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Carbondioxide will increse acidity?

  1. Dec 30, 2006 #1
    The textbook said if you add dry ice (carbon dioxide) to lemonade or soda water, the pH of the substance (which is already acidic) will become even more acidic. Why?
     
  2. jcsd
  3. Dec 30, 2006 #2
    When carbon dioxide is dissoved in water, it forms carbonic acid.

    CO2 + H2O <--> H2CO3
     
  4. Dec 31, 2006 #3
    And then H2CO3 + H2O <-->H30+ HCO3

    so in this way hyronium is increased and so by definition the acidity has increased?
     
  5. Dec 31, 2006 #4

    Gokul43201

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    Yes, with the tiny correction that it's HCO3-

    Also, the second dissociation constant isn't negligible, so include HCO3(-) + H2O <--> H3O(+) + CO3(2-) as well.
     
  6. Dec 31, 2006 #5

    GCT

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    It's actually CO2 + H2O <--> H3O+ + HCO3- (HCO3- is an ampholyte, however, I recall that essential chemistry is that of a base and sometimes employed to neutralize various types of acid solutions).
     
  7. Jan 5, 2007 #6

    Ivan Seeking

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    I was recently made aware of the claim that by heating water in a microwave to just below boiling temp, and then allowing the water to cool and be used for watering house plants, the plant growth is reduced. Apparently is a claim found in the extreme holistic type crowd. A friend's wife actually convinced him to pitch the microwave by demonstrating this using house plants, with one group watered with almost-boiled water, and the other with straight tap water. Of course, if we assume that the demonstration was on the level, the obvious objection was that she never tried using water heated on the stove. Out of curiosity, I intended to check into this but never got around to it.

    By chance I had to boil some tap water that comes from our private well with a known pH of 7.0. This came up for something else entirely, but there you go. :rolleyes: After boiling the water and letting it cool, the pH was well over 8.0. Given that the water has been chlorinated, dechlorinated, filtered twice, then carbon filtered and run through a water softener, and since the solubility of CO2 decreases with increasing temp to about half - over the range of 25C to 100C - am I reasonably safe to assume that the pH change likely results from driving off CO2, or is the potential for other factors is too great to make assumptions about the cause of the pH change?
     
    Last edited: Jan 5, 2007
  8. Jan 5, 2007 #7

    Gokul43201

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    Ivan, unless I'm making a mistake, the numbers don't work out for CO2 being the dominant reason for the change in pH. If I cut the concentration of CO2 by a factor of 4 from its typical value, the pH would increase by not more than 0.5.

    Can't imagine what else it might be though...so I'll put it on your pH meter.
     
    Last edited: Jan 6, 2007
  9. Jan 6, 2007 #8

    Ivan Seeking

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    Whoops, I must have dropped something somewhere. I was shooting from the hip a bit since I don't know what else it could be either. I'm using a common chemical test kit that is accurate over the range of 6 - 8, and it is repeatable - I did it twice just to be sure.

    I just checked and sure enough, after a couple of days or aeration, the pH is still about 8.0. [edit] Although, I think it has dropped a bit. Now it appears to be indicating almost exactly 8.0, whereas initially it was clearly indicated to be > 8.0.

    I still need to buy a pH probe for my Fluke. :redface:
     
    Last edited: Jan 6, 2007
  10. Jan 9, 2007 #9

    chemisttree

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    Was the glass (?) cleaned in an automatic dishwasher using commercial detergents? If so, a layer of sodium silicate could be present that dissolves off the glass when heated.
     
  11. Jan 9, 2007 #10

    Ivan Seeking

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    Good point. I think it might have been the pot used to boil the water. It was cleaned thoroughly, but perhaps a few rounds of boiling will clear things up.

    Thanks. I should mention that Integral made the same suggestion.

    I guess there is always the possibility that we experienced a transient in our water quality. This can happen if we get a heavy flush of minerals and sulfer compounds in the water table.
     
  12. Jan 11, 2007 #11
    Does the weak cabonic acid form in the process?

    Is it more common for H2CO3 -> CO2 + H2O
    or
    CO2 + H2O -> H2CO3 + H2O ->HCO3- + H3O+
     
  13. Jan 12, 2007 #12

    GCT

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    Don't know that at the moment, however, try finding the equilibrium constant for the reaction (CO2 as a reactant) through google ("equilibrium constant for CO2" may suffice) or perhaps the CRC Physical Chemistry Handbook.
     
