BPGHS SJChO Chemical Bonding II

Multiple Choice

1. (Lv. 1) Which of the following compounds does not contain both ionic and covalent bonds?

• NH₄Cl

• Al₂O₃

• MgSO₄

• Fe₃(PO₄)₂

Answer/Feedback

Al₂O₃ is a pure ionic compound. The others' cations or anions have covalent bonds in their structure (as we will see in Chemical Bonding III's F2F session).

Multiple Select

2. (Lv. 1) Which of the following statements are true?

• A covalent bond is represented by a straight line, while a dative bond is represented by an arrow pointing from the acceptor to donor atom.

• Both a covalent and coordinate covalent bond involves 2 atoms.

• A dative covalent bond is much weaker than a normal covalent bond.

• Just like covalent bond formation, a dative covalent bond tends to usually involve non-metals.

Answer/Feedback

The arrow of a dative covalent bond should point from the donor to acceptor atom. Since covalent and dative bonds involve electron sharing, non-metals are usually involved (but do note, metals can engage in electron sharing too!). Both a covalent and dative bond are interatomic bonds, so you will expect both to be strong.

Multiple Select

3. (Lv. 1) Which of the following statements are true?

• For the cyanide ion, CN^-, the formal charge of the N is -1.

• For the hydroxide ion, OH^-, the formal charge on H is -1.

• For the compound Na₂S, the formal charge on Na is +1.

• For the compound CsCl, the formal charge on Cl is -1.

Answer/Feedback

The formal charge of N = 0 in CN^-, based on the triple bond between C and N in CN^-. (For C, no. of electrons in bonded state = 3, no. of electrons in free element = 4, so 3-4 = -1). For the OH^- ion, the formal charge of H = 0, while that on O = -1 (only 5 electrons surround O in the hydroxide ion, so 5-6 = -1). You might want to re-think how you write the charge on the hydroxide ion in the 'O' Levels!

Multiple Choice

4. (Lv. 1) A strong acid is 1 that liberates "H" from a covalent molecule easily. Which of the following is the strongest acid?

• HF

• HCl

• HBr

• HI

Answer/Feedback

The question is hinting to choose the option with the weakest covalent bond. The weakest covalent bond between H and the halogen will mean that it will break off with lesser energy input, this causing H to be liberated the easiest.

Be Prepared!

The techniques listed in the previous 2 slides will be explored more in Chemical Bonding III (F2F Session), where you will showcase your creativity in drawing various covalent structures! For now, we will stick to very simple covalent structures for the content structure.

Multiple Choice

5. (Lv. 1) The electronic configuration of 3 atoms P, Q and R are as follows:

P: 2.8.2, Q: 2.6, R: 2.4. Which of the following formulae shows the correct covalent bond formation of the atoms?

• P=Q

• P-R-P

• R=Q-P

• Q=R=Q

Answer/Feedback

Notice that you can strike off options with atom P? (With that electronic configuration, do you think it can form covalent bonds? This essentially allows you to obtain the answer immediately...). With the structure of Q=R=Q, all elements will be able to attain a stable octet.

Multiple Choice

6. (Lv. 1) A covalent molecule comprising elements P, Q, R and S has the chemical structure as shown by the diagram on the left. Which of the following are likely to represent P, Q, R and S correctly?

• P=S, Q=Cl, R=P, S=C

• P=C, Q=N, R=H, S=O

• P=N, Q=C, R=F, S=Cl

• P=H, Q=O, R=C, S=S.

Answer/Feedback

A carbon atom forms 4 covalent bonds while a nitrogen atom forms 3 covalent bonds. Analysing R and S, R only forms 1 single covalent bond, while S forms a double covalent bond.

Multiple Choice

7. (Lv. 1) Which of the following pairs of covalent molecules have a different number of total valence shell electrons that are not involved in bonding?

• N₂ and H₂O

• H₂O and NH₃

• CO₂ and O₂

• H₂ and CH₄

Answer/Feedback

There are 2 covalent bonds formed in H₂O, while there are 3 covalent bonds formed in NH₃. Total number of valence electrons in H not involved in bonding = 0 (in both NH₃ and H₂O), Number of electrons in O not involved in bonding = 4, number of electrons in N not involved in bonding = 2.

Multiple Choice

8. (Lv. 1) Nitrogen and Phosphorous are both in Group V of the Periodic Table. Which Phosphorous compound is not likely to exist?

• PH₃

• PAt₃

• K₃P

• KP₃

Answer/Feedback

You will expect P to form 3 covalent bonds or an ionic bond with a -3 charge with another metal cation. Therefore, KP₃ is not possible, since +1 and -9 means the charges will not make the molecule neutral overall. Correct answer aside, does anyone see any problem/fallacy/ inaccurate logic with this question though?

