Bonding And Shapes Of Molecules

Question 1

What are intramolecular bonding forces?

Answer 1

Forces of attraction between atoms within a molecule

Question 2

What are intermolecular bonding forces?

Answer 2

Forces of attraction between neighbouring molecules

Question 3

Give examples of intramolecular bonding

Answer 3

• covalent bonding
• polar covalent bonding
• ionic

Question 4

What is covalent bonding?

Answer 4

Sharing of or more pairs of electrons between two atoms

Question 5

What is polar covalent bonding

Answer 5

Unequal sharing of one or more pairs of electrons by two atoms

Question 6

What is ionic bonding?

Answer 6

Electrostatic force of attraction between oppositely charged ions

Question 7

What are weak Van Der Waals forces?

Answer 7

Temporary dipoles in molecule attracting/repelling temporary dipoles in next molecule

Question 8

What are permanent dipole-dipole attractions

Answer 8

Permanently polarised molecules attract polarised neighbour, where opposite ends attract

Question 9

What is hydrogen bonding?

Answer 9

Permanently polarised molecules which have hydrogen and one of the elements N, O or F

Question 10

What is electronegativity?

Answer 10

It’s the relative attraction an atom has for shared pairs of electrons in a covalent bond

Question 11

Use electronegativity values to predict the type of bonding in
phosphine
potassium chloride

Answer 11

i) pure covalent
ii) polar covalent

CHART
0 - 0.7 - 1.7

cov - pol cov - ionic

Question 12

Explain why potassium chloride has a much higher boiling point than phosphine (PH3)

Answer 12

Phosphine is held together by weak Van der Waals forces whereas potassium chloride is held together by ionic bonds

Question 13

Phosphine is virtually insoluble in water, whereas the corresponding hydride of nitrogen which is in the same group as phosphorus is very soluble in water, how so?

Answer 13

The corresponding hydride is NH3, which contains a lone pair making the whole molecule polar covalent [electronegative differences]
This results in bonding with water and therefore dissolves in water
Phosphine is non polar and has little/no attraction to the water

Question 14

What is water of crystallisation?

Answer 14

It’s water chemically combined in definite proportions in a crystalline compound

Question 15

Give two uses of noble gases

Answer 15

Helium - in blimps

Argon - in light bulbs

Question 16

What is the octet rule?

Answer 16

Atoms on reaction tend to reach an electronic arrangement with eight electrons in their outermost shell

Question 17

What are the exceptions to the octet rule?

Answer 17

Beryllium and Boron - few electrons in their outer levels so they cannot gain enough electrons to reach eight
D block elements

Question 18

What is valency?

Answer 18

Number of bonds an atom of an element forms when it reacts

Question 19

What group of elements have variable Valencies and give an example

Answer 19

Transition elements

Iron - 2,3,6
Copper - 1,2

Question 20

What do metal atoms become when they lose one or more electrons?

Answer 20

Positively charged - Cation

Question 21

How is a cation smaller than the corresponding atom?

Answer 21

It has lost electrons and is positively charged and therefore lost a shell

Question 22

Characteristics of Ionic Substances

Answer 22

Form crystal lattices
High melting and boiling points due to strong forces of attraction
Polar
Soluble in water
Conduct electricity in molten/aqueous state

Question 23

Give examples of ionic materials in everyday life

Answer 23

Salt tablets - to replace salt lost in sweating

Brine - curing bacon

Question 24

What factors dictate the extent to which the shared electrons are attracted by different nuclei?

Answer 24

Size of the Atom - smaller atom have a stronger attraction because they can get closer to electrons
Nuclear charge - atoms with bigger nuclear charge will have greater attraction for the shared electrons

Question 25

Characteristics of Covalent Bonding

Answer 25

Low melting and boiling points - gases at room temperature due to weak intermolecular forces Do not conduct electricity due to the fact they consist of neutral molecules Do not dissolve readily in water

Question 26

Give examples of polar materials in every day life

Answer 26

Water - washing clothes | Glucose - lucozade

Question 27

Give examples of non - polar materials in every day life

Answer 27

Petrol | Cooking oil

Question 28

Account for the general trend of electronegativity across a period

Answer 28

Increases - nuclear charge inc, positive attractive force on electrons - atomic radius decreases as electrons are pulled in more tightly - no screening

Question 29

Account for the general trend of electronegativity down a group

Answer 29

- extra levels means there is extra screening | - atomic radius inc caused by extra energy levels means electrons aren’t as tightly held and no attracted as strongly

Question 30

Distinguish between a sigma and a pi covalent bond

Answer 30

Sigma - end on overlap of orbitals | Pi - sideway overlap of p orbitals

Question 31

What does the electron repulsion theory state?

Answer 31

Electron pairs in outer shell of central atom repel each other and end up as far apart as is geometrically possible Lone pairs have a greater repelling effect than bonding pairs

