[MasterMatt]

Hello,
I need help with a prelab for my upcoming inorganic lab. Here is the procedure:

Boron Triflouride as a Lewis Acid:

Boil 80 ml of distilled water. Transfer 50 ml of the hot water into a 100 ml beaker, with a stirring bar and, continuing heating, add approx. 5 g of NH4BF4, if your starting material is not pure, adjust accordingly. When all the NH4BF4 has dissolved, add approx. 5 g KCl dissolved in 20 ml of water. Heat the solution until clear and then let it cool for 5 min. at room temperature. Place the solution in an ice bath and let the product crystallize out (30 min. to 1 hr.). Filter the white crystals, dry them by setting them on top of the oven for 15 min. and calculate the yield.
Place approximately 0.5 g of dry product in a test tube and heat it in a Bunsen flame. What are the white fumes? Observe if they colour a flame and their reaction with ammonia gas. Take an IR of the dry product as a KBr pellet.


I need to give chemical equations for all reactions in my prelab. This is not my strong-suit, so if someone could walk me through the logic of coming up with these equations that would be great.  

[Arkcon]

OK, to begin to solve this for yourself, you have to take that text block, read it, remove the chemical names, and start to build the reactions.  You've been told that boron trifluoride is a Lewis acid.  Use the definition to conclude what reactions it might undergo, and try to predict what you'll see. 

When you leave in text like "Take an IR of the dry product as a KBr pellet."  And say you can't write the reaction, I know you haven't tried at all.  Because taking the IR has nothing to do with the question.

[MasterMatt]

OK, to begin to solve this for yourself, you have to take that text block, read it, remove the chemical names, and start to build the reactions.  You've been told that boron trifluoride is a Lewis acid.  Use the definition to conclude what reactions it might undergo, and try to predict what you'll see. 

When you leave in text like "Take an IR of the dry product as a KBr pellet."  And say you can't write the reaction, I know you haven't tried at all.  Because taking the IR has nothing to do with the question.

Alright, thank you for your response, though somewhat demeaning. Anyways, I've given it a try...

NH4BF4 + KCl  -->NH4Cl + KBF4

So from here, how am I going to get BF3? Is water part of the reaction, or just the solvent?

[Sophia7X]

The thermal decomp of KBF4 produces KF and BF3.

[MasterMatt]

The thermal decomp of KBF4 produces KF and BF3.

Great, thanks!

[Sophia7X]

No problem.

By the way, KF as a solid and BF3 as a gas, I think.

[MasterMatt]

No problem.

By the way, KF as a solid and BF3 as a gas, I think.

Thanks. The next question on my prelab seems to imply that BH3 is also formed...do you know anything about that?

[MasterMatt]

Basically what I have now is:

NH4BF4(s) + KCl(s) --> NH4Cl(s) + KBF4(s) --> NH4Cl(s) + KF(s) + BF3(g)