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Chemical reaction leading to salt water

  1. Feb 8, 2014 #1
    Okay let's say we have water, sodium hydroxide and hydrogen chloride. What do you think is going to happen? Do you think the pH will be the same, a little higher, or a little lower?

    You have this reaction down here:
    H2O+NaOH+HCl(assuming they are equal)= H2O+OH-+Na+(positive charge)+H+(positive charge)+Cl-=H2O+NaCL
    Let me explain what happens. The sodium hydroxide dissasociates completly into Na+ and OH-. The hydrogen chloride dissasociates completly into H+ and Cl-. The H+ and OH- are attracted and form a water molecule. The Na+ and Cl- are attracted and form NaCL or table salt. this table salt formation makes the water have a pH of 8 intead of 7 meaning that when you start off with a neutral, strong base, and strong acid in equal amounts you get a weak base as the result.

    Somebody else said that it will be neutral but I have looked up the pH of saltwater(which is what is forming in this reaction) and it is 8 not 7.
  2. jcsd
  3. Feb 8, 2014 #2
    Are you talking about water with salt dissolved in it, or are you talking about sea water? Sea water contains various species in addition to NaCl which render it slightly basic.

  4. Feb 8, 2014 #3
    I have looked up the pH of saltwater(water with salt dissolved in it) and it says it is 8. That makes sense that if HCl is 1 and NaOH is 14 that H2O + NaCl(which is what forms) that because the acid isn't 0 that raises the pH by 1 so that H2O + NaCl has a pH of 8 instead of 7.
  5. Feb 8, 2014 #4
    HCl does not have a pH. A solution of HCl in water has a pH roughly equal to the molarity of the HCl (because HCl can be considered to dissociate completely). The same goes for NaOH.

    Complete neutralization of a strong acid by a strong base results in a pH of 7 at the end point. This may change a bit if the ionic strength of the solution is high necessitating use of activities instead of concentrations in calculations.

    Can you post up a reference for your statement?
  6. Feb 8, 2014 #5
  7. Feb 9, 2014 #6


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    You are so wrong I don't even know where to start.

    First of all, you are ignoring the most crucial information - what are initial amounts of these substances? Final pH will depend on how much of each you mix.

    Where have you found it? Around 8 is a pH of sea water, which is not equivalent to salt water solution. Plenty of other dissolved substances present as well. Typical NaCl solution will be neutral, unless you leave it in contact with atmospheric air - then, it will soon get acidified by dissolving CO2.

    That's not how it works, see below.

    These are not "NaOH" and "HCl" per se, these are solutions of these substances, at selected concentrations. pH is a property of solution, not of substance. You quote one number from the table, ignoring others, showing different values for the same substance.

    By definition, pH = -log([H+])

    HCl in water solution is a strong acid, so it is fully dissociated. Thus, 0.1 M solution of HCl contains 0.1 M of H+ and its pH is 1. But 0.001 M solution contains 0.001 M of H+ and its pH is 3.

    Because of the way H+ and OH- behave in water solutions, pH + pOH = 14 (explained in more details here). Let's assume NaOH is a strong base, fully dissociated (it almost is). 0.1 M solution of NaOH contains 0.1 M OH-, its pOH is 1, so its pH is 13. 0.001 M solution of NaOH has pH of 11 - again, pH is a function of concentration.

    When calculating pH of the mixture of a strong acid and strong base you need to find which one was in excess, then use this excess to calculate concentration of H+ (OH-) and teh calculate pH from this concentration. Caveat: this approach fails when the amounts were stoichiometric, or concentrations were very low. There are methods of calculating pH in such solutions (compare here), but as a first approximation you can assume pH of such solution is just 7.
  8. Feb 9, 2014 #7
    well the amount of water is = the amount of HCL = the amount of NaOH so no matter how much water I have because all the other substances are equal in amount the pH will stay the same, right?

    and any liquid, solution or not has a pH, right?

    And here the reaction involves 3 liquids turning into 1 liquid and a dissolved solid.

    Particularly HCl + NaOH + H2O -> H2O + NaCl

    The HCl completely disassociates in water and so does NaOH

    The HCl becomes H+ and Cl-

    The NaOH becomes Na+ and OH-

    Via Ionic Bonding Na+ and Cl- become NaCl

    Via covalent bonding H+ and OH- become H2O
  9. Feb 9, 2014 #8


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    No idea what you mean by that. There is water produced, there is water that was used by solvent, final amount is the sum of all those (although typically produced can be ignored, as its amount is negligible compared to the amount of the solvent).

    Again, it doesn't make any sense to me.

    No, water solutions have pH. We sometimes speak of pH in other solvents, but it is neither well defined nor easy to measure.

    What three liquids if you mix two?

    Yes and no. You end with a solution of NaCl, which is completely dissociated, so technically there is no ionically bonded NaCl in the solution - it will appear as a solid when you evaporate water.
  10. Feb 9, 2014 #9
    You have too many misconceptions to tackle in a forum thread. Maybe someone else will take the time to write, essentially, an acid-base chemistry chapter for you but I'm just going to recommend you finding a general chemistry textbook and learning the basics. You can probably just google stuff but it seems that you really need a structured piece of writing to get a handle on the fundamentals. Googling will result in scattered bits and pieces and I believe you need to start from the ground up.

