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(split) Remove an Aluminum Tube

  1. Aug 6, 2018 #1
    Original thread: https://www.physicsforums.com/threads/remove-an-aluminum-tube.952459/

    NaHCO3, i.e. sodium hydrogen carbonate, compared to neutrality, is a weakly alkalinic salt, which, although it can, in the presence of a stronger base, such as sodium hydroxide, act as an acid, by surrendering its hydrogen, is not correctly designated as by itself an acid.

    Adding baking soda to lye, the reaction is NaOH + NaHCO3 → Na2CO3 + H2O, i.e. disodium carbonate (commonly called sodium carbonate, or washing soda) plus water.

    Although that's less alkalinic than NaOH, in solution the disodium carbonate ionizes as 2 sodium ions plus H2CO3 (carbonic acid -- CO2 plus H2O), and that's still more strongly basic than baking soda -- if you add in baking soda alone, you can't fully titrate lye to neutrality.
     
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  3. Aug 6, 2018 #2

    Borek

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    Nope.

    You contradict yourself - you just wrote carbonate ionizes into carbonic acid, now you state this solution of carbonic acid is alkaline.

    Yep. Still better than just lye.
     
  4. Aug 6, 2018 #3
    I didn't go into the fact that the sodium ions don't stay unbound in the solution. Perhaps that omission is what led to your without explanation flatly disagreeing. Otherwise, I would be interested to learn why you disagree, especially if there's something I missed.
    In the previous sentence, I said NaOH + NaHCO3 → Na2CO3 + H2O, i.e. sodium hydroxide plus sodium hydrogen carbonate produces disodium carbonate plus water. I didn't say that disodium carbonate dissolved in water is "a solution of carbonic acid". According to PubChem, Na2CO3 is (among other names) "carbonic acid disodium salt". It's more strongly alkalinic than sodium hydrogen carbonate is, and less strongly so than sodium hydroxide is.

    The dissolution is a double replacement reaction: Na2CO3 + H2O → H2CO3 + Na2O.

    You could also write it as Na2CO3 + H2O → CO2 + 2 NaOH or as
    Na2CO3 + 2 H2O = H2CO3 + 2 NaOH.

    Which balancing is to be preferred would depend on some of the specific characteristics of the solution.
     
  5. Aug 7, 2018 #4

    Borek

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    It is not carbonic acid that is the product of the dissociation, but the carbonate anion CO32-. That's a huge difference. Solution containing the H2CO3 would be solution of a carbonic acid and should have low pH. CO32- is a Bronsted base, and reacts with water increasing the pH, so its solutions have high pH. Still not high enough to be as dangerous as solutions of NaOH.

    No, these are all completely wrong. You can't have Na2O in water, you should write some of these compounds in ionic form, and the dissolution is in general a series of multistep equilibrium processes which can't be written as a single reaction.
     
  6. Aug 7, 2018 #5
    A disodium carbonate (commonly called soda ash when dry, or washing soda when in water) solution contains carbonic acid and sodium hydroxide in a proportion that renders the solution moderately alkalinic.
    It seems to me that it's clear enough that adding a base salt to water produces a solution that is more alkalinic than water. I agree that adding baking soda to a solution of lye tones it down to the much less caustic washing soda level.
    They're all correctly balanced.
    That's true -- Na2O + H2O → 2 NaOH -- sodium oxide is the base anhydride of sodium hydroxide.
    Here you go:
    Na2CO3 → 2 Na+ + CO32-
    CO31- + H2O → HCO3- + OH-
    2 Na+ + 2 OH- ↔ 2 NaOH
    I had originally intended to elaborate only enough to elucidate why sodium hydrogen carbonate (baking soda) is an alkaline salt; not an acid -- it's true that because of its hydrogen, it's a weaker base than disodium carbonate is, and can in intermediary reactions serve the function of an acid, but overall, it's still alkalinic, not acidic.
    I agree that the single equation isn't a complete description, but that doesn't make it incorrect -- for the purpose for which I presented it, I think it's reasonably adequate -- I was pointing out that you can't neutralize lye with baking soda; you can only weaken it.

    Perhaps your statement "Any acid will do, even sodium bicarbonate (baking soda)." might not have caught my attention in the same manner if I hadn't seen it as apparently asserting baking soda to be an acid. If I'd read it as meaning "Any acid will do, and even the mildly basic sodium bicarbonate would suffice to turn it from lye into washing soda.", I wouldn't have fussed over it. Thanks for elaborating.
     
    Last edited: Aug 7, 2018
  7. Aug 7, 2018 #6

    Borek

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    No, salt solution doesn't contain original acid and base. To some extent what you wrote means putting chemistry back into 18th century, or you are just mistaking preparation with the reaction product.

    Some traces of the acid and base will be present because of the hydrolysis, but that's another thing.

    Just because reaction equation can be balanced doesn't make it right.

    Not 1- but -2, I assume it is a typo. Still, you wrote it as if it was a reaction going to completion - it doesn't. It would be better to use ↔ instead of →.

    Why these 2s?

    While this reaction to some extent takes place it doesn't matter much here - actually it makes the solution less alkaline by removing OH-. It is OH- that makes alkaline solutions corrosive, not NaOH.

    Technically it is best referred to as "amphiprotic".

    It's solution are very slightly alkaline, with pH around 8.3. That's about as close to neutral as most water from the natural sources is.

    Yep, a bit careless wording.
     
  8. Aug 8, 2018 #7
    The word "contains" doesn't have to mean "contains as such". For example, saying that baking soda contains carbon dioxide that is released during the baking process isn't wrong, even though baking soda doesn't contain as such the carbon dioxide that was used along with disodium carbonate in its preparation.

    As for revisiting 18th century chemistry, I think that calling sodium hydrogen carbonate "sodium bicarbonate" is rather apt in that matter. The molecule isn't composed of a sodium and two carbonates, as that archaic but still common name suggests. As you are no doubt aware, it's called that because the numeric ratio of carbonate instances to sodium instances in sodium hydrogen carbonate is twice that found in disodium carbonate, which is similarly archaically named "sodium carbonate".
    You're right -- it should be 2- (or -2), as it was in the prior equation.
    I thought about that, and decided to use the ↔ in the 3rd equation only -- I acknowledge your point there.
    It's intended as an oblique preservation of recognition of the fact of there having been 2 sodiums to account for in the predecessor disodium carbonate molecule -- I acknowledge that it's not especially conventional.
    That's true -- baking soda is a weak base, which when added to caustic soda, a strong base, produces washing soda, a moderately alkalinic salt, which in solution could be viewed as if it were soda water with some (ignoring the stoichiometrics) drano in it (please don't anyone let his/her cat or dog try to test that putative equivalency at home).
    As I said earlier, it can act as an acid by surrendering its hydrogen; however, it's still technically a weak base, and not properly (without some qualification of the term) called an acid, despite it being amphiprotic.

    Your bringing up the term "amphiprotic" prompts me to note that the aluminum oxide at the surfaces of the tubes that were to be corroded by the sodium hydroxide solution (as discussed in the parent thread in the Engineering area from which this one was split off) is not amphiprotic, because it doesn't have any hydrogens to contribute, but in that it can act as an acid or a base by receiving or donating electrons, it is still amphiphoteric, as amiphiprotic substances are.
    It is only weakly basic, but not acidic, as some water supplies are -- Icelandic spring water is about 8.4, which squares nicely with your observation.
    It led to what's been at least for me an interesting discussion.
     
    Last edited: Aug 8, 2018
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