Chapter 6 Shapes of Molecules and Intermolecular Forces

Question 1

What is Electron-Pair repulsion theory

Answer 1

Pairs of electrons around a central atom repel each other, so they move as far apart as possible to minimise this repulsion. This means molecules containing 3 or more atoms have shapes

Question 2

What is the molecular shape and bonding angles for:

2 pairs of electrons (or 2 regions of electron density) repelling around a central atom

Answer 2

Linear
180°

Question 3

When working out shapes, how many regions of electron density do double and triple bonds count as

Answer 3

1

Question 4

What is the molecular shape and bonding angles for:

3 pairs of electrons around a central atom

Answer 4

Trigonal Planar
120°

Question 5

What is the molecular shape and bonding angles for:

4 bonding pairs around a central atom with no lone pairs

Answer 5

Tetrahedral
109.5°

Question 6

What is the molecular shape and bonding angles for:

4 pairs of electrons around a central atom with 1 lone pair

Answer 6

Pyramidal
107°

Question 7

What is the molecular shape and bonding angles for:

4 pairs of electrons around a central atom with 2 lone pairs

Answer 7

Non Linear
180°

Question 8

What is the molecular shape and bonding angles for:

5 pairs of electrons around a central atom

Answer 8

Trional Bipyramid
90°
120°

Question 9

What is the molecular shape and bonding angles for:

6 pairs of electrons around a central atom

Answer 9

Octahedral
90°

Question 10

By how many degrees is the bond angle reduced for each lone pair

Answer 10

2.5°

Question 11

Define Electronegativity

Answer 11

Electronegativity is a measure of the ability of an atom in a molecule to attract a pair of electrons in a covalent bond to itself

Question 12

Finish this sentence:

The higher the electronegativity value…

Answer 12

The more strongly the bonding electrons are attracted to that atom

Question 13

Going up a group does electronegativity increase or decrease

Answer 13

Electronegativity increases up a group

Question 14

Across a period, does electronegativity increase or decrease

Answer 14

Electronegativity increases across a period (up to group 7)

Question 15

What is the most electronegative atom

Answer 15

Fluorine

Question 16

What group on the periodic table has the least electronegative atoms

Answer 16

1

Question 17

Which elements have the most electronegative atoms

Answer 17

• Nitrogen
• Oxygen
• Fluorine
• Chlorine

Question 18

What happens to the pull on bonding electrons as nuclear charge increases

Answer 18

It increases

Question 19

Why does the pull on bonding electrons decrese further from the nucleus

Answer 19

Pull falls rapidly as electrons become further from the nucleus and the number of shielding inner shells increases

Question 20

What is a non-polar bond

Answer 20

In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms

Question 21

When is a bond non polar

Answer 21

• The bonded atoms are the same
• The bonded atoms have the same, or similar, electronegativity values.

Question 22

What is a polar bond

Answer 22

In a polar bond, the bonded electron pair is shared unequally between the bonded atoms

Question 23

When is a bond polar

Answer 23

A bond will be polar when the bonded atoms are different and have different electronegativity values, resulting in a polar covalent bond

Question 24

Why is H–F a polar bond

Answer 24

In H-F, F has a higher electronegativity than H so the bonding electrons are more attracted to the F end of the bond. The bond is polarised

Question 25

What is the separation of opposite charges called

Answer 25

A dipole

Question 26

# Finish this sentence: The bigger the difference in electronegativity between the bonding atoms...

Answer 26

The more polar the bond and the greater the ionic character

Question 27

What difference in electronegativity will result in the bond being ionic instead of Polar Covalent | (Give a value)

Answer 27

1.8+

Question 28

How can a molecule with polar bonds be non-polar

Answer 28

The molecule is symmetrical and the dipoles cancel out

Question 29

What has to be true for a molecule with polar bonds to be polar

Answer 29

The molecule has to be unsymmetrical so the dipoles do not cancel out

Question 30

Can polar molecules (eg water) dissolve ions

Answer 30

Yes, polar molecules (particularly water) can form strong enough attractions to ions to permit dissolving in some cases

Question 31

When is dissolving likely

Answer 31

If the energy needed to break the lattice is similar to that released, it will dissolve

Question 32

What is hydration

Answer 32

When attractions form between water moleculesand ions releasing energy

Question 33

Are Nitrates soluble

Answer 33

Yes, all nitrates are soluble

Question 34

Are Chlorides soluble

Answer 34

Yes, all chlorides are soluble, except for AgCl and PbCl₂

Question 35

Are Sulfates soluble

Answer 35

Yes, most sulfates are soluble, except for BaSO₄ PbSO₄ and SrSO₄

Question 36

Are Carbonates soluble

Answer 36

No, all carbonates are insoluble, except (NH₄)₂CO₃ and those of the Group 1 elements

