Topic 2: Bonding and Structure

Question 1

What type of structure does sodium chloride have? [1]

Answer 1

1. Giant ionic lattice

Question 2

Would you expect sodium chloride to have a high or low melting point? Explain your answer. [2]

Answer 2

1. High melting point;
2. A lot of energy is required to overcome the strong electrostatic attraction between the positive and negative ions

Question 3

How would you expect the melting point of NaBr to compare with NaCl? Explain your answer. [3]

Answer 3

1. NaBr would have a lower melting point than NaCl;
2. Bromide ions have one more electron shell than chloride ions, so have a larger ionic radius. This means the ions in NaBr can’t pack as closely together as the ions in NaCl;
3. Ionic bonding gets weaker as the distance between the ions increases, so the ionic bonding in NaBr is weaker than in NaCl

Question 4

Solid CaO does not conduct electricity, but molten CaO does. Explain this with reference to ionic bonding. [3]

Answer 4

1. In solid, ions are held in place by string ionic bonds;
2. When molten, the ions are mobile;
3. so carry charge through the substance

Question 5

In terms of electron transfer, what happens when sodium reacts with fluorine to form sodium fluoride? [3]

Answer 5

1. Sodium loses one electron to form Na+;
2. Fluorine gains one electron to form F-;
3. Electrostatic forces of attraction between oppositely charge ions forms an ionic lattice.

Question 6

Would you expect an N-N single bond to be shorter or longer than an N=N bond? Explain your answer. [3]

Answer 6

1. Longer;
2. There are 4 shared electrons in an N=N bond and only 2 shared in an N-N bond, so the electron density between the two nitrogen atoms is the nitrogen double bond is greater;
3. Increases the strength of the electrostatic attraction between the positive nuclei and the negative electrons in the N=N bond, making the bond shorter

Question 7

Would you expect an N-N single bond to be shorter or longer than an N=N bond? Explain your answer. [3]

Answer 7

1. Longer;
2. There are 4 shared electrons in an N=N bond and only 2 shared in an N-N bond, so the electron density between the two nitrogen atoms is the nitrogen double bond is greater;
3. Increases the strength of the electrostatic attraction between the positive nuclei and the negative electrons in the N=N bond, making the bond shorter

Question 8

Explain why the shapes of NCl3 and BCl3 are different. [3]

Answer 8

1. BCl3 has 3 electron pairs around B;
2. NCl3 has 4 electron pairs around B;
3. including 1 lone pair

Question 9

Explain what is meant by metallic bonding. [1]

Answer 9

1. The attraction between positive metal ions and a sea of delocalised electrons between them

Question 10

Explain why calcium has a higher melting point than potassium. [1]

Answer 10

1. Calcium has two delocalised electrons per atom but potassium only has one delocalised electron per atom. So calcium has more delocalised electrons and therefore stronger metallic bonding

Question 11

Silicon dioxide is a covalent compound that melts at 1610°C. Explain the high melting point of silicon in terms of its bonding. [2]

Answer 11

1. Silicon dioxide has a giant covalent lattice structure;
2. so, to melt it, lots of strong covalent bonds must be broken, which requires high temperatures

Question 12

Explain why graphite is able to conduct electricity. [2]

Answer 12

1. Graphite consists of sheets of carbon atoms, where each carbon atom is bonded to three others;
2. So each atom has one free electron not involved in bonding and these free electrons allow graphite to conduct electricity

Question 13

Electrical grade copper must be 99.99% pure. If sulphur and oxygen impurities react with the copper ions, its electrical conductivity is reduced. Use your knowledge of metallic and ionic bonding to explain this. [3]

Answer 13

1. Copper is metallically bonded and so delocalised electrons are free to move (carry electric charge);
2. Oxygen and sulphur form copper oxide/sulphur, fixing some electrons (as anions);
3. This prevents them from moving and carrying charge

