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Sodium Fluoride Reaction

  1. Feb 23, 2016 #1
    I have been reading about how NaOH (Sodium Hydroxide) reacting with HF (Hydrofluoric Acid) yields the products H20 (Water) and NaF (Sodium Fluoride). If I were to do this, would I need the EXACT same quantity of NaOH as HF to get H20 and NaF?
     
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  3. Feb 23, 2016 #2

    Borek

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    Have you heard about limiting reagents?
     
  4. Feb 23, 2016 #3

    SteamKing

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    The first thing to do when trying to find out what happens in a chemical reaction is to write a balanced chemical reaction formula.

    Is
    NaOH + HF → NaF + H2O

    a balanced chemical reaction formula?
     
  5. Feb 23, 2016 #4
    I know that it is the first reactant that is used up in an reaction. But that is pretty much it. Is that how this would work with different amounts?
     
  6. Feb 23, 2016 #5
    Yes it is.
     
  7. Feb 23, 2016 #6

    SteamKing

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    Count the number of different atoms on each side of the equation. If they're not equal, the reaction equation is not balanced.
     
  8. Feb 24, 2016 #7

    Borek

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    Yes.
     
  9. Feb 24, 2016 #8

    SteamKing

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    With the reaction:

    NaOH + HF → NaF + H2O

    you want to start with an equal number of moles of HF and NaOH, otherwise you'll have either some HF or NaOH left over after everything has reacted.

    1 mole of HF = 1.008 + 18.998 = 20.006 grams +
    1 mole of NaOH = 22.990 + 15.994 + 1.008 = 39.992 grams

    will yield

    1 mole of NaF = 22.990 + 18.998 = 41.988 grams +
    1 mole of H2O = 2*1.008 + 15.994 = 18.010 grams
     
  10. Feb 24, 2016 #9

    Borek

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    Technically yes, but in practice this is unrealistic, so we always use an excess of one reagent. That's why purification of the product is an important part of each synthesis procedure.
     
  11. Feb 24, 2016 #10
    So if I were to have 1 mole of NaOH and 1.01 moles of HF, would the extra 0.01 moles of HF simply be mixed in with the Water and Sodium Fluoride? If so, could I purify it by boiling out the extra HF (which I believe has a boiling point of about 67°F or 19.5°C)?
     
  12. Feb 24, 2016 #11

    Borek

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    Apart from the fact I don't like the idea of an airborne HF you are right.

    Much safer (but also more difficult) approach is to use excess of NaOH, to be sure no HF is left. HF is quite nasty, not only highly corrosive but also pretty toxic. Trick is, while HF can be easily boiled off, getting rid of excess NaOH is not trivial and may require recrystallization. To be on the safe side you can use excess HF and pass the gases through a scrubber which will filter out HF. Or perhaps just distilling off water with excess HF will be safe enough, no idea what is the best approach here.
     
  13. Feb 24, 2016 #12
    I think the best option would be to distill the water if possible. Thanks for all the help!
     
  14. Feb 25, 2016 #13

    Borek

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    Don't forget you are distilling water AND hydrogen fluoride.
     
  15. Feb 25, 2016 #14
    Mixing what ever quantity of NaOH and HF you want to, the reaction will occur. However, it will only produce the amount of NaF you are seeking that corresponds to the reactant that is in less quantity with respect ratio of the reactants required. For example if you started with 1 gram of NaOH and 1 million grams of HF and mixed them, you would produce about 1.05 grams of NaF. (1 gram of NaOH is 0.025 moles, NaOH and HF react in 1:1 ratio to produce NaF in a 1:1 ratio, thuse 0.025 moles of NaF which is ~ 1.05 grams).

    Think of it like your car and gasoline, you car can hold about 10-15 gallons of gasoline, which reacts with oxygen to make among other by products, CO2 and water (gas phase which, with the heat, expand to make your car go). Now, which runs out first your gas tank or all the oxygen on the earth? This is the concept of a limiting reagent.
     
    Last edited: Feb 25, 2016
  16. Feb 25, 2016 #15
    Borek is correct, you cannot simply distill water to easily separate it from HF, you will get some fraction of HF. Your best bet was stated above, use enough NaOH to react all the HF. Also, do this reaction in the aqueous phase. I.e. a solution of HF and NaOH. Do not use gaseous HF and solid NaOH. Pure HF is gaseous but it dissolves readily in water as a weak acid. In the aqueous phase this is a pretty easy acid-base reaction.

    Don't mess with HF, it is quite harmful. HF does not quickly cause burning sensation on the skin like strong acids (such as HCl or H2SO4) as such you cannot really tell if you have been exposed. HF does absorb through the skin and leaches calcium from your bones. I do not recommend handling it if you are not trained and aware of the dangers.
     
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