On the right is a graph showing the boiling points of the group 4,5,6 and 7 hydrides. For the most part the trend is exactly the same in all four groups except that the boiling point of the compound of hydrogen with the first element in group 4 is abnormally low.
Explain why from your knowledge of intermolecular bonds and non-polar molecules?
The polarity of the bonds formed when hydrogen bonds with oxygen, nitrogen or fluorine is much greater than the polarity of the bond formed between hydrogen and carbon.
Such strong poles create relatively strong intermolecular forces of attraction known ashydrogen bonding. Hydrogen bonding is an especially strong form of dipole-dipole bonding that forms when a hydrogen is bonded to an oxygen, nitrogen or fluorine atom. The animation on the right shows how the dipoles from different molecules interact, in other words how the hydrogen from one molecule is attracted to the lone pairs on the neighbouring molecule to form hydrogen bonds. Now looking at the graph of boiling points, above, you would think that hydrogen fluoride (HF) would have a much higher boiling temperature than water, since its dipoles are so much greater considering the difference in electronegativity between fluorine and hydrogen. This can be explained by the fact that a water molecule has two hydrogen atoms that are slightly positive and two lone pairs, so it can form greater interactions (four hydrogen bonds per molecule) and networks within the liquid and solid states. HF on the other hand can only form one other strong interaction and so its ability to form cohesive networks is limited.
View the video on the right to get a better understanding of the polarity of the ammonia and water molecules and how each molecules interacts with other molecules in the liquid and solid states.
Now let's list a few properties of water and see how hydrogen bonding can explain these.
- Water is a liquid with very high surface tension. Or in chemical terms it is cohesive and adhesive. Cohesive because molecules attach to each other and adhesive because they also stick to other substances. This is key for a very important biological phenomenon called capillary action.
- For a small molecule it has a very high boiling and melting temperature. The strength of the hydrogen bonds between water molecules also means that it has a high boiling point. Only at a high 100℃ is there enough energy present to break the cohesive bonds between the water molecules and form a gas.
- It dissolves most ionic and polar molecules. Consider the animation on the right. The small charged water particles are attracted to the charged particles of solutes and breaks apart the crystal structure. This is crucial in providing nutrients in a form readily absorbed by plants and animals.
- Water is the only molecule of its size that can form pools of liquid on the surface of the earth. Once again this is due to strong intermolecular (H-bonding) between water molecules.
- For such a small molecule water has a very high specific heat capacity. This is the amount of energy required to raise a given mass of substance by one degree Celsius. In actual fact it is 4.18 J/g/oC, very high for such a small molecule. So, it takes 4.18 Joules of energy to raise one gram of water by one degree Celsius. Pretty good for such a small little molecule and yes it is due to strong intermolecular bonding, H-bonding.
- Water is the only substance that expands when cooled and becomes less dense. Ice is less dense than liquid water and this is why icebergs float. This is again due to the strong H-bonding that pulls the molecules into a rigid matrix that is full of space to create a low density solid crystal structure. If ice was more dense than water it would sink, and cause the freezing of entire lakes and oceans. You see, by ice floating on the surface it actually insulates the water beneath the surface and prevents it from freezing. If this didn't happen ocean life would not be possible as all bodies of water would freeze. This is the reason why sea life exists in the waters under the frozen polar regions.
