Bonding - Part 2

Question 1

4 things about London forces that should come to mind when I see one?

Answer 1

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.

Question 2

What does the strength of the London force depend on?

Answer 2

• number of electrons + shape of molecule

- some shapes reduce the accessibility of electrons to neighbouring molecules

Question 3

3 things about permanent dipoles including an example?

Answer 3

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!

Question 4

3 things about hydrogen bonds

Answer 4

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

Question 5

How to draw a hydrogen bond [4]

Answer 5

• delta +/-
• lone pair of e- on N, O or F
• line like IIIIIIII
• 180 degree H-O-H bond angle

Question 7

Effect of branching in the carbon chain on the boiling temperatures of alkanes.

Answer 7

• 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.

Question 10

Trends in boiling points of alkanes with increasing chain length

Answer 10

Increasing chain length means more carbons and hydrogen’s so more electrons. This means there are more temporary dipoles (London forces) which are stronger.

Question 12

Why does ice have a lower density than water?

Answer 12

• orientation of hydrogen bonds pushes the water molecules further apart
• to make a hexagonal lattice
• which is more spread out

Question 13

Dipole definition

Answer 13

A pair of equal and oppositely charged poles separated by a distance

Question 14

Why do alcohols and alkanes with a similar number of electrons have different boiling points?

Answer 14

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.

Question 15

Higher boiling point of alcohols means :

Answer 15

They have a lower volatility than alkanes

Question 16

Why are alcohols and sugars soluble in water?

Answer 16

Hydrogen bonds form between the water molecule and the molecule being dissolved.

Question 17

Polarised meaning

Answer 17

formation of a bond between two polar atoms (unequal distribution of electrons)

Question 18

When is a compound purely covalent?

Answer 18

When it contains elements of similar electronegativities and hence there is a small difference in electronegativity.

Question 19

When is a compound ionic?

Answer 19

when it contains elements with very different electronegativities and hence there is a large difference in electronegativity. (they become not dipoles but full ions)

Question 20

a permanent dipole is…

Answer 20

a type of covalent bond - ‘polar covalent bond’. Forms when elements have different electronegativities.

Question 21

arrangement of electrons in a dipole?

Answer 21

unequal distribution of electrons producing a charge separation

Question 22

what type of molecule is definitely not polar?

Answer 22

symmetrical molecules (eg. Carbon Dioxide).
[all bonds must be identical + no lone pairs]

Question 23

testing if a liquid is polar

Answer 23

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).

Question 24

Polarity in molecules
(+ 2 examples that are not polar
+ 1 example that is polar)

Answer 24

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.

Question 25

What does the strength of an instantaneous dipole-induced dipole depend on?

Answer 25

because they are instantaneous, they depend on the number of electrons in the molecule, and its shape at that particular time

Question 26

main factor effecting strength of london force

Answer 26

the more electrons in the molecule, the higher chance of temporary dipoles forming. The more electrons means stronger london forces

Question 27

4 key bonds in order of decreasing strength

Answer 27

ionic bonds > hydrogen bonds > permanent dipole-dipole > london forces (instantaneous dipoles)

Question 28

How should you draw a hydrogen bond?

Answer 28

• 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

Question 29

how many h bonds can water form

Answer 29

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

Question 30

Trend in boiling points down hydrogen halides and why?

Hint - what does the graph look like

Answer 30

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

Question 31

boiling points of noble gases

Answer 31

all very low - small atoms - limited movement of electrons - less london forces

Question 32

metallic bonding

Answer 32

strong electrostatic attraction between metal ions and delocalised electrons

Question 33

What is solubility of a solute in a solvent?

Answer 33

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

Question 34

how ionic substances dissolve in water

Answer 34

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

Question 35

ionic substances dissolving in water - hydration enthalpy fact

Answer 35

the higher the charge density the greater the hydration enthalpy as the ions attract the water molecules more strongly.

Question 36

trend in alcohol solubility

Answer 36

the longer the carbon chain, the less soluble

smaller alcohols can form hydrogen bonds with water more easily

Question 37

two examples of compounds that are insoluble in water and why?

Answer 37

polar halogenoalkanes
non polar hexane

cannot form h bonds with water nor break hydrogen bonds already existing in water

Question 38

When do compounds generally dissolve?

Answer 38

If they have similar intermolecular forces to those in the solvent.

Question 39

Name a useful solvent

Answer 39

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

Question 40

From what length are alcohols insoluble?
Which are partially soluble?
Up to which length are alcohols soluble?

Answer 40

insoluble from 5 carbon chain
butanol partially soluble
alcohols miscible with water up to and including 3 carbons

Question 41

When will larger alcohols (with longer carbon chains) dissolve more?

Answer 41

if they have more OH groups - eg. glucose has a long chain but equally lots of OH groups so it is soluble in water

Question 42

Why does a lone pair cause polarity in a molecule

Answer 42

it is an area of higher density of electrons increases the strength of the dipole