Chapter 6 Shapes Of Molecules And IM forces

Question 1

What is the valence shell electron pair repulsion theory a model for?

Answer 1

Determining / predicting the Shapes of molecules/polyatomic ions

Question 2

So what is the valence shell electron pair repulsion theory

1) general
2) what this means, how will bonded atoms be in a definitive shape?

Answer 2

• where the shape of a molecule is determined by the electron pairs surrounding the atom =

=1. electrons are negative so the pairs will repel each other such that they are arranged AS FAR APART as possible

(2. Multiple bonds = means more repulsion = smaller bind angles )
(3. Lone pairs REPEL more than bonded pairs of electrons )

• the arrangement of the electron pairs is where it’s at least repulsion, meaning bonded atoms are held in a DEFINITIVE shape due to electrons

Question 3

Why are lone pairs repel stronger than bonded pair (not need to know)

Answer 3

• Slightly closer to central atom,
• occupies more space
  (Compared with bonded pair)

= thus repels more

Question 4

What is the order of repulsion , weakest to strongest, amongst pairs.

Answer 4

Bonded pair /bonded pair < bonded pair / lone pair < lone pair / lone pair

Remember lone pair powerful

Question 5

What happens if there are 5 electron pairs around central atom

Name
Bond angle

Answer 5

Bipyramidal

• 120 in one plane
• 90 in other

Question 6

Which ions have dative bonds (in case you are stuck)

Answer 6

NH4 + And HCO3+

Question 7

What is electronegativity

Answer 7

• the ability of an atom to attract the electron pair in a covalent bond

-

Question 8

What makes an atom more electronegative

How you measure

Answer 8

1. Higher the atom number = means more protons, which means a stronger attraction
2. Less shells = mean less atomic radius = means less distance needed to attract these and so stronger attraction

• the more right you go in a period the more protons and slightly lower radius as the higher nuclear charge, the radius DECREASES slightly DUE TO THEM PULLING IN ON IT.
  2) Pauling scale

Question 9

What will make a bond covalent , polar , or ionic basically

Answer 9

The electronegativity differences , if it is 0, so diatomic then it’s covalent,

• but more than that to 1.8 so say atleast 0.5 to 1.8, one atom will pull electrons closer making it negative and other slight positive so polar
• more than 1.8 it pulls so much it might aswell be ionic bind

Question 10

Polar bond

Dipole

Need to know

Answer 10

Polar bond = a covalent bond between atoms with different electronegativies where there is a partial positive and negative charge on the atoms

Dipole: is just the name given to the separation of opposite changes between atoms over short distance

Question 11

Permanent dipole

Answer 11

Separation of partial changes across a polar bond, arising from DIFFERENT ELECTRONEGATIVTIES ( Not random movement of electrons) is a permanent dipole . It will not change

Question 12

What are three types of IM binds

What are each relative strength

Answer 12

PERMANET dipole dipole strong

Induce dipole dipole (London forces)weak

Hydrogen middle

Covalent strongest

Question 13

How do induced dipole dipole interactions , known as London forces, come about

Answer 13

• at any time, the random movement of electrons means the distribution of them may by uneven in an atom
• this results in a instsneous dipole in an atom, meaning at any time an INSTANEOUS DIPOLE WILL EXIST, but it’s position changing
• the instaneous dipole will induce a dipole on a neighbouring particle based on its charge
• which can then Induce another and another
• this is all random, one moment there other not
• the attraction between induced dipoles is called an induced dipole dipole interaction

Happens in all molecules

Question 14

How does the strength of London forces change?

Answer 14

Based on number of electrons

Question 15

How do more electrons mean stronger London forces

How do these affect boiling point

Answer 15

The more electrons

• the LARGER the instsneous dipole, which also means larger induced dipole
• this means more amount of induced dipole dipole interactions
• and this also means the stronger ATTRACTION between the induce dipole dipole

2) more electrons mean stronger London forces, meaning more energy is needed to overcome these and thus molecules will have a higher melting snd boiling point, as it’s the intermolecular forces thst break not the covalent binds

Question 16

Van der Waal forces

Answer 16

For both PERMANET and induced, but London specifically for induced

Question 17

PERMANET dipole dipole

Boiling point?

What happens in liquid

Answer 17

Remmeber a dipole is where opposite partial charges are spread out in a polar bond.

When two PERMANET dipoles come close a permanent dipole dipole interaction happens between partially negative of one atom and partially positive of another.