Question 14

Carborundum (silicon carbide) has the formula SiC and is almost as hard as diamond.
What sort of structure would you expect carborundum to have as a solid? [1]

Answer 14

1. Giant covalent

Question 15

Carborundum (silicon carbide) has the formula SiC and is almost as hard as diamond.
Apart from hardness, give two other physical properties you would expect carborundum to have. [2]

Answer 15

Any two from:
1. High melting point;
2. Electrical insulator;
3. Insoluble;
4. Good thermal conductor

Question 16

Define the term electronegativity. [1]

Answer 16

1. The ability of an atom to attract the bonding electrons in a covalent bond

Question 17

What are the trends in electronegativity as you go across a period and down a group in the periodic table? [1]

Answer 17

1. Electronegativity increases across a period and decreases down a group

Question 18

Explain why the boiling point of pentane is higher than that of 2-methylbutane, which is higher than that of 2,2-dimethylpropane, even though they all have the molecular formula C5H12. [3]

Answer 18

1. Boiling point depends on energy needed to overcome intermolecular forces between molecules;
2. Pentane is the most linear molecule, so has greatest surface constant, so the strongest London forces, and so the highest boiling point;
3. Surface contact if 2,2-dimethylpropane is smaller than that if 2-methylbutane, which is lower than that if pentane, so these substances have weaker London forces and so lower boiling points

Question 19

What intermolecular forces are present in chloroethane (CH3CH2Cl)? [1]

Answer 19

1. London forces and permanent dipole-permanent dipole bonds

Question 20

N2 and NO are both gases are room temperature. Predict, with reasoning, which has has a higher boiling point. [2]

Answer 20

1. NO has higher boiling point. Both molecules have similar number of electrons so strength of London forces will be similar;
2. NO is a polar molecule so also forms permanent dipole-permanent dipole bonds, so has stronger intermolecular forces between molecules than N2, which can only form London forces

Question 21

Explain why water’s boiling point is higher than expected in comparison to other similar molecules. [2]

Answer 21

1. Contains hydrogen covalently bonded to oxygen so can form hydrogen bonds;
2. Hydrogen bonds are stronger than other types of intermolecular forces, so more energy is needed to break them

Question 22

Explain why alcohols often dissolve in water but halogenoalkanes don’t. [4]

Answer 22

1. Hydrogen bonds;
2. form between the alcohol and water molecules;
3. The (hydrogen) bonds between water molecules are stronger;
4. than bonds that would form between water and the halogenoalkane molecules

Question 23

Explain the process by which potassium iodide dissolves in water to form hydrated ions. [4]

Answer 23

1. K+ ions attracted to the δ- ends of the water molecules;
2. I- ions are attracted to the δ+ ends;
3. The ions are pulled away from the lattice;
4. and surrounded by water molecules forming hydrated ions

Question 24

An unknown substance, X, is suspected to be a non-polar simple covalent molceule. Describe how you could confirm this by testing with two different solvents. Name the solvents chosen and give the expected results. [3]

Explain these results in terms of the intermolecular bonding within X and the solvents. [4]

Answer 24

1. Try to dissolve the substance in water;
2. and hexane;
3. If X is non-polar, it is likely to dissolve in hexane, but not in water.
4. X and hexane have London forces between their molecules;
5. and form similar bonds with each other;
6. Water has hydrogen bonds;
7. which are much stronger than the bonds it could form with a non-polar compound

Question 25

Iodine, I2, and graphite are both solid at r.t.p.. At 500K, iodine exists as a gas, while graphite remains solid. Explain this difference in the properties of iodine and graphite in terms of their structures. [4]

Answer 25

1. Iodine is a simple molecular substance;
2. To melt or boil iodine, you only need to overcome the weak intermolecular forces holding the molecules together, which doesn’t need much energy;
3. Graphite is a giant covalent substance;
4. Graphite will remain solid unless you can overcome the string covalent bonds between atoms, which needs a lot of energy

Question 26

Diamond, graphene and graphite are different forms of carbon.
One way in which diamond differs from graphene and graphite is that only diamond has:

A a high melting temperature
B a precise molecular formula
C poor electrical conductivity
D a giant structure [1]

Answer 26

1. C

Question 27

What type of reaction occurs when ammonia gas reacts with hydrogen chloride gas?

