Water boils at 373 K and hydrogen sulphide at 213 K, although an H₂S molecule is almost twice as…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Water boils at and hydrogen sulphide at , although an molecule is almost twice as heavy. Find the difference between the two boiling points, and explain it.
Need a reminder? The key ideaShow
A hydrogen bond is the attraction between a hydrogen atom joined to fluorine, oxygen or nitrogen and a lone pair on another such atom. Intermolecular hydrogen bonds join molecules and raise boiling points; intramolecular ones form within one molecule. Water boils at , far above , and .
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The difference. .
Step 2. The reason. Oxygen is small and highly electronegative, so water molecules hold each other by hydrogen bonds. Sulphur is larger and less electronegative, so forms almost none. Breaking the hydrogen bonds needs extra energy.
Answer: water boils higher, because of the hydrogen bonds between its molecules.
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