Structure & Bonding

Question 1

Explain why MgO has exactly the same structure as NaCl, but a much higher mp & bp

Answer 1

To form an ionic bond, Mg transfers 2 electrons to O, whereas Na transfers only 1 electron to Cl. The 2+ & 2- ions in MgO attract each other more strongly than Na+ attracts Cl- in NaCl

Question 2

Explain why RbI melts and boils at slightly lower temperatures than NaCl

Answer 2

Both rubidium and iodide ions are bigger than sodium and chloride ions so have weaker electrostatic attractions so less heat energy is needed to separate them

Question 3

Describe macromolecular covalent bonding

Answer 3

Large lattice of many atoms held together by strong covalent bonds

Question 4

Describe the structure of diamond, giving some of its physical properties

Answer 4

• 3D tetrahedral structure of C atoms, with each C atom bonded to 4 others
• High mp & bp, as it takes a lot of energy to overcome many strong covalent bonds
• Doesn’t conduct electricity as all electrons are used in bonding

Question 5

What’s the name for a ‘large lattice of many atoms held together by strong covalent bonds’?

Answer 5

Macromolecular covalent bonding

Question 6

Describe the structure of graphite, giving some of its physical properties

Answer 6

• Macromolecular covalent structure where each C atom is bonded to 3 others and arranged in layers
• Weak vdW forces of attraction between layers mean they can slide over each other, making it soft & slippery
• One electron from each C is delocalised and can carry charge so graphite can conduct electricity

Question 7

Describe & explain the solubility of metallic compounds

Answer 7

Insoluble in water or organic solvents unless they undergo a reaction with them

Question 8

Describe & explain the properties of diamond

Answer 8

• Very hard and high mp & bp from very strong carbon-carbon covalent bonds that have to be broken throughout the structure before melting occurs
• Doesn’t conduct electricity as all the electrons are held tightly between the atoms and aren’t free to move
• Insoluble in water and organic solvents as there are no possible attractions which could occur between each of the substances’ molecules which could outweigh the attractions between the covalently bound carbon atoms

Question 9

Describe the structure of graphite

Answer 9

• Graphite is made entirely of carbon, where each carbon atom uses 3 of its electrons to form simple bonds to its 3 close neighbours, leaving the 4th electron delocalised and free to move around the whole of the sheet of atoms in one layer. It’s formed in layers rather than a 3-dimensional lattice
• There’s no direct contact between the delocalised electrons in one sheet and those in neighbouring sheets
• The atoms within a sheet are held together by strong covalent bonds
• As the delocalised electrons move around in the sheet, very large temporary dipoles (VdW forces) can be set up, which will induce opposite dipoles in the sets above and below - and so on throughout the whole graphite crystal

Question 10

Which are stronger between the covalent bonds in graphite or diamond?

Answer 10

Graphite because of the additional bonding caused by delocalised electrons

Question 11

Describe the distances between the layers in graphite

Answer 11

The distance between the layers is about 2.5 times the distance between the atoms within each layer

Question 12

Does graphite have a high or low mp?

Answer 12

High mp because it isn’t enough to loosen one sheet from another so it takes a lot of heat energy to break the strong covalent bonds throughout the structure

Question 13

Describe and explain the density of graphite compared to diamond

Answer 13

It has a lower density than diamond because of the relatively large amount of space that’s wasted between the sheets

Question 14

Describe & explain the electrical conductivity of graphite

Answer 14

It conducts electricity as the delocalised electrons are free to move throughout the sheets. If a piece of graphite is connected into a circuit, electrons can fall off one end of the sheet and be resolved with the new ones at the other end

Question 15

What arrangement does frozen water take

Answer 15

Many arrangements but the 2 most common ones are hexagonal ice & cubic ice

Question 16

What’s the theory of electron repulsion?

Answer 16

Electron pairs will take up positions as far away from each other as possible to minimise the repulsive forces

Question 17

What is the shape and bond angle in a molecule with 2 bp & 2 lp?

Answer 17

V-shaped & 104.5°

Question 18

What is the shape and bond angle in a molecule with 5 bp & 1 lp?