    I really hope this doesn't come off as me/us bashing you, but this forum is not here to replace whole textbooks or classes. Rather we are here to help guide people with small misconceptions or details when the fundamentals are more or less in place.

    Just to get you a bit of a head start, a solution of HCl (or NaOH) in water does not have [HCl] = [H2O]. When water is the solvent, its concentration is ~55.6M, assuming a relatively dilute solution where the density is approximately that of pure water. You can work this out for yourself by using the density of water and converting from g/L to mol/L using the MW of water.

    Also NaOH and HCl are not liquids. Pure NaOH is solid and pure HCl is a gas. They each have high solubility in water allowing for us to use them in solution but, as above, the concentration of water is much greater than the solute concentration.

    Just to reiterate, when a strong base titrate a a strong acid to the end point the resulting solution is near pH 7 with the products being a salt (composed of the counter ions) in water. You can try this for yourself by dissolving NaCl in water and noticing that pH won't change appreciably. It may change due to other factors such as contaminants etc.
  11. Feb 20, 2014 #10
    NaOh and HCl can be pure substances in the liquid state just like CO2 can. Now what I said about videos and textbooks giving misconceptions and forums not doing that is true. For example textbooks and videos say "1 glucose molecule is = to 38 ATP" when actually it is closer to 30 because NADH produces 2.5 and FADH2 produces 1.5 and the 2 ATP cost of transporting the NADH from glycolysis into the mitochondrion.
  12. Apr 8, 2014 #11
    Now lets assume you have equal parts of each, that is 1/3 is NaOH, 1/3 is HCl, and the other 1/3 is water. What fraction of water and what fraction of NaCl do you get? 2/3 water and 1/3 NaCl because you already had that 1/3 water. If you didn't you would have a racemic mixture of water and salt(meaning 50/50).

    Now also HCl and NaOH have to be liquids to optimally react. Why?

    Well solids don't react unless they dissolve or oxidize(like Fe + H2O = FeOx(the x stands for unknown oxygen atoms since there are multiple iron oxides) or NaCl + H2O = Na^(+) + Cl- + H2O for an example of dissolving)

    Gases move so fast they have little time to react.

    Thus for reactions liquids are optimal.

    Now if you were given a value like 2 moles of NaOh and 1 mole of NaCl could you tell whether there was excess NaOh or limiting HCl or if not all of it has turned into sodium chloride and water?
  13. Apr 8, 2014 #12


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    If there was an excess of NaOH, HCl was - by definition - the limiting reagent. No "or" here.
  14. Apr 8, 2014 #13
    okay but if you were given 2 moles of NaOH and 1 mole of NaCl could you tell whether there was an excess of NaOH or if not all of it has turned into sodium cloride and water(which is not quite the same thing as excess NaOH)?
  15. Apr 8, 2014 #14


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    No idea what you mean. If there is NaOH left after neutralization, it was in excess.
  16. Apr 8, 2014 #15
    What I mean by not all turning into sodium chloride is that there is still some HCL and NaOH that hasn't reacted.
  17. Apr 8, 2014 #16
    Acid/base neutralizations go to completion, especially in the case of strong acids and strong bases. Also you should keep in mind that when someone writes that NaCl is formed, in the case of an aqueous solution, they don't actually mean that all the Na+ and Cl- are actually together. Each ion is solvated by a shell of water so that if we imagine zooming in very closely, we wouldn't actually see any NaCl. We would instead see shells of water around each ion.

    Also, racemic doesn't mean simply a 50/50 mixture of anything. It refers specifically to a mixture of enantiomers which are 50/50 in a sample. Another way of saying it, is that it is a mixture of two enantiomers which doesn't rotate plane polarized light.

    Also, NaOH and HCl are not liquids at STP. Just because something is in solution, doesn't mean it becomes liquid. Like I said above, pure HCl is a gas (at STP) and pure NaOH is a solid (at STP). You can verify this by looking at the physical properties of each molecule. The melting point of NaOH is 318° C and the boiling point of HCl is -85° C (ref Wikipedia).
  18. Apr 9, 2014 #17
    Liquids are optimal for reactions because of the reason I said so a few posts above. That includes liquid solutions.
  19. Apr 9, 2014 #18


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    Liquids are liquids, solutions are solutions, NaOH is a solid at STP, HCl is a gas at STP. You are juggling terms not paying attention to the context, no wonder you feel confused.
  20. Aug 11, 2014 #19
    Why isn't the NaCl made from HCl + NaOH + H2O safe to eat? I mean couldn't you just evaporate the water and excess acid or base to get NaCl that is safe to eat?

    Or is there a chance that the acid or base will be trapped in the NaCl crystals and thus the NaCl would still not be safe?

    And does whether the NaCl in aqueous solution is safe after evaporation or not depend on the amount of water, acid, and base so that a 50/50 mixture of acid and base in aqueous solution means no acid or base left and all that could be trapped in the ionic crystals is H2O whereas if 1 is in excess the NaCl would likewise be acidic or basic in aqueous solution due to the acid or base being trapped in the ionic crystals?
  21. Aug 11, 2014 #20


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    Staff: Mentor

    Who told you so?
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