OZ6: Haloalkanes + Polarity

Question 1

What is a haloalkane?

Answer 1

An alkane with at least one hydrogen atom replaced by a halogen

Question 2

How do you name a haloalkane?

Answer 2

• Longest part of the carbon chain = last part of the compound’s name
• Add “chloro-“, “bromo-“, “iodo-“ or “fluoro-“ depending on which halogen is bonded to it
  (If more than one list in alphabetical order)
• Show the halogen position by adding numbers
• If more than one of the same halogen, use “di” for 2, “tri” for 3, “tetra” for 4

Question 3

How does boiling point change down Group 7?

Answer 3

Increases down the group

Question 4

Why do larger molecules have higher boiling points than smaller ones?

Answer 4

• Larger surface area
• Bigger exposed electron cloud
• Stronger instantaneous dipole-induced dipole bonds

Question 5

Why does boiling point increase down Group 7?

Answer 5

• Atomic radius increases
• Number of electron shells increase
• Larger electron clouds
• Stronger instantaneous dipole-induced dipole bonds=harder to break

Question 6

Define “electronegativity”

Answer 6

The measure of the ability of an atom in a molecule to attract electrons in a covalent bond to itself

Question 7

How is electronegativity measured?

Answer 7

• Using the Pauling Scale

- The higher the electronegativity value, the more electronegative the element

Question 8

How does electronegativity change across a period?

Answer 8

Increases from left to right

Question 9

Why does electronegativity increase across a period?

Answer 9

• Atomic cores are attracted to shared electrons in covalent bonds
• The two atoms bonded together have different sized cores
• The core of a smaller atom is closer to the shared electron and so exerts a stronger pull on them and is more electronegative
• Atomic radius decreases across a period

Question 10

How does electronegativity change down a group?

Answer 10

Decreases down the group

Question 11

How does electronegativity decrease down a group?

Answer 11

• Atomic cores are attracted to shared electrons in covalent bonds
• The two atoms bonded together have different core charges
• Shared electrons are attracted more strongly by the atom with the greater core charge
• Core charge decreases down the group

Question 12

Why are molecules with similar/identical electronegativities non polar?

Answer 12

• Electrons sit midway between the two nuclei, at equal distance between them
• Because they are equally attracted to both nuclei

Question 13

Why are molecules with different electronegativities polar?

Answer 13

• Bonding electrons are pulled more towards the more electronegative atom
• The electrons are spread unevenly so each atom has a partial charge

Question 14

Define “dipole”

Answer 14

Difference in charge between two atoms caused by a shift in electron density in the bond

Question 15

How does the difference in electronegativity between the two atoms affect the polarity of the bond?

Answer 15

The greater the difference in electronegativity, the greater the shift in charge, so the more polar the bond

Question 16

If the difference in the electronegativity of two atoms according to the Pauling Scale is more than x, then they will form a polar bond. What is x?

Answer 16

0.4

Question 17

What does a molecule need to have in order to be polar?

Answer 17

A permanent charge across the molecule

Question 18

How must the polar bonds be orientated in a molecule for it to be polar?

Answer 18

Pointing in the same direction

Question 19

How must the polar bonds be orientated in a molecule for it not to be polar and why?

Answer 19

• Pointing in the opposite direction

- The charges cancel each other out so there is no permanent charge across the molecule

Question 20

What type of molecules are always non polar?

Answer 20

Symmetrical molecules

Question 21

What is a permanent dipole?

Answer 21

Two atoms in a bond have substantially different electronegativities

Question 22

What is an instantaneous dipole?

Answer 22

• Electrons in charge clouds are always moving in constant motion
• Electrons in an atom at a particular instant are not evenly distributed
• One end has a greater negative charge than the other
  at one instant

Question 23

What is an induced dipole?

Answer 23

• An initially unpolarised molecule ends up next to a dipole
• Dipole repels/attracts electrons in unpolarised molecules, inducing a dipole in it - two dipoles then attract
• If the delta positive end faces the unpolarised molecule, electrons will move to the closest end of the molecule polarising it

Question 24

What are intermolecular bonds?

Answer 24

Forces between molecules

Question 25

Why do stronger intermolecular bonds have higher melting points?

Answer 25

More energy needed to break them

Question 26

Name 3 types of intermolecular bonds?

Answer 26

• Instantaneous dipole - induced dipole bonds
• Hydrogen bonding
• Permanent dipole - permanent dipole bonds

Question 27

Where are instantaneous dipole - induced dipole bonds present?

Answer 27

In between all molecules

Question 28

What are permanent dipole - permanent dipole bonds?

Answer 28

Weak electrostatic forces of attraction between polar molecules

Question 29

What is the relationship between the length of a carbon chain and the alkane’s boiling point?

Answer 29

The longer the carbon chain the higher the hydrocarbon boiling point

Question 30

Why do longer carbon chained alkanes have a higher boiling point?

Answer 30

• Stronger instantaneous dipole - induced dipole bonds
• Because more molecular surface area leads to a bigger exposed electron cloud
• More energy needed to break these bonds

Question 31

What is the relationship between the “branchedness” of an alkane and its boiling point?

Answer 31

• The more branched the alkanes the lower the boiling point

Question 32

Why do more branched alkanes have a lower boiling point?

Answer 32

• Branched alkanes cannot pack as closely together
• So there are less points of contact between molecules so less instantaneous dipole-induced dipole bonds
• Molecular surface area of branched alkanes is also smaller so there are fewer instantaneous dipole-induced dipole bonds

Question 33

Describe a method for comparing strengths of intermolecular bonds between substances.

Answer 33

• Wrap a piece of filter paper around a thermometer’s bulb
• Dip it in one of the liquids to be tested
• Record the initial temperature
• Remove the thermometer and saturated filter paper from the liquid
• Leave them at room temperature and record the temperature again
• After 5 minutes, calculate the temperature change
• The greater the change in 5 mins, the faster the rate of evaporation, so the weaker the intermolecular bonds in the liquid