Dr Bob

Question

Can you please confirm for me that only NH3 has a boiling point that does not fit the general trend from these choices
NH3
PH3
SbH3
AsH3
many thanks andy
I am picking NH3 because of the reason that the larger the compound the greater the boiling point
Am I right with my choice???

Answer 1

You are right with your choice but wrong in the reasoning. If you looked up the boiling point of each, you would see that the boiling point changes from higher to lower as we go from SbH3, AsH3, PH3, etc and that is due to the decrease in size and but mostly because of decrease in mass (from Sb to P) BUT, NH3 is much higher than it would be expected to be. The reason is that H bonding makes it much harder for NH3 molecules to break apart to form vapor. The same trend occurs for H2Te, H3Se, H2S, H2O. By all estimates, H2O should be a gas at room temperature but it's MUCH higher than expected. After posting, let me look for a chart that will show you in a graph how it looks.

Answer 2

That would be great if i could see the chart thanks Andy

Answer 3

I found a site for the H2O---H2Te series.
Notice that if the trend continued, as we might expect, from H2Te to H2Se to H2S, to H2O, we would expect water to boil about -65 C or so BUT look at the pink line as see that it is MUCH higher than expected (in fact about 165 degrees higher) to boil at +100. That is attributed to hydrogen bonding. You have this same effect in the NH3 series (group 15), the H2O series (group 15) and the HF series (group 17).
http://www.elmhurst.edu/~chm/vchembook/163boilingpt.html

Answer 4

thank you so much for that wonderful chart
Andy