration, and pKa1 = 6.37, pKa2 = 10.25, this gives CO32- concentration of 5.6*10-11 - way below the precipitation.
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it will reduce the number or frequency of collision with water molecules, therefore resulting in a decrease in the rate of the reaction.
Water concentration in water is about 55M, NaCl (or CaCl2) concentration in the reacting mixture is about 0.08M. I would expect frequency of colissions to decrease by about 1/700i (where i is the number of ions produced in the dissociation) and similar changes in reaction ratio. Unmeasurable with the accuracy attainable with this equipment.
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Borek thanks for the extra on the collision numbers, I wasn't sure but if right was a good reason.
So then what do you suppose was the driving force that made the NaCl and CaCl2 decrease the rate of reaction over pure dH20?
ADDED: What if the liquid used was pure H2CO3???? would that make the [CO3-2], or [HCO3-] high enough for some precipitation to occur?
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So then what do you suppose was the driving force that made the NaCl and CaCl2 decrease the rate of reaction over pure dH20?
I still don't understand WHY anything evolved in the first place. If the solution was just saturated and no acid was added, nothing forces dissolved CO2 out from the solution.
According to the original description of the experiment - Alka Seltzer water was moved to graduated cylinder, slightly diluted with pure water and lo - CO2 evolved. IMHO nothing should happen (unless AS water contains some more reagents).
ADDED: What if the liquid used was pure H2CO3???? would that make the [CO3-2], or [HCO3-] high enough for some precipitation to occur?
Pure carbonic acid (if it exists) is an extremally exotic animal. Not a thing that stands in a bottle in the students lab :)
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My guess is that the solution was actually this:
water + vinegar + sodium bicarbonate.
H2O + HCOOH + NaHCO3
There for the vinegar adds acetic acid to solution for the catalysis of CO3-2 anion.
Then CO3-2 (aq) + 2H+ (aq) -> CO2 (g) + H2O (l)
Does that sound more reasonable?
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water + vinegar + sodium bicarbonate.
Does that sound more reasonable?
No. All these reagents where in the bottle before - they should have reacted long ago, not now.
(HCOOH is a formic or methanoic acid, vinegar, or acetic, or ethanoic acid is CH3COOH).
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As stated above Ca2+ has greater polarising power than Na+ .
What does this mean?
Well, it means that the solvated ions [Ca(H2O)6]2+ will tend to hydrolyse into [Ca(OH(H2O)5]+ + H+ ions (although the Ca2+ does not form a complex ion as such it is solvated - this is analogous to the aluminium hexaaqua III ion that is formally recognised) The polarising Ca2+ attracts electrons from the H-O-H coordinated to the ion weakening the O-H bonds.
Consequently salt solutions containing Ca2+ ions are slightly acidic (they contain free H+ ions)
These free H+ ions can react with the carbonate ions in a solution (of hydrogen carbonate ions) releasing CO2 gas - it's an acid - base reaction.
As the Na+ is only poorly polarising it cannot hydrolyse when solvated (hydrated) and is consequently neutral - there is no tendency to react with carbonates or hydrogen carbonates.
Although I have never seen this experiment I am familiar with the use of aluminium sulphate solution mixed with liquid detergent and sodium hydrogen carbonate solution to provide foam for fire extinguishers. Try it - it's safe and great fun!
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Consequently salt solutions containing Ca2+ ions are slightly acidic (they contain free H+ ions)
These free H+ ions can react with the carbonate ions in a solution (of hydrogen carbonate ions) releasing CO2 gas - it's an acid - base reaction.
So - if CaCl2 solution is slightly acidic - CO2 should evolve. Not long, as the pH of CaCl2 solution is about 6.76 - not much H+ ions to be used. pH calculated using BATE, so only hydrolysis taken into account, no complex equilibrium - although I suppose it is already accounted for through pKb values.
What about other solutions? Why does CO2 evolve from the solution made by just diluting AS?
Not to mention contradiction - if CaCl2 is acidic, why does CO2 evolve slowest from CaCl2 solution?
It doesn't make sense, no matter which way you look at it. There is some puzzle element missing.
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OK point taken Borek.
Tablets that 'fizz' always contain a mixture of carbonate (either magnesium or calcium) and citric / ascorbic acids.
It's an acid - base reaction - they are not going to react in the solid state but in the presence of water the acids dissolve releasing the H+ ions that can then react with the base.
OK so why the dissolved salt effects?
I would suggest that it's an ionic product effect as mooted above - more ions dissolved less ability for the carbonate/hydrogen carbonate to dissolve in order to react.
CaCl2 produces three ions per ionic unit
NaCl only two
water clearly has none.
or, as the weak acid dissolves HA + H2O <=> H3O+ + A-
Slight removal of the H2O by the polarising Ca2+ may have a buffering effect on the acid by pulling the equilibrium of the weak acid to the left hand side.
The calcium citrate produced in citric acid solution will be a buffer and resist change in pH. This will slow the reaction down by not allowing the H+ concentration to increase as the acid dissolves...
more ideas?
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Tablets that 'fizz' always contain a mixture of carbonate (either magnesium or calcium) and citric / ascorbic acids.
Tablets were not used in this experiment. Please read the thread carefully.
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Well thanks guys, but remember this is a General Chemistry course and it was also the first lab of the semester. I highly doubt that what is going on is so confusing and complex. I mean of course tehre are millions of interactions going on, and many differen't things happening, however I think we are working with a simple idea here.
Thanks for the help though, some of it has been very helpful :)
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Well I do know that the carbonic acid was somewhat pressurized.
I have to take a break. I missed this information on Wednesday. The crucial one :(
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OMG
Sorry guys, I just realized something.
I was over helping the person with another lab today and found out that it was SELTZER WATER not Alka Seltzer water. So it is just carbonic acid we are dealing with.
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But anyways guys, now that we know that we are dealing with SELTZER water (carbonic acid) and not alka seltzer water, I think we can finish this.
I understand how the CaCl2 decreases the amount of CO2 gas produced. I'm still unsure about how the NaCl affects it. Is it really just decreasing the interaction of the H2CO3 and the H2O?
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Hey you know what! Check this out:
H2CO3 is a stable compound alone.
However when it interacts with water it readily decomposes into H2O and CO2.
Now the reason this occurs i'm not 100% sure but I thought it may have to do with the polarity of water and the fact that water is a product of the decomposition.
When you throw NaCL in and get the Na + and Cl- ions, don't they stick to the water molecules based on the fact that water is polar? Thus t his decreases the amount of water molecules that can interact with the H2CO3? As a result the decomposition decreases?
What do you guys think?
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When you throw NaCL in and get the Na + and Cl- ions, don't they stick to the water molecules based on the fact that water is polar? Thus t his decreases the amount of water molecules that can interact with the H2CO3? As a result the decomposition decreases?
Traces of water are enough for carbonic acid decomposition. Concentration of water in pure water is 55.51M. In the CaCl2/NaCl solutions concentration of water is lowered to - say - 55.43M. That's 0.14% difference. It is very doubtfull to me that's what changes speed of CO2 evolution in measurable degree (taking into account accuracy of the experiment on the whole).