Textbook question 2.25

[Paige Lee 1A]

Which bond is longer: (a) the CN bond in HCN or in H3CNH2? (b) The NF bond in NF3 or the PF bond in PF3?

For part A, I got that the bond in H3CNH2 is longer, is this the correct answer?
For part B, I got that the NF bond in NF3 and the PF bond in PF3 were the same length--is this correct?

[Baoying Li 1B]

Which bond is longer: (a) the CN bond in HCN or in H3CNH2? (b) The NF bond in NF3 or the PF bond in PF3?

For part A, I got that the bond in H3CNH2 is longer, is this the correct answer?
For part B, I got that the NF bond in NF3 and the PF bond in PF3 were the same length--is this correct?

You got the correct answer for part A since the single bond of CN in HCN is longer than the three bond of CN in H3CNH2.
However, for part B, the PF bond in Pf3 would be expected to be longer because the atomic radius of phosphorus is larger than the nitrogen. Bond distance increases with atomic size.

[VPatankar_2L]

I think for the first question, the answer is that the CN bond in H3CNH2 is longer than the CN bond in HCN because Carbon and Nitrogen share a triple bond in HCN while they only share a single bond in H3CNH2. Single bonds are longer than triple bonds because less electrons are being shared.
For part b, although it may seem that the Lewis structure for PF3 and NF3 are the same, Phosphorus has a greater atomic radius which means that the atoms will be more spread out.