Multiple Choice

9. (Lv. 1) The elements T, X and Y have increasing atomic numbers. If element X is a noble gas, what will be the chemical formula of the compound formed between T and Y?

• T₂Y

• YT₂

• TY

• YT

Answer/Feedback

Type of bonding between T and Y = ionic.

For ionic compounds, we usually write the cation 1st, followed by the anion.

Multiple Choice

10. (Lv. 1) An unknown compound formed by 2 elements Y and Z have a chemical formula YZ₃. Further experiments gave the following observations:

1. YZ₃ is a solid at room temperature.

2. No changes occur when YZ₃ is added to ethanol.

3. YZ₃ conducts electricity in the aqueous state.

• Y is a Group III element, while Z is a Group VII element.

• Y has 5 valence electrons, while Z has 7 valence electrons in their outermost shells.

• Y is an element located in Period 3, while Z is an element located in Period 2.

• Y and Z are likely to be Boron and Hydrogen respectively.

Answer/Feedback

The observations made reflect the physical properties commonplace in ionic compounds. Thus, options which hint of covalent bonding between Y and Z can be eliminated quickly. We do not have enough evidence to identify Y and Z as Boron and Hydrogen Respectively.

Multiple Choice

11. (Lv. 1) Which of the following explains why both copper and graphite can conduct electricity in the solid state?

• Both copper and graphite contain mobile ions.

• Both copper and graphite contain free electrons

• Both copper and graphite contain mobile ions and free electrons.

• Copper contains free electrons and mobile ions while graphite contains free electrons.

Answer/Feedback

Copper has free electrons from the structure of metallic bonding (recall Chemical bonding I), while the free electrons of Graphite come from the 1 electron which has not been used for bonding with 3 other carbon atoms in its structure.

Multiple Choice

12. (Lv. 1) Silicon Carbide (carborundum) is a shiny, hard, chemically inert material with a very high melting point. It can be used to sharpen knives and mae crucibles. Which type of structure explains these properties?

• A giant structure with covalent bonds between silicon and carbon atoms.

• A giant structure containing metallic bonds.

• A giant structure with covalent bonds between atoms and intermolecular forces between layers.

• A simple molecular structure with covalent bonds between the atoms of silicon and carbon.

Answer/Feedback

The structure of silicon carbide cannot possibly have intermolecular forces between the layers, or else, it will have the property of being soft, thus not able to allow it to "sharpen knives or make crucibles".

Multiple Choice

13. (Lv. 1) Silicate minerals make up the largest class of rock-forming minerals and constitute about 90% of the Earth's crust. Quartz (Silicon Dioxide) is an example of a silicate material.

Talc is the softest silicate mineral known and can be ground into powder form. It is widely used in body and baby talcum powders due to its slippery nature. It has a crystal lattice with a chemical formula of Mg₃Si₄O₁₀(OH)₂.

Which of the following is likely to be true about the structure of talc?

• It is simple molecular, with weak intermolecular attractive forces.

• It is macromolecular, with strong covalent bonds similar to Silicon Dioxide.

• It is ionic, with weak electrostatic attraction between the cations and anions.

• It is layered and macromolecular, with weak attractive forces between the layers,

Answer/Feedback

Since talc and quartz belong to the same silicate family, we can expect talc to have the macromolecular structure which is similar to that of quartz. Since it is soft and slippery, its structure is likely to be layered with weak attractive forces between the layers.

Multiple Select

14. (Lv. 2) Silicon Carbide and steel are carbon-containing compounds that display different properties. Which of the following statements about Silicon Carbide and steel are correct?

• Both compounds are covalently bonded.

• Both compounds have high melting points.

• Both compounds are insoluble in water.

• Both compounds conduct electricity in the solid state.

Answer/Feedback

Silicon Carbide has a giant molecular structure while steel is an alloy with metallic nature. Both compounds thus have high melting and boiling points but do not conduct electricity.

Multiple Choice

15. (Lv. 2) Boron is an element that belongs to Group III of the Periodic Table. It forms a nitrogen containing compound, known as Boron Nitride that has a similar structure to graphite. Which of the following statements is incorrect?

• Boron Nitride is a good thermal insulator.

• Boron Nitride comprises layers of elements that are arranged in the form of hexagonal rings.

• Boron Nitride is soluble in water but not in organic solvents

• The electrical conductivity of Boron Nitride is superior across the planes of hexagonal rings.

Answer/Feedback

Utilise the hint from the question that Boron Nitride resembles graphite. This can assist you in a few clues:
(a) It likely has hexagonal planar rings

(b) Being a macromolecule, is not soluble in anything

(c) It has high melting and boiling points, enabling it to withstand high temperature, due to covalent bonds throughout the entire structure.

