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I am finding different equations for the synthesis. My lecture handout suggests a reaction with Sodium tetrafluroborate and Sulfuric acid. This is fine but I have to remember this for an exam and that reaction looks extremely hard to remember/balance in an exam. A second way seems to be reaction of boron oxide with Calcium fluoride and Sulfuric acid. However, I cannot find consistent equations for this...

Wikipedia simply suggests that Boron oxide reacts with $\ce{HF}$ which is made in situ by the reaction between sulfuric acid and calcium fluoride:

$$\ce{B2O3 + 6HF -> 2BF3 + 3H2O}$$

It's easy to remember but I am reluctant to accept this because it excludes two of the reactants (sulfuric acid and calcium fluoride). Would it be acceptable to merge the formation of $\ce{HF}$ into the above equation - thus including all reactants:

$$\ce{B2O3 + 3CaF2 + 6H2SO4 -> 2BF3 + 3H2O + 3Ca(HSO4)2}$$

It's a little tricky to balance but not too hard to remember. Is there anything wrong with the above equation? I haven't seen it anywhere but only variations on it.

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    $\begingroup$ $\ce{CaF2} + {H2SO4}$ will give you $\ce{HF}$ so why bother if you add directly the $\ce{HF}$ instead ? $\endgroup$
    – Babounet
    Commented Mar 24, 2015 at 21:05
  • $\begingroup$ I don't know but that is how it is synthesised $\endgroup$
    – RobChem
    Commented Mar 24, 2015 at 22:02
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    $\begingroup$ @Babounet Because anhydrous hydrofluoric acid is more expensive and more difficult to handle than a mixture of fluorspar and sulfuric acid. $\endgroup$
    – Klaus-Dieter Warzecha
    Commented Mar 25, 2015 at 6:37
  • $\begingroup$ I agree Klaus, but since it's an exam and not a lab experimentation, who cares ? $\endgroup$
    – Babounet
    Commented Mar 25, 2015 at 15:11

1 Answer 1

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The reaction of boron trioxide with sulfuric acid and calcium fluoride to obtain boron fluoride is given here.

$$\ce{B2O3 + 3CaF2 + 3H2SO4 ->[\Delta] 2BF3 + 3CaSO4 + 3H2O}$$

Conditions

Boron trioxide react with calcium fluoride and sulfuric acid to produce boron trifluoride, calcium sulfate and water. Sulfuric acid - concentrated solution.

The reaction will be easy to understand if you break the reaction into two reaction.

Babounet says:-

$\ce{CaF2 + H2SO4}$ will give you $\ce{HF}$ so why bother if you add directly the $\ce{HF}$ instead ?

Indeed $\ce{CaF2 + H2SO4}$ will indeed yield $\ce{HF}$ along with calcium sulfate as indicated here.

$$\ce{CaF2 + H2SO4 ->[\Delta] 2HF + CaSO4}$$

This $\ce{HF}$ will now react with boron trioxide to yield boron trifluoride as indicated here.

$$\ce{B2O3 + 6HF → 2BF3 + 3H2O}$$

Thus the products are boron trifluoride, calcium sulfate and water and not calcium bisulfate.

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