Permanent dipole dipole also have London’s forces, so there boiling point will be HIGHER

3) CONSTANTLY BREAK AND REFORM

Question 18

Simple molecular substances

What and properties

Answer 18

Made up of simple moleuculed (small units with definite number of atoms), form regular structure called simple molecular lattice.

• these held by weak IM bonds
• but string covalent between atoms

2; properties include = low melting and boiling point

Question 19

Solubility of non polar+ non polar simple molecular substance

Answer 19

Simple + non polar
= IM forces are made between the molecules and the solvent
- these interactions WEAKEN the IM forces already in simple molecular substance
= thus the IM forces break and the compound DISSOLVES

THEREFORE NON POLAR SOLVENT IS SOLUBLE IN NON POLAR SIMPLE MOLECULAR

Question 20

Solubility of simple molecular and polar

Answer 20

When simple molecular + polar added

= little interaction between molecules in lattice and solvent molecules
- thus IM bonding remains too strong in molecular substance to break
= thus simple molecular is not soluble in POLAR substances

Question 21

Polar substances in polar solvents ?

Answer 21

Will dissolve
-polar solute molecules and the solvent molecules WILL attract each other, thus they can dissolve

• it’s like ions dissolving in water, sugar is polar with OH so interacts with water OH and dissolve

Hydrophilic will be polar and contain something electronegative like oxygen thst can interact with water in biological molecules..,

Question 22

Solubility summary

Answer 22

Non polar substance with non polar simple molecular = yes
Polar with non polar molecular = no
Polar with polar = yes

Like dissolves like

Question 23

What are hydrogen bonds

What are they between

How shown hydrogen bond and what is the shape around it ?

Answer 23

• an especially strong PERMANENT dipole dipole interaction

2) occur between
- an electronegative (so bit partially negative ) atom with a lone pair of electrons (oxygen, nitrogen fluorine )
- a hydrogen atom attached to an electronegative atom (H-O)

3) dashed line + linear + strongest IM bond

Why?

• h needs to be next to electronegative atom so overall it’s positive
• lone pairs need to be in electronegative atom so it can go positive to negative

Question 24

Common hydrogen bond (don’t need to know)

Answer 24

Water, ammonia Hydrogen fluoride

Question 25

Anamolous properties of water = density Why?

What happens when it melts then

Answer 25

Solid ice denser then liquid water when normally this isn’t true.

• hydrogen bonds hold water molecules at a fixed sirs cells apart in an OPEN LATTICE STRUCTURE.
• here the maximum close distance it can get is at 4°= maximum density
• Binds extend outwards, holding molecules apart and forms TETRAHEDRAL LATTTICE OF HOLES, decreasing density
• water molecules in ice are further apart thus than in water , so solid ice less dense and floats
  2) when ice melts, this tetrahedral lattice of holes in ice collapses allowing molecules to move closer together. So density increases

Question 26

What use is ice being less dense for us

Answer 26

Ice floats in lakes, forming INSULATING layer for animals below where temperatures are normally constant
- and also preventing water from freezing allows a habitat for aquatic animals

Question 27

Anamalous properties : high melting and boiling

Answer 27

Has higher boiling and melting because

• along with LONDON FORCES, they have hydrogen (which is stronger permanent permanent dipole)
• thus a lot of energy needed to break hydrogen bonds , so higher melting boiling .

Without it it would be -75 °lol

Question 28

Why water can have these properties and not other hydrogen bonded molecules

Explain the trends of graph

Answer 28

One reason why water has higher boiling point than HF and NH3 is because it can form 2 hydrogen binds due to two lone pairs, and be bonded with two lone pairs as has two hydrogens. The others all have some uneven distribution of lone to hydrogen bonds.

This graph, the first all hydrogen bonded, so higher boiling point, then not so they drop, but then they have more electrons = higher boiling point due to larger instanaeous dipoles, stronger + greater induced etc

Question 29

Other anamlous properties = surface tension, viscosity (don’t need to know)

Answer 29

SHC, viscosity, surface tension

Question 30

DNA AT how many bonds CG how many

Why always paired correctly

Answer 30

AT = 2
CG = 3

Because of chemical structure and shape these 4 bases are always paired correctly

Question 31

Electrical conductivity with IM forces

Answer 31

In simple molecular, as they all IM forces and no mobile charges, it csn’t conduct electricity