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Reaction between sodium bicarbonate and acetic acid

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Sophie Amelia

Mon Jan 09 2012, 06:08AM

Registered Member #4201 Joined: Wed Nov 09 2011, 07:42AM
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Posts: 20

Hey,

I have been doing laboratory test between sodium bicarbonate and vinegar. I realised that reacting 1M of acetic acid with solid sodium bicarbonate will not produce any pressure. In fact, the lower the concentration of acetic acid, the better it's reactivity. The lowest concentration i ventured into was 0.37M.

Could someone explain to me why the lower the concentration of acetic acid, the more reactive the reaction is? I understand it is an acid-base reaction.

Thanks

Fulmen

Mon Jan 09 2012, 01:13PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

That is odd, could you elaborate on how the experiment was performed? Apparatus, state of the bicarbonate (lump/powder) etc would help.

One possibility is that the sodium acetate produced in the reaction precipitated to form a protective layer over the bicarbonate, but I thought sodium acetate was fairly soluble in water.

radiotech

Tue Jan 10 2012, 04:21AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Could it have anything to do with the sodium bicarbonate buffering the acetic acid
solution at the higher molarity and using up all the H+ ?

Sophie Amelia

Tue Jan 10 2012, 06:45AM

Registered Member #4201 Joined: Wed Nov 09 2011, 07:42AM
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Posts: 20

Hmmmm... i pounded the finely powdered sodium bicarbonate and placed it in a vessel. Sodium bicarbonate is two times in excess. Then, i poured in the acetic acid.

There are two experiments:
1. Test out pressure produced by 400ml of 0.37M acetic acid and 24g of sodium bicarbonate, Pressure produced = 18PSI
2. Test our pressure produced by 400ml of 1M acetic acid and 60g of sodium bicarbonate, Pressure produced = 0PSI

Our vessel has an space volume of 5L

Sooo, could you help explain why lower concentration of acetic acid is better? Im really confused.

Thanks :)

Fulmen

Wed Jan 11 2012, 11:11AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Sorry, Sophie. I am at a complete loss here, I can't think of any reason why this happened. Have you repeated the experiment? Could be a mistake somewhere, did you make the solutions yourself from the same stock? The reaction should be quite vigorous, did you observe the reaction?

radiotech

Thu Jan 12 2012, 04:32AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Have you a way of measuring the pH of both vessels?

Wastrel

Fri Jan 13 2012, 12:42PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

Either the container was not gas tight for the second run or it had finished by the time it was sealed.

randommscience117

Fri Jan 13 2012, 11:27PM

Registered Member #4274 Joined: Mon Dec 19 2011, 03:10AM
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Posts: 47

If you're talking about the concentration of the Acetic acid to water, here's the problem: Acids have what is called an "Iso-corrosion Curve" which says that at higher concentrations, acids aren't active enough to really do much until they reach a certain concentration. An example of this is Sulfuric acid. Sulfuric acid can be stored in steel tanker cars at concentrations of around 100%, but drop the concentration enough (40% is around the top for H2SO4), and it will eat through that tank in no time. This may be what you are experiencing with the Acetic acid and sodium bicarbonate. We learned about this briefly in Chem 101 today.

Sophie Amelia

Sun Jan 15 2012, 08:25AM

Registered Member #4201 Joined: Wed Nov 09 2011, 07:42AM
Location:
Posts: 20

Both experiments were conducted using the same vessel... and i didnt measure the pH.

Well,i previously described about the two experiments and the second one did not give off any pressure. Strangely, when i simply add more water to the second experiment (to prepare to throw them into waste), it immediately starts to fizz more vigorously. haha, so it kinda supports that lesser concentration is more vigorous.

On the other hand, i did more research and found out:

where if you were to scroll down to the point on "More complicated titration curves: Adding hydrochloric acid to sodium carbonate solution", they mentioned something about not giving off carbon dioxide at the initial stage.

I know they are not exactly mentioning about pressure or about acetic acid. However, there seem to be some kind of a link. Unfortunately, they did not explain much into the reasoning behind the chemical behavior.

I just wish someone could explain to me whats happening in terms of molecular level.

Thanks

Sophie Amelia

Sun Jan 15 2012, 08:29AM

Registered Member #4201 Joined: Wed Nov 09 2011, 07:42AM
Location:
Posts: 20

randommscience117 wrote ...

If you're talking about the concentration of the Acetic acid to water, here's the problem: Acids have what is called an "Iso-corrosion Curve" which says that at higher concentrations, acids aren't active enough to really do much until they reach a certain concentration. An example of this is Sulfuric acid. Sulfuric acid can be stored in steel tanker cars at concentrations of around 100%, but drop the concentration enough (40% is around the top for H2SO4), and it will eat through that tank in no time. This may be what you are experiencing with the Acetic acid and sodium bicarbonate. We learned about this briefly in Chem 101 today.

Does it have something to do about the dissociation of acid in water? Could you explain more in terms of molecular level?

Thanks

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