Answer 18

Square pyramidal & 90°

Question 19

What’s the shape and bond angle of NH2 and how many lone pairs does it have?

Answer 19

V-shaped, 104.5 degrees, 2 lp

Question 20

What’s the shape and bond angle of PF4 and how many lone pairs does it have?

Answer 20

Trigonal Bipyramidal, 90 & 120 degrees, 1 lp

Question 21

State what is meant by the term polarised

Answer 21

Electron cloud is unequally distributed

Question 22

What type of bonding is in AlCl3?

Answer 22

Ionic Bonding with a high degree of covalency

Question 23

Describe & explain the trend between hydrogen bonding and the size of the bonded atom

Answer 23

The bigger the atoms paired with hydrogen the more shielding the electrons have from the nuclei so the freer the electrons and the more polarised the molecules can get so the stronger the van der Waals bonds between them so the more energy needed to break the bonds therefore the higher the melting and boiling points.

Question 24

Between NH3, HF and water explain which molecule has the highest bp

Answer 24

Water has the highest boiling point because each molecule can potentially form 4 hydrogen bonds with other water molecules. It has 2 lone pairs for exactly 2 δ+ hydrogens, therefore all of the hydrogens are bonded and the resulting bond between the molecules is stronger. Not all of the δ+ hydrogens are so easily satisfied for NH3 and HF.

Question 25

Describe and explain why the H bonding between H and the first elements of groups 5, 6 and 7 is stronger than that between H and the rest of the elements

Answer 25

- The hydrogen is attached directly to one of the most electronegative elements, causing the hydrogen to acquire a significant amount of positive charge. - There is at least one lone pair of electrons on the central atom. --- These electrons sit on the second shell, which is relatively small in space, so the electron density is high - The δ+ hydrogen is strongly attracted to the lone pair. The attraction is significantly stronger than an ordinary dipole-dipole interaction.

Question 26

Describe and explain water's unique properties

Answer 26

- Water is strongly affected by hydrogen bonding - because Hydrogen bonds attract water molecules to one another, water is more tightly packed in liquid form than in solid form (where the molecules are held further apart in a lattice). Water is one of the few known substances whose solid form is less dense than the liquid. - Water's tightly packed liquid structure also alters its boiling point sufficiently so that solid, liquid and gaseous water can exist at the same temperature; this is known as the “triple point.”

Question 27

What are van der Waals forces of attraction? Are these forces greater in smaller or larger molecules?

Answer 27

Temporary dipoles are created by the random movement of electrons leading to induced dipoles in neighbouring molecules and cause temporary induced dipole-dipole attraction aka van der Waals forces of attraction. It is stronger in larger molecules as there are more electrons

Question 28

Fluorine reacts with bromine to form liquid bromine trifluoride (BrF3). State the type of bond between Br and F in BrF3 and state how this bond is formed

Answer 28

Covalent bond which is formed from shared pair(s) of electrons / one electron from Br and one electron from F *Not 2 electrons from 1 atom and not shared pair between ions/molecules*

Question 29

Draw a diagram to show how two molecules of hydrogen fluoride are attracted to each other by H bonding. Include all partial charges and all lone pairs of electrons in your diagram.

Answer 29

*- 3 marks in total - One mark for 4 partial charges (H^δ+ and F^δ-) - One mark for 6 lone pairs (3 around each F) - One mark for H bond from a lone pair of F to the H^δ+ - Allow Fl - If more than 2 molecules are shown they must all be correct. Treat any errors as contradictions within each marking point. CE = 0/3 if incorrect molecules shown*

Question 30

Out of metallic and ionic bonding, which is stronger?

Answer 30

Ionic

Question 31

Explain why the bond angle of any molecule is lower when there are lone pairs than when their aren't

Answer 31

- Electron pairs repel to be as far apart as possible - lp/lp repulsion > lp/bp repulsion > bp/bp repulsion

Question 32

Which statement is not box correct? A. CuCl4^2– is square planar. B. NH4^+ is tetrahedral. C. [Co(H2NCH2CH2NH2)3]^2+ is octahedral. D. [Fe(H2O)6]^2+ is octahedral.

Answer 32

CuCl4^2- is square planar