A level examiners love this topic. But they can be quite picky about the difference between a bond and an intermolecular force - which makes it all the stranger that the strongest intermolecular forces are called Hydrogen bonds.
You should know:
- that Hydrogen bonds can only form between covalent molecules in which a Hydrogen atom is bonded to either a Fluorine, Oxygen or Nitrogen atom.
- that this is mostly due the large difference in electronegativity (but that atomic radius has an effect or else there would be Hydrogen bonding in H-Cl which is also a very polar bond)
- that the strength of Hydrogen bonds decreases from H-F to H-O to H-N bonds.
- that the more Hydrogen bonds there are in a molecule the higher the boiling point of that substance will be.
- that substances that Hydrogen bond are likely to be soluble in water unless it is a long molecule with only one Hydrogen-bonding site.
- the evidence for Hydrogen bonding. (See below)
- the consequence of Hydrogen bonding on the density of ice. (See below).
- how to describe the formation of Hydrogen bonds.
- how to draw Hydrogen bonds.
The evidence for Hydrogen bondsEdit
The above graph shows the boiling points of the Hydrides of the elements in Groups 4,5,6 and 7 - the compounds formed when the element combines directly with Hydrogen.
The bottom (dark blue) line shows the expected trend of an increase of boiling point down the group due to an increase in the strength of Van der Waals forces with molecular mass.
This trend would be found in all groups except the Hydrides of Groups 5,6 and 7 where there is a clear increase in boiling point for NH3, H2O and HF.
The only explanation for this is that these molecules contain a different type of force.
You should be able to sketch this "tick-shape" graph and use it to show the existence of Hydrogen bonding.
You do not have to know the actual boiling point of any of these compounds (except water).
Drawing Hydrogen bondsEdit
When asked to draw a Hydrogen bond you are generally expected to label the δ- and δ+ atoms (partial charges). Note: the Hydrogen atoms are always δ+.
You will be asked to draw any lone-pairs of electrons. Obviously, these are on the N, O or F atom which will generally have 1, 2 or 3 lone-pairs respectively.
The examiner generally asks for a dashed line to represent the Hydrogen bond - from the lone pair to the Hydrogen.
Explaining Hydrogen bondingEdit
Very polarising atoms like N, O and F pull the bond-pair so far from the Hydrogen atom that its 1s orbital is almost empty.
This means that there is room to accept electron-density from the lone-pair.
This produces an intermolecular attraction similar to a co-ordinate bond.
The fact that ice floats is unusual.
Most substances become denser as they are cooled. Generally, this is true of water but ice floats because it is less dense than cold water, which is at its densest at 4oC.
You should be able to explain that when ice crystalises it freezes the water molecules into position at the perfect distance apart from each other to maximise the Hydrogen bonds.
Given enough energy to melt, the water molecules begin to move and can actually be slightly closer together now that they are not forced to stay in the same position.(See above).
This lowers density.
At higher temperatures the molecules will be moving faster and so will be further apart, lowering the density again - hence the peak density at 4oC.
Other Hydrogen Bonding Exam questions about your laundry. No, really!Edit
- At A2 you study Condensation Polymers - a closely related topic is proteins and you also learn that the secondary structure of proteins is largely maintained by Hydrogen bonds within the chain.
- So, ironing works because the heat from the iron breaks hydrogen bonds holding the fabric in the creased form and allows them to reform (temporarily, at least) in the pressed form.
- Equally, to destroy the jumper your Grandmother knits you for Christmas simply wash it and hang it while still very wet. The weight will overcome the hydrogen bonds holding the jumper in the original form, allowing it to stretch until the arms are long enough for a chimp. It will then form new Hydrogen bonds keeping it like this.
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