  14. Jan 12, 2007 #13

    Gokul43201

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    Carbon acid (H2CO3) is actually not a very weak acid (its Ka is not that small). The pH of carbonic acid is usually pretty low because (i) the solubility product for CO2 in water is low, and (ii) only a small fraction of the dissolved CO2 forms H2CO3. But among the H2CO3 that forms, a relatively significant fraction (nowhere near a majority, but a bigger fraction than HF) dissociates to H+ and (HCO3)- and a very tiny fraction of the (HCO3)- further dissociates into H+ and (CO3)2-.
     
  15. Jan 13, 2007 #14
    So this
    H2CO3 + H2O ->HCO3- + H3O+ is more likely

    than this
    H2CO3 -> CO2 + H2O

    What about
    CO2 + H2O -> H2CO3 + H2O -> HCO3- + H3O+
     
  16. Jan 13, 2007 #15

    GCT

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    ADD THE FOLLOWING REACTIONS

    CO2 + H2O---->H2CO3 + H2O

    H2CO3+H2O---->HCO3- + H3O+
    --------------------------
    CO2+H2O ---->HCO3-+H3O+

    ....The overall reaction for the dissolution of CO2 to become HCO3- and H3O+ to increase the acidity of the aqueous solution.

    The two reactions are essentially dependent on each other, so the question of whether one occurs to a greater extent is somewhat irrelevant.
     
  17. Jan 15, 2007 #16
    let's clear things up:
    so you got CO2 in H2O:

    CO2 + H2O -> H2CO3 and H2CO3 + H2O -> HCO3- + H3O+

    so that gives us:
    CO2 + 2 H2O <-> HCO3- + H3O+ (K1= 4.16 x 10^(-7))
    and because HCO3- is still an acid you get:
    HCO3- + H2O <-> CO3(2-) + H3O+ (K2= 4.84 x 10^(-11))

    so we have these concentrations:
    [CO2]= C (given normally if you calculate pH, before the reaction)
    [HCO3-]= X (for first reaction) and X-Y for both
    [CO3(2-)]= Y
    [H3O+]= X + Y

    so again reactions with concentrations underneath:
    CO2 + 2 H2O <-> HCO3- + H3O+
    [CO2]= C-X
    [HCO3-]= X
    [H3O+]= X

    HCO3- + H2O <-> CO3(2-) + H3O+
    [HCO3-]= X-Y
    [CO3(2-)]= Y
    [H3O+]= Y

    this means that K1 and K2 have these expressions:

    K1= ( (X-Y).(X+Y) ) / ( C-X )

    K2= ( Y.(X+Y) ) / ( X-Y )

    you might not know this but there is a rule like this:
    IF C > 100.K1 then X is negligible compared to C (which is most likely the case)
    and also in this case X > 10^(-7),
    so we can say that X > 100.K2 (which says that Y is negligible compared to C)

    this means that we can calculate X for expression for K1
    => K1= ( X . X ) / (C) so: X= sqrt( K1 . C )

    same for Y:
    => K2= ( Y . X ) / (X) so ofcourse: Y= K2

    (i probably lost you long ago but for people who are still with me:)

    if you take C= 0.04 M (before CO2 is put into the H2O)

    [HCO3-]=X-Y= 1.29 x 10^(-4)
    [CO3(2-)]=Y= 4.84 x 10^(-11)
    [H3O+]= X + Y= 1.29 x 10^(-4)

    so pH= 3.89 (pH=-log([H3O+]) <=> first pH= 7 (H2O)

    clearly Belgium is owning again :rolleyes:
     
    Last edited: Jan 15, 2007
  18. Jan 23, 2007 #17
    BartDT, I haven't really learnt what you wrote but I will post what I have realised from what people like Gokul and GT have said:

    It is more likely for H2CO3 to form H2O+CO2 than vice versa so the reaction that occur most often is

    H2CO3 + H2O -> 2H20 + CO2 -> HCO3- + H3O+ -> 2H3O+ + (CO3)2-

    Although if no H2CO3 or carbonic acid is involved than we could start with water and carbon dioxide and start the reaction from there. Ultimately, it will lead to increase in H+ ions in the solution hence to answer the question of the thread Will carbondioxide will increse acidity? YES. Is my analysis correct?
     
  19. Jan 23, 2007 #18

    Gokul43201

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    This is not what I said. In fact, such a statement actually makes no sense from a chemistry point of view. The correct way to talk about reactions is to discuss the equilibrium conditions and the kinetics that lead to it. At equilibrium, the H2CO3 is as likely to make H2O + CO2 (in a given window of time) as the reverse reaction (by definition).

    If you have not studied chemical equilibria and kinetics, you should wait until you do, before you try to answer questions about chemical equilibria.
     
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