Problem 1

Nitrogen trifluoride is a surprisingly stable compound that was first prepared by the melt electrolysis of a mixture of ammonium fluoride and hydrogen fluoride. (a) At which electrode does

\ce{NF3}

form? Write the balanced half-reaction for its formation. (b) The related compounds

\ce{NH2F}

and

\ce{NHF2}

are very unstable and form as side products. Which of

\ce{NF3}

\ce{NHF2}

\ce{NH2F}

is expected to condense at the lowest temperature? (c) The N–F bond lengths in these molecules are 136, 140 and 142 pm; assign them using a simple electrostatic (partial-charge) model. (d) When

\ce{NHF2}

is bubbled through a solution of KF in HF, a binary nitrogen–fluorine compound is obtained as a mixture of two geometric isomers; write a balanced equation for its formation. (e)

\ce{NF4+}

salts form from

\ce{NF3}

and

\ce{F2}

in the presence of a suitable reagent — propose one and write the equation. (f)

\ce{NF4+}

hydrolyzes to

\ce{NF3}

and

\ce{O2}

, but less

\ce{O2}

than expected is often obtained; write the hydrolysis equation and a possible side reaction lowering the

\ce{O2}

\ce{NF3}

ratio. (g) A tetrafluoroammonium salt (a candidate solid rocket fuel) contains 65.6% fluorine by mass, all released as

\ce{NF3}

and

\ce{F2}

on heating, with

n(\ce{F2}) = 2.5\,n(\ce{NF3})

. Determine the formula of the salt.

Step 4 of 6: Synthesis of the

\ce{NF4+}

ion

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\ce{NF3 + F2 + SbF5 -> NF4+ + SbF6-}

Analysis

A strong fluoride-ion acceptor (Lewis acid) such as $\ce{SbF5}$ removes $\ce{F-}$ , allowing $\ce{F2}$ to fluorinate $\ce{NF3}$ to the $\ce{NF4+}$ cation; $\ce{AsF5}$ or $\ce{BF3}$ work similarly.