How will the bonding be like between Boron Nitride?

Multiple Choice

16. (Lv. 2) 2 statements about Diamond and Silicon Dioxide made by students A and B during their Chemistry lesson are as shown:

Student A: There are delocalised electrons in Silicon Dioxide as there are electrons in Oxygen which are not involved in bonding.

Student B: The covalent bonds in Silicon Dioxide are greater in strength than that in Diamond as there is a greater electronegativity difference between Si and O.

Which of the following is correct?

• Student A is correct.

• Student B is correct.

• Both students are correct.

• Both students are incorrect.

Answer/Feedback

Electrons in oxygen atoms are not delocalised in the lattice of Silicon Dioxide although they are not involved in bonding. This is due to the fact that the SiO₂ structure does not show conjugation throughout as compared to that of graphite. Recall from the earlier section that, should there be a charge separation, it helps in increasing the strength of a covalent bond.

Multiple Choice

17. (Lv. 1) Which of the following best indicates that diamond and graphite are forms of carbon?

• Diamond can be formed from graphite under high pressure and temperature.

• Complete combustion of both solids yields Carbon Dioxide as the only product.

• Both diamond and graphite are giant lattices.

• Diamond is hard, while graphite is soft and slippery.

Answer/Feedback

Complete combustion: completely reacting the compound with oxygen. Since only carbon dioxide is produced, we are confident that the carbon has to originate from both diamond or graphite.

Multiple Choice

18. (Lv. 1) Buckyballs are structures of carbon used in nanotechnology. 1 type of Buckyball has the chemical formula C₆₀ and is shown on the diagram on the left. Which of the following can a Buckyball be classified as?

• An atom of an element.

• An atom of a compound.

• A molecule of an element.

• A molecule of a compound.

Answer/Feedback

As the question hinted, Buckyballs are individual balls of C₆₀ atoms. Each ball is not joined together covalently with one other (therefore the structure is not described as "giant covalent") and since these are made up of only Carbon, it is classified as an element.

Multiple Choice

Refer to the following to answer questions 19 and 20.

Scientists are investigating the properties and potential uses of tube-shaped structures of Carbon called nanotubes as shown on the left.

19. (Lv. 1) Which of the following describes the structure of a carbon nanotube?

• It contains immobile electrons.

• It is macromolecular and identical to that of graphite.

• Its carbon atoms are covalently bonded to 3 other carbon atoms.

• It has a simple molecular structure with weak intermolecular attractive forces.

Answer/Feedback

Observe the diagram provided, and you should have noticed that each carbon is connected to 3 other carbon atoms. Note that unlike graphite, the layers of the carbon nanotubes are not layered!

Multiple Choice

20. (Lv. 1) A single perfect carbon nanotube can be about 10-100 times stronger than steel (an alloy) per unit weight. Which of the following is the most likely explanation for this property of carbon nanotubes?

• They cannot slide over 1 another.

• They have a more rigid structure than steel.

• The attractive forces between the macromolecular nanotubes are very strong.

• The Carbon-Carbon covalent bonding in the nanotubes is stronger than the bonding in steel.

Answer/Feedback

The strength of a material depends on the strength of the bonding between particles that constitute it. Steel is an alloy that consists of metallic bonding, and the only other types of bonding which are comparable/stronger are ionic or covalent bonding.

Multiple Select

Refer to the following to answer questions 21 and 22.

Graphene is a new allotrope of Carbon recently isolated by scientists. It has been described as a 2-dimensional sheet of Carbon atoms arranged in a hexagonal pattern like a honeycomb as shown by the diagram on the left.

21. (Lv. 1) Which of the following statements describe the properties of graphene?

• It is soluble in water.

• It is very strong and rigid.

• It has a high melting point.

• It is an electrical conductor.

Answer/Feedback

The structure resembles a single layer of graphite and thus, electrical conductivity should be expected. With covalent bonds throughout the structure, a high melting point is expected like graphite.

Multiple Choice

22. (Lv. 1) Since the discovery of graphene, scientists have been able to convert it to another material known as graphane by attaching 1 hydrogen atom to each carbon atom as shown by the diagram on the left.

Graphane has the same honeycomb structure as graphene, and retains most of its properties too. Which property of graphene is not likely to be shared by graphane?

• It is insoluble in water.

• It is very strong and rigid.

• It has a high melting point.

• It is an electrical conductor.

Answer/Feedback

The ability of graphene (and graphite) to conduct electricity is due to the availability of 1 electron from each carbon atom not used in bonding. Adding hydrogen to the structure will make use of this electron to form covalent bonds with H, thus robbing the property of electrical conductivity.