CHAPTER 6 - SHAPES OF MOLECULES AND INTERMOLECULAR FORCES

Question 1

What is the electron-pair repulsion theory

Answer 1

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

Question 2

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

Answer 2

Methane (CH4)

Question 3

What is the bonding angle in a tetrahedral shape?

Answer 3

109.5 degrees

Question 4

What lines are drawn to show what dimension

Answer 4

Solid line - Straight

Solid wedge - Out of paper (towards viewer)

Dashed Wedge - into paper (away from viewer)

Question 5

What is the increasing repulsion of lone pairs and bonded pairs

Answer 5

Bonded pair to bonded pair least repulsive

Bonded pair/lone pair - Moderate Repulsion

Lone pair/lone pair - High repulsion

Question 6

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

Answer 6

2.5 degrees

Question 7

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

Answer 7

4 bonded pairs (bonding regions)

0 Lone pairs

Question 8

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

Answer 8

107 degrees
NH3

Question 9

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

Answer 9

3 Bonded Pairs (bonding regions)

1 Lone Pair

Question 10

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

Answer 10

104.5 Degrees

Water (H2O)

Question 11

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

Answer 11

2 Bonded Pairs (bonding regions)

2 Lone Pairs

Question 12

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

Answer 12

180 degrees

CO2

Question 13

How many bonding regions does a linear molecule have?

Answer 13

2 Bonded regions

Question 14

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

Answer 14

120 degrees

3 bonding regions

BF3

Question 15

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

Answer 15

90 degrees

6 bonding regions

SF6
(forms shape with 8 sides)

Question 16

What is the most electronegative element?

Answer 16

Fluorine - 4.0
electronegativity decreases further away

Question 17

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

Answer 17

Nuclear charge increases
Atomic radius decreases

Question 18

What is the difference in electronegativity for a covalent bond

Answer 18

none

Question 19

What is the difference in electronegativity for a polar covalent bond

Answer 19

0-1.8

Question 20

What is the difference in electronegativity for an ionic bond

Answer 20

Greater than 1.8

Question 21

What is a non-polar bond

Answer 21

Bonded electron pair is shared equally between the bonded atoms

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

Question 22

What is a polar bond?

Answer 22

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

Question 23

Define Electronegativity

Answer 23

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

Question 24

Define Polar covalent bond

Answer 24

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

Question 25

Define Dipole

Answer 25

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

Question 26

What are the 3 categories of Intermolecular forces?

Answer 26

Induced dipole-dipole interactions (London forces) Permanent dipole-dipole interactions Hydrogen Bonding

Question 27

What are London forces (induced dipole-dipole interactions)

Answer 27

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

Question 28

What determines the strength of induced dipole-dipole interactions

Answer 28

Number of electrons Strength of attractiveness number of interactions

Question 29

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

Answer 29

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

Question 30

What are simple molecular substances

Answer 30

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

Question 31

What structures do simple molecules form?

Answer 31

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

Question 32

What are properties of simple molecular substances

Answer 32

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

Question 33

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

Answer 33

IMF forces form between molecules and solvent IMF are weakened by interactions and therefore break and compound dissolves

Question 34

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

Answer 34

Little interaction occurs Hence bonds are too strong to be broken

Question 35

Are simple molecular substances conductors of electricity

Answer 35

No mobile charged particles Nothing to move electrical current

Question 36

Explain how an induced dipole forms

Answer 36

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

Question 37

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

Answer 37

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

Question 38

What are Hydrogen bonds?

Answer 38

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

Question 39

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

Answer 39

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

Question 40

Why does Water have a relatively high MP+BP

Answer 40

Contain Hydrogen bonds over and above the London Forces Requires more energy to break bonds

Question 41

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)

Answer 41

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

Question 42

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

Answer 42

Increase electrons increases the strength of the London forces

Question 43

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

Answer 43

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

Question 44

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

Answer 44

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

Question 45

State and explain two anomalous properties of water

Answer 45

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