OZ6: Haloalkanes + Polarity Flashcards
What is a haloalkane?
An alkane with at least one hydrogen atom replaced by a halogen
How do you name a haloalkane?
- 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
How does boiling point change down Group 7?
Increases down the group
Why do larger molecules have higher boiling points than smaller ones?
- Larger surface area
- Bigger exposed electron cloud
- Stronger instantaneous dipole-induced dipole bonds
Why does boiling point increase down Group 7?
- Atomic radius increases
- Number of electron shells increase
- Larger electron clouds
- Stronger instantaneous dipole-induced dipole bonds=harder to break
Define “electronegativity”
The measure of the ability of an atom in a molecule to attract electrons in a covalent bond to itself
How is electronegativity measured?
- Using the Pauling Scale
- The higher the electronegativity value, the more electronegative the element
How does electronegativity change across a period?
Increases from left to right
Why does electronegativity increase across a period?
- 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
How does electronegativity change down a group?
Decreases down the group
How does electronegativity decrease down a group?
- 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
Why are molecules with similar/identical electronegativities non polar?
- Electrons sit midway between the two nuclei, at equal distance between them
- Because they are equally attracted to both nuclei
Why are molecules with different electronegativities polar?
- Bonding electrons are pulled more towards the more electronegative atom
- The electrons are spread unevenly so each atom has a partial charge
Define “dipole”
Difference in charge between two atoms caused by a shift in electron density in the bond
How does the difference in electronegativity between the two atoms affect the polarity of the bond?
The greater the difference in electronegativity, the greater the shift in charge, so the more polar the bond
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?
0.4
What does a molecule need to have in order to be polar?
A permanent charge across the molecule
How must the polar bonds be orientated in a molecule for it to be polar?
Pointing in the same direction
How must the polar bonds be orientated in a molecule for it not to be polar and why?
- Pointing in the opposite direction
- The charges cancel each other out so there is no permanent charge across the molecule
What type of molecules are always non polar?
Symmetrical molecules
What is a permanent dipole?
Two atoms in a bond have substantially different electronegativities
What is an instantaneous dipole?
- 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
What is an induced dipole?
- 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
What are intermolecular bonds?
Forces between molecules
Why do stronger intermolecular bonds have higher melting points?
More energy needed to break them
Name 3 types of intermolecular bonds?
- Instantaneous dipole - induced dipole bonds
- Hydrogen bonding
- Permanent dipole - permanent dipole bonds
Where are instantaneous dipole - induced dipole bonds present?
In between all molecules
What are permanent dipole - permanent dipole bonds?
Weak electrostatic forces of attraction between polar molecules
What is the relationship between the length of a carbon chain and the alkane’s boiling point?
The longer the carbon chain the higher the hydrocarbon boiling point
Why do longer carbon chained alkanes have a higher boiling point?
- 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
What is the relationship between the “branchedness” of an alkane and its boiling point?
- The more branched the alkanes the lower the boiling point
Why do more branched alkanes have a lower boiling point?
- 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
Describe a method for comparing strengths of intermolecular bonds between substances.
- 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
