OZ 6 The CFC story

dichlorodifluoromethane

CCl₂F₂

Uses of CFCs

1. Refrigerants

2. Air conditioning

3. Aerosol propellants

4. Blowing agents

5. Dry cleaning solvents

Carbon- halogen bonds is

polar

Polar boding occurs when two atoms with

different electronegativities form a covalent bond

dipole

charge separation

The atom with the __________ _____________ pulls the shared electrons closer, becoming slightly ______ , while the other atom becomes slightly ________. This creates a _________ within the molecule.

higher electronegativity

negative δ⁻

positive δ⁺

dipole

_________ molecules dissolve in water

polar

_________ molecules do not dissolve in water

non-polar

OH bond is

polar

Electronegativity

relative tendency of an atom in a covalent bond in a molecule to attract electrons in a covalent bond to itself

Name 4 of the most electronegative atoms

F, O, N, Cl

Electronegativity __________ across a period

increases

Why does electronegativity increase across a period?

1. number of protons increases

2. atomic radius decreases- electrons in the same shell are pulled in more

Electronegativity _________ down a group

decreases

Why does electronegativity decrease down a group?

1. distance between nucleus and outer electrons increases

2. shielding of inner electrons increases

What kind of compounds are purely covalent?

elements of similar electronegativity- small electronegativity difference

All haloalkanes are _______ with water

immiscible

The _______ the halogen atom in a haloalkane and the _______ halogen atoms there are, the __________ bpt

bigger

more

higher

longer molecules have ________ intermolecular bonds

stronger

Why do straight chain alkanes have higher bpt?

more contact between molecules

more opportunities for intermolecular bonds to form

Why does the bpt of noble gases increase down the group?

1. Atoms get bigger

2. more electrons

3. stronger id-id bonds

4. more energy to break

dipole

charge separation

When does a polar covalent bond form?

elements in bond have different electronegativities

When are dipoles formed in a polar covalent bond?

bond has unequal distribution of electrons producing a charge separation

When are compounds ionic?

very different electronegativities and large electronegativity difference

Why does instantaneous dipoles occur?

electrons are in constant and random motion

electron density can fluctuate

parts of molecule become more or less negative

When do induced dipoles form?

a dipoles cause dipoles to form in a neighbouring molecule

induced dipole has the ______ charge to a temporary/permanent dipole

opposite

How does the number of electrons affect the size of id-id bonds?

• more electrons

• higher chance of temporary dipoles

• id-id bonds stronger

• higher bpt

The boiling point of halogen________ down the group

increases

Why does the bpt of halogen increase down the group?

• bigger molecules have more electrons

• bigger id-id bonds

• more energy needed to break bonds

Why does the bpt of alkane homologous series increase down the series?

• bigger molecules have more electrons

• bigger id-id bonds

• more energy needed to break bonds

The boiling point of alkanes ___________ across the series

increases

Why do long chain alkanes have a high bpt?

• larger surface area of contact between molecules

• more induced dipole- instantaneous dipole interactions

When do induced dipoles occur?

when an unpolarised molecule is next to a dipole

When do permanent dipoles occur?

between polar molecules

Order the dipoles from weakest to strongest

1. instantaneous dipole- induced dipole

2. permanent dipole- permanent dipole

3. permanent dipole- induced dipole

When is the molecules non-polar even if it has permanent dipoles?

symmetrical

Reaction for methane removing chlorine radicals in stratosphere

CH₄ + Cl → CH₃ + HCl