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OCR A LEVEL CHEMISTRY > SHAPES OF MOLECULES AND INTERMOLECULAR FORCES > Flashcards

SHAPES OF MOLECULES AND INTERMOLECULAR FORCES Flashcards

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1
Q

The electron pair repulsion theory

A

Electron pairs surrounding central atom determines the molecule / ion shape

Pairs repel each other so they are arranged as far as possible

Arrangement minimises repulsion, so hold atoms in definite shape

Different number of electron pairs = different shape

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2
Q

What 2 factors determines the shape of a molecule / ion

A

Bonding regions

Lone pairs

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3
Q

Lone pair

A

Lone pair slightly closer to central atom

Occupies more space than a bonded atom because they repel more strongly than a bonding pair

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4
Q

How does a lone pair affect the bond angle?

A

Reduces the bond angle by 2.5 per lone pair

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5
Q

What is the bond angle of a tetrahedral molecule

A
  1. 5

e. g. CH4

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6
Q

What is the bond angle of a pyramidal molecule

A

107

e.g. NH3

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7
Q

What is the bond angle of a non linear molecule

A

104.5

Non-linear

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8
Q

A molecule with 2 bonding regions

A

Bond angle : 180
linear

e.g. CO2

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9
Q

A molecule with 3 bonding regions

A

Bond angle: 120
Trigonal planar

e.g. BF3

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10
Q

A molecule with 4 bonding regions

A

Bond angle: 109.5
Tetrahedral

e.g. CH4

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11
Q

A molecule with 6 bonding regions

A

Bond angle: 90
Octahedral

e.g. SF6

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12
Q

Examples of tetrahedral molecules

A

CH4
Ammonium ions
SO4^2-

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13
Q

Examples of trigonal planar molecules

A

CO3 ^2-

NO3 ^-

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14
Q

What is meant by electronegativity

A

The attarcted of a bonded atom for the pair of electrons in a covalent bond

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15
Q

What scale is used to compare electrongetavitivties

A

Pauling scale

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16
Q

What is the most electronegative element?

A

Fluorine

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17
Q

Electronegativity trend

A

Electronegativity increases upwards and across towards fluorine

Group 1 is the least electronegative elements

18
Q

What kind of bond is formed where there is a large difference in electronegativities?

A

Ionic rather than Covalent

19
Q

Non polar bond

A

Bonded electron pair shared equally between bonded atoms

When atoms are the same element / have similar electrongetiavity

20
Q

Pure covalent bond

A

Bonded atoms are the same element

21
Q

Polar covalent bond

A

Bonded electron pair shared unequally

Different atoms with different electronegativity.
More electronegative has greater attraction for bonded pair electrons

22
Q

Polar bonds are polarised with…

A

Dipoles

23
Q

Permanent dipole

A

Dipole in a polar covalent bond which doesn’t change

24
Q

What can polar bonds do?

H2O and CO2

A

Polar bonds may reinforce one another to produce larger dipole over whole molecule or cancel out

H2O:
Polar
OH bonds have permanent dipoles
Act in opposite directions but don’t exactly oppose each other
Overall O end is delta neg and H end is delta positive

CO2:
Non-polar
C=O have permanent dipole
Act in opposite directions and exactly oppose each other
Overall dipoles cancel
25
Q

Ionic lattices dissolving in polar solvents

A

Water molecules attract +/- ions
Ionic lattice breaks down as dissolves
Water molecules surround ions
+ ions attracted towards delta negative oxygen of water
- ions attracted to delta positive hydrogen of water

26
Q

What are intermolecular forces

A

Weak interactions between dipoles of different molcules

27
Q

Induced dipole-dipole interactions (London forces)

A

Exist between all molecules, only temporary

Movement electrons produces changing dipole in any molecule, at any instant an instantaneous dipole will exist but position shifts constantly

Instantaneous dipole induces a dipole on neighbouring molecule, which induces dipoles on further molecules, they then attract one another.

More electrons in each molecule
Larger instantaneous and induced dipoles
Greater induced dipole - dipole interactions
Stronger attractive forces

(Explains increased bp of noble gases)

28
Q

Permanent dipole - dipole interactions

A

Act between permanent dipoles in polar molecules

Mean boiling point of polar molecules is much greater:
Have both london and permanent dipole-dipole interactions
Extra energy needed to break additional permanent interactions
So bp is higher

29
Q

Why is the mean boiling point of polar molecules high

A

Mean boiling point of polar molecules is much greater:

Have both london and permanent dipole-dipole interactions

Extra energy needed to break additional permanent interactions

So bp is higher

30
Q

What are simple molecules?

A

Simple molecules contain only a few atoms held together by covalent bonds.

e.g CO2

31
Q

What do simple molecules form in a solid state?

A

Simple molecular lattices

Held together by weak intermolecular forces
But atoms within molecules bonded strongly with covalent bonds

32
Q

The bp and mp of of simple molecular substances

A

Low mp/bp

Only weak intermolecular forces break
Not strong covalent bonds

33
Q

Nonpolar simple molecules are soluble in…

A

Nonpolar solvents

34
Q

Why are nonpolar simple molecules soluble in nonpolar solvents

A

Nonpolar simple molecules tend to be soluble in non-polar solvents (hexane) because intermolecular forces form between molecules and solvent, weakening intermolecular forces in simple lattice, so they break and the compound dissolves

35
Q

Simple molecules are insoluble in…

A

Polar solvents

36
Q

Why are simple molecules insoluble in polar solvents?

A

Tend to be insoluble in polar solvents (water) because of little interaction between molecules in lattice and solvent molecules, intermolecular bonding in solvent too strong to be broken

37
Q

What determines the solubility of polar simple molecular substances?

A

Depends on strength of dipole
May dissolve because polar solute / solvent molecules can attract each other

Some with part polar and part non-polar dissolve

38
Q

Do simple molecular structures conduct electricity?

A

No
No mobile charged particles within structure
So nothing to complete an electrical circuit

39
Q

How many lone pairs does Nitrogen, Oxygen and Fluorine have?

A

Nitrogen:
Three bonds and 1 lone pair

Oxygen
Two bonds and 2 lone pairs

Fluorine (and all halogens):
One bond and 3 lone pairs

40
Q

Where is hydrogen bonding found?

A

Found in molecules containing Electronegative atom with lone pair electrons (O,N,F) attached to a hydrogen atom

41
Q

Why does ice float?

A

Solid is less dense than liquid water

Hydrogen bonds hold water molecules apart in open lattice structure, water molecules in ice is held further apart than in water, so ice is less dense that liquid water and floats

2 lone pairs on oxygen and 2 hydrogens, so each molecule can form 4 bonds = open tetrahedral lattice full of holes

OCR A LEVEL CHEMISTRY (48 decks)

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