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CHEMISTRY A LEVEL > CHAPTER 6 - SHAPES OF MOLECULES AND INTERMOLECULAR FORCES > Flashcards

CHAPTER 6 - SHAPES OF MOLECULES AND INTERMOLECULAR FORCES Flashcards

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1
Q

What is the electron-pair repulsion theory

A

Electrons have a negative charge, so they repel each other

Determines she of molecule or ion

Repulsion causes the electrons to move as far away as possible

Electron pairs minimise repulsion so the bonds hold a definite shape

Different numbers of electron pairs result in different shapes

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2
Q

What is an example of a molecule that forms a tetrahedral shape

A

Methane (CH4)

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3
Q

What is the bonding angle in a tetrahedral shape?

A

109.5 degrees

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4
Q

What lines are drawn to show what dimension

A

Solid line - Straight

Solid wedge - Out of paper (towards viewer)

Dashed Wedge - into paper (away from viewer)

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5
Q

What is the increasing repulsion of lone pairs and bonded pairs

A

Bonded pair to bonded pair least repulsive

Bonded pair/lone pair - Moderate Repulsion

Lone pair/lone pair - High repulsion

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6
Q

For each Lone pair of electrons, ho much does the bond angle decrease by?

A

2.5 degrees

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7
Q

How many bonded pairs and Lone pairs does a tetrahedral molecule have?

A

4 bonded pairs (bonding regions)

0 Lone pairs

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8
Q

What bonding angle does a Pyramidal have and what is an example of a pyramidal molecule

A

107 degrees
NH3

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9
Q

How many bonded pairs and Lone pairs does a Pyramidal molecule have?

A

3 Bonded Pairs (bonding regions)

1 Lone Pair

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10
Q

What bonding angle does a non-linear molecule have and what is an example o a non-linear molecule?

A

104.5 Degrees

Water (H2O)

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11
Q

How many bonded pairs and Lone pairs does a non-linear molecule have?

A

2 Bonded Pairs (bonding regions)

2 Lone Pairs

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12
Q

What bonding angle does a linear molecule have and what is an example o a linear molecule?

A

180 degrees

CO2

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13
Q

How many bonding regions does a linear molecule have?

A

2 Bonded regions

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14
Q

What is the bonding angle, number of bonding regions and an example of a Trigonal Planar

A

120 degrees

3 bonding regions

BF3

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15
Q

What is the bonding angle, number of bonding regions and an example of an octahedral molecule

A

90 degrees

6 bonding regions

SF6
(forms shape with 8 sides)

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16
Q

What is the most electronegative element?

A

Fluorine - 4.0
electronegativity decreases further away

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17
Q

What happens to nuclear charge and atomic radius across the periodic table

A

Nuclear charge increases
Atomic radius decreases

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18
Q

What is the difference in electronegativity for a covalent bond

A

none

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19
Q

What is the difference in electronegativity for a polar covalent bond

A

0-1.8

20
Q

What is the difference in electronegativity for an ionic bond

A

Greater than 1.8

21
Q

What is a non-polar bond

A

Bonded electron pair is shared equally between the bonded atoms

Bonded atoms are the same or the atoms have same/similar electronegativity

22
Q

What is a polar bond?

A

A difference in electronegativity, which leads to partial charges and a permanent/induced dipole - variations of electron inhabitance time

eg H - Cl 2.1 - 3.0

23
Q

Define Electronegativity

A

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

24
Q

Define Polar covalent bond

A

Shared pair of electrons where the pair is not shared equally between the two bonded atoms

25
Q

Define Dipole

A

Charge separation across a bond with one atom having a slightly positive charge and another with a slightly negative charge

26
Q

What are the 3 categories of Intermolecular forces?

