[Donaldson Tan]

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.

[Blueshawk]

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.

[Borek]

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 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 CO₃^2- 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 CO₃^2- concentration of 5.6*10^-11 - way below the precipitation.

[Borek]

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.

[Blueshawk]

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?

[Borek]

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

[Blueshawk]

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?

[Borek]

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).

[charco]

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!

[Borek]

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.

[charco]

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?

[Borek]

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.

[MilchstrabeStern]

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

[Borek]

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

[MilchstrabeStern]

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.