A acid-base
B displacement
C redox
D substitution [1]

Answer 27

1. A

Question 28

What is the bond angle in BF3? [1]

Answer 28

1. 120°

Question 29

Which molecule has a linear shape?

A H2S
B SO2
C CO2
D CH2=CH2 [1]

Answer 29

1. C

Question 30

What is ionic bonding the strong electroststic attraction between? [1]

Answer 30

1. Anions and cations

Question 31

State what is meant by the term covalent bond. [2]

Answer 31

1. (Strong electrostatic) attraction;
2. between two nuclei and the shared pair of electrons

Question 32

State the type of bond that joins two AlCl3 molecules together. [1]

Answer 32

1. Dative (covalent) bonds

Question 33

NH3 and BF3 react to form NH3BF3 which contains a dative covalent bond. Explain how the dative covalent bond is formed. [2]

Answer 33

1. Donation of lone pair (of electrons) from nitrogen;
2. to boron (atom) which only has 6 electrons in outer shell

Question 34

State what is meant by the term electronegativity and hence explain the polarity, if any, of the bonds in chlorine trifluoride, ClF3. [3]

Answer 34

1. Electronegativity is the ability of an atom to attract the (bonding) electrons (in a covalent bond);
2. Fluorine is more electronegative than chlorine;
3. so fluorine is δ- and chlorine is δ+

Question 35

Explain why the melting temperature of silicon(IV) oxide is much higher than that of iodine, even though the bonding in both is covalent. [3]

Answer 35

1. Silicon(IV) oxide (is a giant structure so) contains many (strong covalent) bonds;
2. Iodine - (only) weak intermolecular forces must be broken;
3. More energy is needed to break the stronger bonds in silicon(IV) oxide

Question 36

Explain why both water and carbon dioxide molecules have polar bonds but only water is a polar molecule. [4]

Answer 36

1. Oxygen is more electronegative than hydrogen and carbon;
2. which results in a polar bond with oxygen δ- so carbon and hydrogen δ+;
3. Carbon dioxide is a symmetrical/linear molecule and so the dipole moments cancel;
4. The lone pairs of electrons of oxygen mean that the dipole moments don’t cancel

Question 37

Explain why silicon has a higher melting temperature than chlorine. [3]

Answer 37

1. Silicon - giant covalent and covalent bonds;
2. Chlorine - (simple) molecular and contains London forces;
3. (Covalent) bonds in silicon are stronger than intermolecular forces in chlorine

Question 38

Explain why diamonds has a much higher melting temperature than iodine. [5]

Answer 38

1. Iodine is (simple) molecular;
2. Diamond is a giant (covalent) structure;
3. Iodine molecules are held together by weak London forces;
4. Carbon atoms in diamond are held together by (strong) covalent bonds;
5. String covalent bonds require more energy to break than intermolecular forces

Question 39

B and Al are both in group 3 and form compounds with Cl and F.
Alf and AlCl are both crystalline solids at room temperature. AlF sublimes at 1291°C but AlCl sublimes at 178°C.
Use the Pauling electronegativity values in the Data Booklet to explain these differences in sublimation temperature. [6]

Answer 39

1. Aluminium and chlorine electronegativity difference 2.5 and aluminium and fluorine electronegativity difference 2.5;
2. Aluminium chloride (mostly) covalent;
3. Aluminium fluoride (bonds) more polar;
4. Aluminium chloride molecular so weak intermolecular forces;
5. Aluminium fluoride is a giant structure;
6. More energy needed to break the stronger bonds to cause sublimation in aluminium fluoride