A

Induced dipole-dipole interactions (London forces)

Permanent dipole-dipole interactions

Hydrogen Bonding

27
Q

What are London forces (induced dipole-dipole interactions)

A

Weak IMF that exist between all molecules, whether polar or non-polar

Movement of electrons produces changing dipole, instantaneous

Induces dipole in neighbouring molecule

Induced dipole induces further dipoles in neighbouring molecules which then attract one another

temporary

28
Q

What determines the strength of induced dipole-dipole interactions

A

Number of electrons
Strength of attractiveness
number of interactions

29
Q

What is the difference between LDFs and Van Der Waals forces?

A

LDF - only induced/temporary dipole-dipole interactions
VDW - Both induced and permanent dipole-dipole interactions

30
Q

What are simple molecular substances

A

Molecules made up of small units containing a definite number of atoms with a definite molecular formula, such as Ne, H2, H2O or CO2

31
Q

What structures do simple molecules form?

A

Simple molecular lattice
Held by weak IMF, atoms in each molecule held strongly together by covalent bonds

32
Q

What are properties of simple molecular substances

A

Low MP + BP
soluble in non-polar non-simple substances

33
Q

why are non-polar simple molecular substances soluble in non-polar non-simple solvent (eg hexane)

A

IMF forces form between molecules and solvent

IMF are weakened by interactions and therefore break and compound dissolves

34
Q

why are non-polar simple molecular substances insoluble in polar non-simple solvent

A

Little interaction occurs
Hence bonds are too strong to be broken

35
Q

Are simple molecular substances conductors of electricity

A

No mobile charged particles
Nothing to move electrical current

36
Q

Explain how an induced dipole forms

A

Fluctuation in the electron density around a molecule creates an instantaneous dipole in a molecule. The instantaneous dipole induces a dipole in a neighbouring molecule

37
Q

Explain why simple molecular compounds:
a) have low MP + BP
b) Doesn’t dissolve in water
c) Poor Electrical conductivity

A

a) Weak IMF are broken by the energy present at low temperatures
b) Little interactions between the molecules and lattice
c) no mobile charged particles

38
Q

What are Hydrogen bonds?

A

A permanent dipole-dipole interaction from a Hydrogen to either an Oxygen, Nitrogen or Fluorine atom

39
Q

Why is solid (ice) less dense than water (liquid)

A

Hydrogen bonds hold water molecules further apart in ice than in liquid

in an open lattice structure

Therefore it is less dense - so it floats

40
Q

Why does Water have a relatively high MP+BP

A

Contain Hydrogen bonds over and above the London Forces

Requires more energy to break bonds

41
Q

Water, Hydrogen Fluoride and ammonia do not follow the trend shown by the other hydrides in each group.
a) Estimate what the boiling points of water, hydrogen fluoride, and ammonia would be if they were to follow the group trends.
b) explain why water, hydrogen, fluoride, and ammonia, do not follow the group trends
(pg 82)

A

a)
water: -75 degrees C
HF: - 90 degrees C
NH3: -100 degrees C
b)
Three. Hydrogen bonding, which is a stronger intermolecular force than other dipole interactions. Greater energy needed to overcome the intermolecular forces, so boiling points are much higher

42
Q

Explain all groups show an increase in boiling point from period 3 to period 6

A

Increase electrons increases the strength of the London forces

43
Q

What can be drawn about the relative strengths of London forces and permanent dipole-dipole interactions for the hydrides of group 14 to 17

A

From period 3 to period six, difference in electronegativity between hydrogen and element decreases, decreasing the permanent dipole-dipole interactions. Number of electrons increases, increasing London forces. Boiling point increases, so London forces are stronger and more significant than a permanent dipole-dipole interactions

44
Q

Suggest why pairing doesn’t take place between two purine bases or between two Pyramidine bases

A

Two bases would be too close together.
Two Pyramidine bases will be too far apart.

45
Q

State and explain two anomalous properties of water

A

ICE is dense than liquid water because hydrogen bonds hold molecules apart in open lattice structure.

Higher milk and boiling points than expected because appreciable energy is needed to break the hydrogen bond

CHEMISTRY A LEVEL (14 decks)

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