Explain why the boiling temperature of PH3 is greater than that of CH4.

How do you know that PH3 has dipole-dipole and the other one doesn't just from a question like this and no other information given?

Reply 1

charco

Study Forum Helper

Original post by yaya1

How do you know that PH3 has dipole-dipole and the other one doesn't just from a question like this and no other information given?

You should be able to draw out the Lewis structure of each molecule and from that derive the shape of the molecule. Once you do that you see that PH₃ is trigonal pyramidal, while CH₄ is tetrahedral.

From the shapes you get the polarity.

Reply 2

yaya1

So does trigonal pyramidal have more dipole than tetrahedral is there trend?

Reply 3

charco

Study Forum Helper

Original post by yaya1

So does trigonal pyramidal have more dipole than tetrahedral is there trend?

Trigonal pyramidal structures are not totally symmetrical. They are symmetrical about a vertical centra of rotation, but they have a non-symmetrical horizontal plane.

Put simply, there are three groups on one side of the central atom and none on the other. This makes them polar if the attached groups are of different electronegativity to the central atom.

In this case the phosphorus and the hydrogen have different electronegativity and the hydorgen atoms are relatively positive.

This means that at the bottom of the molecule there is a region of relative positive charge while at the top there is a region of relative negative charge. This charge separation is called a molecular dipole.

In the case of CH₄ all of the individula dipoles cancel out (although there is very little electronegativelty difference between C and H anyway), and the molecule does not have an overall dipole.