Question 37

Are sodium, potassium, and ammonium salts soluble

Answer 37

Yes all sodium, potassium and ammonium salts are soluble

Question 38

What do the solubility rules depend on

Answer 38

Amount of solute and/or solvent and temperature

Question 39

Define intermolecular forces

Answer 39

Weak interactions between dipoles of different molecules

Question 40

Which atoms or molecules London forces (induced dipole dipole forces)

Answer 40

All atoms and molecules

Question 41

Explain London Forces

Answer 41

* Electrons in an atom or molecule are constantly moving * A temporary uneven distribution of electrons can occur for an instant * This creates an instantaneous dipole * The instantaneous dipole induces a dipole in its neighbours * leading to an attraction * Each dipole can induce many others

Question 42

How long do london forces last

Answer 42

Induced dipoles are only temporary. In the next instant of time, the induced dipoles may disappear, only for the whole process to take place amongst other molecules

Question 43

Which intermolecular forces do non-polar molecules have

Answer 43

London Forces Only

Question 44

What happens to the london force as the molecule gets larger

Answer 44

Number of electrons increases so size of the induced dipoles also get larger, giving stronger forces

Question 45

Why do the melting and boiling points increase as the strength of the London force increases

Answer 45

As the strength of the London forces increases, more energy is needed to overcome them, so the melting and boiling point increases

Question 46

Define Permanent Dipole-Dipole interactions

Answer 46

Dipole-Dipole interactionshese are formed between molecules which are polar (molecules with both polar bonds and a shape that does not allow the dipoles to cancel each other out)

Question 47

How long do permanent dipole-dipole interactions last

Answer 47

They are permanent and exert stronger forces. Opposites attract

Question 48

What are simple molecular substances

Answer 48

Simple molecular substances are made up of discrete molecules containing a specific number of atoms and having a consistent molecular formula e.g CH₄, N₂

Question 49

What is the regular structure of solid simple molecular substances called

Answer 49

Simple Molecular Lattice

Question 50

Are the intermolecular forces holding simple molecular substances together strong or weak

Answer 50

Weak

Question 51

What are the properties of simple molecular substances | Summarised

Answer 51

* Low melting and boiling points * Non-polar molecules tend to dissolve in non-polar solvents, whereas polar molecules tend to dissolve in polar solvents * Do not conduct electricity

Question 52

Why do simple molecular substances have low melting and boiling points

Answer 52

Intermolecular forces usually do not need a lot of energy to overcome so most simple molecular substances have low melting and boiling temperatures with many being gases or liquids at room temperature

Question 53

Describe the solubility of simple molecular substances

Answer 53

Like dissolves like Non-polar molecules tend to dissolve in non-polar solvents. Polar molecules tend to dissolve in polar solvents

Question 54

Why can't simple molecular substances conduct electricity

Answer 54

There are no mobile charged particles

Question 55

What is Hydrogen bonding

Answer 55

A special type of permanent dipole-dipole interaction containing: * An electronegative atom with a lone pair of electrons, e.g. oxygen, nitrogen, or fluorine * A hydrogen atom attached to an electronegative atom, e.g. H-O, H-N, or H-F

Question 56

What is the strongest type of intermolecular force

Answer 56

Hydrogen Bonding

Question 57

What are the anomalous properties of water

Answer 57

* Solid (ice) is less dense than liquid (water) * Water has a relatively high melting point and boiling point * Water has a relatively high surface tension

Question 58

Why is ice less dense than water

Answer 58

* Hydrogen bonds hold water molecules apart in an open lattice structure * The water molecules in ice are further apart than in liquid water * Solid ice is less dense than liquid water and floats

Question 59

How many hydrogen bonds can each water molecule form

Answer 59

4

Question 60

The hydrogen bonds extend outwards, holding water molecules slightly apart in an open tetrahedral arrangement

Answer 60

The hydrogen bonds extend outwards, holding water molecules slightly apart in an open tetrahedral arrangement

Question 61

What is roughly the bond angle for each Hydrogen bond

Answer 61

180⁰

Question 62

DO General trend – boiling point increases. As molecule size (and number of electrons) increases, the strength of the London forces between molecules increases and more energy is needed to overcome them.

Question 63

FINISH H2O has the highest boiling point as it can form more extensive hydrogen bonding between its molecules

Question 64

FINISH H2O, NH3, and HF do not follow the trend. They have stronger hydrogen bonds between their molecules which require more energy to break

Question 65

FINISH urface tension is caused by molecules on the surface experiencing unbalanced hydrogen bonding forces pulling them in. Molecules in the bulk experience balanced forces in every direction.

Question 66

FINISH The double helix structure of DNA is held together by hydrogen bonds between base pairs.