Bonding - Part 2 Flashcards
4 things about London forces that should come to mind when I see one?
London forces are the ONLY forces which occur in non polar molecules OR noble gases (because they exist as single atoms)
London forces are present in all molecules
London forces are temporary dipoles (IDIDS: instantaneous dipole induced dipole) - they arise as a result of a temporary dipole inducing a complementary dipole in an adjacent molecule.
What does the strength of the London force depend on?
- number of electrons + shape of molecule
- some shapes reduce the accessibility of electrons to neighbouring molecules
3 things about permanent dipoles including an example?
Only present in polar molecules
Molecules with permanent dipoles usually have higher melting and boiling points than non polar molecules because non polar molecules have only temporary dipoles whereas polar molecules have temporary dipoles AND permanent dipoles.
Eg. H-F ~~~~~H-F
The dash is the covalent bond which joins hydrogen and fluorine. The wavy dash is the intermolecular force (in this case a permanent dipole) between the partially negative fluorine and partially positive hydrogen.In this particular example it’s actually a HYDROGEN BOND!
3 things about hydrogen bonds
If there is a hydrogen bond there is usually a permanent dipole-dipole force.
Hydrogen bonds always from between H and another electronegative atom which is either F, N or O. (it is attracted to a lone pair of electrons on the atom)
Strongest intermolecular force
How to draw a hydrogen bond [4]
- delta +/-
- lone pair of e- on N, O or F
- line like IIIIIIII
- 180 degree H-O-H bond angle
Effect of branching in the carbon chain on the boiling temperatures of alkanes.
- long straight chain alkanes have a higher boiling point (than spherical shaped branched alkanes)
- because larger surface area of contact between molecules for london forces to form.
Trends in boiling points of alkanes with increasing chain length
Increasing chain length means more carbons and hydrogen’s so more electrons. This means there are more temporary dipoles (London forces) which are stronger.
Why does ice have a lower density than water?
- orientation of hydrogen bonds pushes the water molecules further apart
- to make a hexagonal lattice
- which is more spread out
Dipole definition
A pair of equal and oppositely charged poles separated by a distance
Why do alcohols and alkanes with a similar number of electrons have different boiling points?
Eg. Methane and methanol.
Although they have a similar number of electrons and London forces, the only additional factor is that methanol has an oxygen. This oxygen produces hydrogen bonds in the methanol which are much stronger than London forces therefore more energy is required to break them hence alcohols have higher boiling points.
Higher boiling point of alcohols means :
They have a lower volatility than alkanes
Why are alcohols and sugars soluble in water?
Hydrogen bonds form between the water molecule and the molecule being dissolved.
Polarised meaning
formation of a bond between two polar atoms (unequal distribution of electrons)
When is a compound purely covalent?
When it contains elements of similar electronegativities and hence there is a small difference in electronegativity.
When is a compound ionic?
when it contains elements with very different electronegativities and hence there is a large difference in electronegativity. (they become not dipoles but full ions)
a permanent dipole is…
a type of covalent bond - ‘polar covalent bond’. Forms when elements have different electronegativities.
arrangement of electrons in a dipole?
unequal distribution of electrons producing a charge separation
what type of molecule is definitely not polar?
symmetrical molecules (eg. Carbon Dioxide). [all bonds must be identical + no lone pairs]
testing if a liquid is polar
place a charged rod near a jet of the liquid. If the liquid is polar the jet will be deflected (the stronger the dipole, the larger the jet deflection).
Polarity in molecules
(+ 2 examples that are not polar
+ 1 example that is polar)
some molecules will have polar covalent bonds but they won’t be polar overall because their bond polarities cancel out due to the shape of the molecule.. (Eg. BCl3 - trigonal planar shape means even though it has polar bonds, these polarities are cancelled out and overall it isn’t a polar molecule)
Eg. CCl4
NH3 is an example where it does have polar bonds and the overall molecule is STILL POLAR because its pryamidal shape doesn’t cause bond polarities to cancel.
What does the strength of an instantaneous dipole-induced dipole depend on?
because they are instantaneous, they depend on the number of electrons in the molecule, and its shape at that particular time
main factor effecting strength of london force
the more electrons in the molecule, the higher chance of temporary dipoles forming. The more electrons means stronger london forces
4 key bonds in order of decreasing strength
ionic bonds > hydrogen bonds > permanent dipole-dipole > london forces (instantaneous dipoles)
How should you draw a hydrogen bond?
- lone pair on the O, F or N
- delta +/- on appropriate molecule
- should have a 180 degree bond angle with a bond in one of the molecules its bonded to
how many h bonds can water form
2 per molecule because the electronegative oxygen atom has two lone pairs of electrons on it.
so water can form stronger hydrogen bonds and has a high bp
Trend in boiling points down hydrogen halides and why?
Hint - what does the graph look like
From HF to HCl there is a big drop in boiling point because, since fluorine is highly electronegative it forms hydrogen forces with the hydrogen, which are very strong and require more energy to break.
From HCl down the group boiling point increases because as you go down there are more electrons which means increasing london forces
Graph: down loads then gradually up
boiling points of noble gases
all very low - small atoms - limited movement of electrons - less london forces
metallic bonding
strong electrostatic attraction between metal ions and delocalised electrons
What is solubility of a solute in a solvent?
a balance of energy required to:
- break bonds in the solute AND solvent
AGAINST THE energy GIVEN OUT
-when new bonds BETWEEN the solute and the solvent form
how ionic substances dissolve in water
ionic bonds in the lattice break up and new bonds between the metal ions and water molecules form.
negative ions attracted to delta + hydrogens positive ions attracted to delta negative oxygens
ionic substances dissolving in water - hydration enthalpy fact
the higher the charge density the greater the hydration enthalpy as the ions attract the water molecules more strongly.
trend in alcohol solubility
the longer the carbon chain, the less soluble
smaller alcohols can form hydrogen bonds with water more easily
two examples of compounds that are insoluble in water and why?
polar halogenoalkanes
non polar hexane
cannot form h bonds with water nor break hydrogen bonds already existing in water
When do compounds generally dissolve?
If they have similar intermolecular forces to those in the solvent.
Name a useful solvent
propanone - polar and non-polar characteristics means it can form london forces with non-polar compounds through its CH3 group and it can also form hydrogen bonds with polar compounds through its C=O bond
From what length are alcohols insoluble?
Which are partially soluble?
Up to which length are alcohols soluble?
insoluble from 5 carbon chain
butanol partially soluble
alcohols miscible with water up to and including 3 carbons
When will larger alcohols (with longer carbon chains) dissolve more?
if they have more OH groups - eg. glucose has a long chain but equally lots of OH groups so it is soluble in water
Why does a lone pair cause polarity in a molecule
it is an area of higher density of electrons increases the strength of the dipole
