OZ - Hydrogen bonding Flashcards
What elements have to bond for there to be hydrogen bonding?
F-H
N-H
O-H
Hydrogen bonding only happens if what?
Hydrogen is covalently bonded to fluorine, nitrogen or oxygen.
What is the strongest type of intermolecular force?
Hydrogen bonding
Why does hydrogen bonding occur?
Because fluorine, nitrogen and oxygen are very electronegative, so they draw the bonding electrons away from the hydrogen atom.
The bond is polarised, and the hydrogen has such a high charge density because it is so small, that the hydrogen atoms form weak bonds with lone pairs of electrons on the fluorine, nitrogen or oxygen atoms of other molecules.
How do hydrogen bonds explain why ice is less dense than water?
In ice, the water molecules are arranged so that there is the max number of hydrogen bonds - the lattice structure formed in this way ‘wastes’ lots of space.
As the ice melts, some of the hydrogen bonds are broken and the lattice breaks down - allowing molecules to ‘fill’ the spaces.
This effect means ice is much less dense than water - which is why ice floats.
What are common properties of substances that form hydrogen bonds?
High mp/bp because a lot of energy is required to overcome the intermolecular bonds.
Soluble in water because they can form hydrogen bonds with the water molecules, allowing them to mix and dissolve.
Which substances have the highest BP/MP’s?
Hydrides of nitrogen, oxygen and fluorine because of the extra energy needed to break the hydrogen bonds.
Why do molecules that form hydrogen bonds have high mp/bp?
Because a lot of energy is required to overcome the intermolecular bonds.
Why are molecules that form hydrogen bonds soluble in water?
Because they can form hydrogen bonds with the water molecules, allowing them to mix and dissolve.
Describe an experiment that shows the strength of different intermolecular bonds?
Water, ethanol and hexane.
- Wrap a piece of filter paper around the bulb of a thermometer and dip it into one of the liquids.
- Wait until the temp has stabilised and record it (initial temp).
- Remove the thermometer and saturated filter paper from the liquid and leave it at room temp. After 5 mins, record the temp again. Calculate the temp change.
- Repeat method for each of the three liquids.
Explain the results of the experiment used to show the strengths of different IM bonds
The weaker the IM bonds in a liquid, the faster its rate of evaporation will be, so a greater change in temp will be observed.
How many hydrogen bonds does water form per molecule?
2 strong hydrogen bonds.
How many hydrogen bonds does ethanol form per molecule?
1 per molecule.
How many hydrogen bonds does hexane form per molecule?
Non-polar so only forms very weak instantaneous dipole-induced dipole bonds.
Which of the group 7 hydrides has the highest bp?
Hydrogen fluoride.
