NJC 2023 Chemical Bonding Tutorial Answer

National Junior College SH1 H2 Chemistry 1 CHEMICAL BONDING TUTORIAL Interatomic Bonding 1(a) Fill in the following table by matching the following substances to their physical properties. graphite, diamond, copper, iodine, sodium chloride Properties substance structure Describe bonding and relate it to the physical properties (a) A hard and brittle solid. NaCl Giant ionic lattice Strong ionic bonds require great amt of energy to overcome. When the regular arrangement of the oppositely charged ions are disrupted, repulsion between like charges causes the lattice to break down. (b) A solid that conducts electricity and melts at 1085oC. Cu Giant metallic structure Strong Metallic bond require great amt of energy to overcome. Sea of delocalised electrons are the mobile charge carriers than enable electrical conductivity. (c) A solid that cuts through metal and does not conduct electricity. Diamond Giant covalent lattice (Giant molecular lattice) Form strong c ovalent bonds with 4 other C atoms in a 3D tetrahedral arrangement, making the lattice very strong. All valence e used for sigma bond formation, absence of mobile charge carriers. (d) A substance that sublimates when heated. I2 Simple molecular structure • Covalent bonding within molecule • Weak intermolecular force of attraction between molecules

National Junior College SH1 H2 Chemistry 2 that can be easily overcome with heat. (e) A solid that can be used as a lubricant. Graphite Giant covalent lattice (Giant molecular lattice) • Forms covalent bond with 3 other C atoms in a 2D network hexagonal carbon rings => graphene layer • Weak intermolecular force of attraction between layers causing the layers to slide over each other easily. 1(b) 2010 MCQ 6 Which diagram best represents the structure of solid magnesium oxide? 1(c) The melting point of calcium, 839oC, is much higher than the melting point of sodium, 98 oC. Which statement is most relevant in explaining the difference? A The calcium atom is larger than the sodium atom. B The calcium atom is heavier than the sodium atom.

National Junior College SH1 H2 Chemistry 3 C The calcium ion, Ca2+, has a higher charge than the sodium ion, Na+. D The calcium ion, Ca2+, contains more electrons than the sodium ion, Na+. Calcium and sodium have giant metallic lattices. Melting point magnitude reflects metallic bond strength. Giant metallic lattice consists of cations in a sea of delocalized electrons, thus metallic bond strength increases with 1) Greater number of valence electrons involved in delocalization. 2) Cation of higher charge over size ratio when factor 1 is the same. Statement C is the answer as it implies that there are greater number of valence electrons involved in the sea of delocalised electrons for calcium than sodium. 2 Which statement(s) are correct? I HCl (aq) cannot conduct electricity. II Covalent bond is a weak bond compared to ionic bond as boiling point of H2O is lower than that of MgO. III Ionic bonds and covalent bonds can both occur in the same compound. IV Metals can be distinguished from ionic compounds by their electrical conductivity in the solid state. A Statements I and III are correct. B Statements II and IV are correct. C Statement III and IV are correct. D Statements I, III and IV are correct I False. Simple covalent molecule with acidic or basic properties will dissociate in water to form H+ or OH− ions. E.g HCl, HNO3, NH3. Thus HCl is an electrical conductor in aq medium. II False. Magnitude of B.pt of H 2O is not indicative of the covalent bond strength in H 2O since boiling overcomes the intermolecular hydrogen bonding NOT the covalent bond. Ionic bond and covalent bond are interatomic bonds, they are relatively strong compared to IMF. Since ionic bonds are broken in the giant ionic lattice of MgO, its b,pt is much higher compared to H2O. III True. Eg. Na2SO4. Ionic bonds between Na+ and SO42– ions, covalent bonds between S and O in SO42– ion. IV True. Metals conduct electricity in solid state due to the mobile sea of delocalised electrons, while ionic compounds DO NOT conduct electricity in solid state as the ions are fixed in positions. Hence, this can be used to distinguish metal from ionic compounds. 3(a) Boron trifluoride and ammonia reacts in a 1:1 mole ratio to form a single compound. Describe the type of bond formed during this reaction. N atom in ammonia forms a dative bond to B atom in BF3. B in BF3 is short of 2 electron to attain octet while N in NH 3 has a lone pair. hence BF3 and NH3 reacts 1:1 ratio where lone pair of N in NH 3 is used to form a coordinate (dative) covalent bond with B in BF3.

National Junior College SH1 H2 Chemistry 4 3(b) 2015 MCQ 2 Which row about sigma () bonds and pi () bonds in hydrocarbons is correct? () bonds () bonds A Can be formed by either s or p orbitals Can be formed by either s or p orbitals B Can be formed by either s or p orbitals Can be formed by p orbitals but not s orbitals C Can be formed by p orbitals but not s orbitals Can be formed by either s orbitals but not p orbitals D Can be formed by s orbitals but not p orbitals Can be formed by p orbitals but not s orbitals In diatomic molecules  bonds can be formed from the overlap of either s and/or p orbitals  bonds in can only be formed from the side-on overlap of p orbitals, not s orbitals For this question, hydrocarbon is used as the example: -  bonds in C -C and C -H bonds can be formed from the overlap of either s and/or p orbitals Note: C atoms should be forming  bonds using sp / sp2 / sp3 hybrid orbitals depending on the shape wrt C atom. -  bonds in C=C bonds can only be formed from the side-on overlap of p orbitals, not s orbitals B is the best ans 4 Explain the following observations. i. Magnesium oxide is used in making bricks for lining high temperature furnaces; while sodium chloride cannot be used for this purpose. Explain this observation in terms of structure and bonding. Both MgO and NaCl have giant ionic lattice structures. • Product of charges (q + × q−) is larger for MgO and interionic distance (r+ + r−) is smaller for MgO. • Since |L.E| ∝ | q+ × q– r+ + r– |, magnitude of lattice energy of MgO is greater than NaCl. • More energy is needed to overcome the stronger ionic bonds in MgO than in NaCl.

National Junior College SH1 H2 Chemistry 5 • MgO has a higher mp than NaCl, hence can be used in lining of high temp. furnaces. ii. At room temperature and pressure, CO 2 is a gas, while SiO 2 is a solid of high melting temperature. CO2 exists as simple molecular structure with weak intermolecular forces/ instantaneous dipole - induced dipole (id-id) interactions between CO2 molecules. SiO 2 exists as a giant covalent structure whereby all atoms are bonded to each other by strong covalent bonds. The energy supplied at room temperature is sufficient to overcome weak intermolecular forces between CO2 molecules to allow CO2 to exist as a gas. However, energy is insufficient to overcome the strong covalent bonds in SiO 2 hence, SiO 2 exist as solids at room temperature. Note: must relate back to physical states of the compounds at rtp. The question is not focusing about the mp/bp. iii. SiC has a higher melting point (2730 C) than SiGe (1176 C) Both SiC and SiGe have giant covalent lattice structure. C atom has smaller atomic radius than that of Ge and hence there is more effective overlap of orbitals between Si and C atoms as compared to Si and Ge. More energy is required to break the stronger covalent bonds in SiC than that of SiGe. There fore, SiC has higher melting point than SiGe.

National Junior College SH1 H2 Chemistry 6 Dot-and-Cross Di