Question 32

What are the posible shapes sit molecules of general formula : AB2

Answer 32

Valency = 2 2 bonds Linear, V-shaped

Question 33

Account for the difference in shape of NH3 and BF3

Answer 33

Draw out NH3 - three bond pairs and one lone pair BF3 - three bond pairs

Question 34

Describe the bonding in the nitrogen molecule in terms of sigma and pi bonding

Answer 34

``` Sigma = 1 Pi = 2 ```

Question 35

What type of intermolecular forces would you expect to find in nitrogen gas? Explain your answer

Answer 35

Van Der Waals, it is non polar

Question 36

If a molecule contains two bonds, what shapes can it be?

Answer 36

No lone pairs - Linear | Lone pairs - V-shaped

Question 37

If a molecule contains three bonds, what shapes can it be?

Answer 37

No lone pairs - Trigonal Planar | Lone pairs - Pyramidal

Question 38

If a molecule contains four bonds, what shapes can it be?

Answer 38

Lone pairs - Tetrahedral

Question 39

What bond angle does linear have?

Answer 39

180

Question 40

What bond angle does v shape planar have?

Answer 40

104.4

Question 41

What bond angle does trigonal planar have?

Answer 41

120

Question 42

What bond angle does pyramidal have?

Answer 42

107.3

Question 43

What bond angle does tetrahedral have?

Answer 43

109.5

Question 44

Identify in ammonia, the type of intermolecular bonding and intermolecular forces present

Answer 44

Intramolecular - polar covalent bonding | Intermolecular - hydrogen bonds

Question 45

What is the molecular formula of benzene

Answer 45

C6H6

Question 46

Account for the difference in the shapes of BF3 and NH3 molecule

Answer 46

Boron - three bond pairs | Nitrogen - three bond pairs and one lone pair

Question 47

How do you know how many sigma and pi bonds there are?

Answer 47

Every element forms only a single covalent bond. That is sigma bond. If there are 3 bonds, one must be covalent and other 2 pi Look at valency to see how many bonds

Question 48

What is bond energy?

Answer 48

Its the average energy required to break a bond and seperate atoms in gaseous state

Question 49

The boiling points of hydrogen and oxygen are 20.0K and 90.2K. Account for the higher boiling point of oxygen

Answer 49

Stronger intermolecular forces (dipole-dipole)

Question 50

Which are the non polar bond shapes?

Answer 50

Linear, trigonal planar, and tetrahedral.

Question 51

Which are the polar bond shapes?

Answer 51

V-shaped and Pyramidal

Question 52

``` If the eletronegativy difference is i) 0-.7 ii).7-1.7 iii) 1.7< what kind of bonds does it have? ```

Answer 52

i) Covalent (non-polar) ii) Polar covalent iii) Ionic

Question 53

Give one property that is affected by the presence of intermolecular bonding?

Answer 53

Melting point Boiling point Density Solubility in water

Question 54

Would you expect i) the B-Cl bonds and ii) the BCL3 molecules to be polar or non polar? Justify

Answer 54

i) Non-polar -> large e.n difference ii) Polar -> unequal sharing of electrons between B and Cl is cancelled due to symmetry of the molecule as the centres of positive and negative coincide

Question 55

Indicate on your dots and crosses bonding diagram for chlorine monofluoride any full or partial charges that you would expect to result from the formation of the bond between chlorine and fluorine? E.N values : Cl (3.0) and Fl (4.0)

Answer 55

The atom with greater electronegativity - partial negative The atom with smaller electronegativity - partial positive In this case, chlorine is partially positibe and fluorine is partially negative

Question 56

NH3, PH3, AsH3 | Which if any of the three hydrides would you expect to have intermolecular hydrogen bonding and justify

Answer 56

NH3 | Hydorgen is bonded to Nitrogen - small and very electronegative

Question 57

NH3, PH3, AsH3 Suggest a reason why ammonia has the highest boiling point of the three hydrides and phosphine's boiling point is lower than that of arsine

Answer 57

i) Hydrogen bond - strongest of intermolecular forces | ii) Smaller mr

Question 58

How would you know if a compound is water soluble or not?

Answer 58

Find electronegativity values and see if nonpolar or polar

Question 59

Account for the difference in bond angle between the two molecules, 107.3 in ammonia and 109.5 in silane

Answer 59

Lone pair in ammonia - greater repulsion power than bond pair and pushes bonds closer to form a smaller bond angle

Question 60

Give the reason why a molecule with polar bonds can be non-polar

Answer 60

Symmetry of molecule - Centres of positive and negative charges coincide

Question 61

Why do ionic substances conduct electricity when molten or dissolved in water but not in solid state?

Answer 61

Molten - ions free to move | Solid - not free, fixed

Question 62

Show that the ammonia molecule has polar covalent bonding

Answer 62

The electronegativity difference between N and H show unequal sharing

Question 63

Describe the processes involved when ammonia dissolves in water

Answer 63

- Breaking of hydrogen bonds in water | - Forming of hydrogen bonds between ammonia and water

Question 64

Suggest a reason why the boiling point of ammonia (-33) is significantly lower than that of water (100)

Answer 64

Weaker bonding in ammonia (small e.n difference)

Question 65

Draw a diagram illustrating hydrogen bonding in ammonia

Answer 65

H H H N - H N H H

Question 66

Use your knowledge of intermolecular forces to explain why methane has a very low boiling point

Answer 66

very weak intermolecular forces

Question 67

The relative molecular mass of methane is only slightly lower than that of water but the boiling point of water is much higher. Suggest a reason for this

Answer 67

much stronger hydrogen bonds between water molecules

Question 68

A thin stream of liquid flows from the burette. A stream of water is deflected towards a positively chargd rod whereas a stream of cyclohexane is undeflected. Account for these observations

Answer 68

polarity of water causes attraction to charged rod | non-polarity of cyclohexane means it is not affected by charged rod

Question 69

Explain what would happen in the case of the stream of water if the positively charged rod were replaced by a negatively charged rod

Answer 69

stream of water still attracted to rod as molecules (dipoles) arrange themselves with positive pole towards rod

Question 70

(weakly polar) Would you expect hydrogen sulfide to be soluble in water? Explain your answer

Answer 70

Only slightly - weakly polar

Question 71

Describe the bonding in the nitrogen molecule in terms of sigma and pi bonding

Answer 71

one sigma and two pi

Question 72

Why will water always be deflected by any charged rod?

Answer 72

Because it had a dipole moment i.e the centre of positive charge does not coincide with centre of negative charge