What Causes Water to Boil and What Bonds Need to Be Broken?

[bjoyful]

Homework Statement

QUESTION: Methane (CH4) is a gas at room temperature and pressure while water is a liquid. In both molecules hydrogen atoms are bonded to a non-metal element. Water also has a much higher heat of vaporization than methane even though the molecular weights of these two molecules aren't that different.

I am to:
-account for these differences by describing the bonding between atoms within these two molecules, and the bonding and intermolecular interactions between molecules in each.
-indicate how electronegativity differences between the elements, and geometry of the molecules, are both important.
-explain what hydrogen bonding is.
-indicate what is happening on the molecular level when water boils, and what kind of bonds or interactions need to be broken.

The Attempt at a Solution

This question contains many little ones! I believe that hydrogen bonding involves dipole forces. I assume that in order to indicate how electronegativity differs between the elements, I would use the elements of methane (CH4). I really need a lot of help on this. Hopefully it makes sense to someone out there!
Thanks!

 

[BlindSpot]

At standard pressure, the molecules of a liquid are much closer together than those in a gas (just look at their relative densities). For some reason, water molecules are much more comfortable with this than are methane molecules. These two have similar molecular masses so you probably want to start some place else when figuring out why methane exists as a gas at standard temperature and pressure (STP) but water is a liquid. Your instincts here are right so follow them as far as your textbook. Read up on hydrogen bonding and ask yourself how it applies to both CH4 and H2O.

It may be useful to answer the questions in an order diffent from that presented above. Try starting with the third one: "explain what hydrogen bonding is"

Once you have done this, consider the last question. Consider that water - whether it is solid, liquid, or gas - is always H2O just as methane remains CH4. This should help you determine what kinds of bonds are being broken when each transitions from one phase to another.

Now tackle the first question. H bonding plays an important role in the interactions between molecules of one species but not the other. Why is that? What is it about the shape (geometry) of each that determines if H bonding is prominent or not? What impact does H bonding have on the boiling point?

Finally, answer the question "indicate how electronegativity differences between the elements, and geometry of the molecules, are both important." You'll notice in your previous answer that geometry is important. Do you know why a molecule of water is shaped the way that it is?

 

[bjoyful]

Thank you so much for your explanation and suggestion to rearrange the order of questions. I have found that helpful. I have been working on the first question about explaining hydrogen bonding. I have come up with a paragraph attempting to explain hydrogen bonding, but I feel like something is missing…am I really explaining hydrogen bonding or merely “stating facts.”

My paragraph...

Hydrogen bonding is when an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule interact. Compared with covalent bonds, hydrogen bonds’ have less strength; however, hydrogen bonds are considerably strong compared with other intermolecular forces. Hydrogen bonding is when extremely strong polar molecular interactions take place. Because the hydrogen atom is usually associated with strong polar groups that exhibit very strong molecular interaction, hydrogen is used in the term explaining this process. Hydrogen bonding applies to CH4 and H2O, because of how the hydrogen in the molecule exists.

 

[klile82]

Your explanation of H bonding is right on the money but I think you are missing an important concept. Hydrogen bonding occures between + and - poles of polar molecules (this is analogous to the attraction between + and - poles on a magnet). Hydrogen bonding is an important force in water because water molecules are highly polar but methane molecules are highly non-polar. Do you know why this is?

 

[bjoyful]

thanks for reading over and commenting on my paragraph on H bonding. In response to your question, does it have something to do with intermolecular forces??

 

[bjoyful]

I have been working through the questions and applying what Blindspot wisely advised! So on to the next one (which was - indicate what is happening on the molecular level when water boils, and what kind of bonds or interactions need to be broken) --

Hydrogen bonds must be broken in order for water to boil. So the bonds that need to be broken in order for boiling to occur would be the hydrogen and oxygen bonds of water (H2O)? Water will start boiling when the kinetic energy of the water molecules is great enough to enable them to break away from each other.

I have read and googled on this and... Am I including the correct information as well as all the stages? I am scared that I won’t actually answer the question directly, so I need a *little* help?!