NJC 5 H1 Chemical Bonding Tutorial Answer

National Junior College SH1 H1 Chemistry 1 CHEMICAL BONDING TUTORIAL Interatomic Bonding 1 Using the list below, give one example to each of the following solid lattices showing the given properties. Name the structure and type of bonding found in each lattice. graphite, diamond, copper, iodine, sodium chloride (a) A hard and brittle solid. (b) A solid that conducts electricity and melts at 1085oC. (c) A solid that cuts through metal and does not conduct electricity. (d) A substance that sublimates when heated. (e) A solid that can be used as a lubricant. Structure Bonding type Example (a) ionic lattice Ionic bonding NaCl (b) metallic lattice Metallic bonding Cu (c) Giant molecular structure Covalent bonding Diamond (d) Simple molecular structure • Covalent bonding within molecule • Weak intermolecular force of attraction between molecules I2 (e) Giant molecular structure • Covalent bonding between atoms within a layer • Weak intermolecular force of attraction between layers Graphite 2 Which statement(s) are correct? I HCl (g) can 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 and liquid 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. HCl (g) has simple molecular structure with no mobile charge carrier. HCl can dissociate into ions H+(aq) and Cl−(aq) in water, and conduct electricity in aq state. II False. Both are strong bonds. The boiling of H 2O involves only the breaking of the weak hydrogen bonds between the H 2O molecules. However, the strong ionic bo nd is overcome in MgO when it undergoes phase change during boiling point.

National Junior College SH1 H1 Chemistry 2 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 molten and solid state, while ionic compounds conduct electricity in molten state, not in solid state. Hence, this can be used to distinguish metal from ionic compounds. 3 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 atom in BF3 is electron deficient, hence B attains the octet by accepting a lone pair from N in ammonia into its empty p-orbital. 4 Explain the following observations in terms of structure and bonding. i. MgO has a higher melting point than NaCl. Both MgO and NaCl have 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 NaC l. • More energy is needed to overcome the stronger ionic bonds in MgO than in NaCl. • MgO has a higher mp than NaCl. 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. SiO2 exists as a giant molecular 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, SiO2 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.

National Junior College SH1 H1 Chemistry 3 Dot-and-Cross Diagram and VSEPR 5 Fill up the table below for the following compounds: compound ‘dot-and-cross’ diagram (i) Draw and name the shape of the covalent molecule or polyatomic ion (ii) Indicate the bond angle in the diagram Bonds or intermolecular forces broken during melting or boiling process (i) Al2O3 (m.p. 2072C) Ionic compound NA Ionic bond (ii) AlCl3 (m.p. 192C) Covalent molecule Al Cl Cl Cl Trigonal planar 120o Instantaneous dipole-induced dipole (iii) PCl3 (l) trigonal pyramidal, 107o Permanent dipole-permanent dipole & Id-id (iv) CH2Cl2(l) tetrahedral, 109.5o Permanent dipole-permanent dipole & Id-id (v) PCl5(s) Trigonal bipyramidal Instantaneous dipole-induced dipole (vi) HCN Linear Permanent dipole-permanent dipole & Id-id H C N 180 o

National Junior College SH1 H1 Chemistry 4 compound ‘dot-and-cross’ diagram (i) Draw and name the shape of the covalent molecule or polyatomic ion (ii) Indicate the bond angle in the diagram Bonds or intermolecular forces broken during melting or boiling process (vii) SO2 Bent, 118o (<120o) Permanent dipole-permanent dipole & Id-id (viii) SO3 S O O O Trigonal planar 120 Instantaneous dipole-induced dipole (ix) H2O2 (H-O-O-H arrangement) around each O Bent 105 Hydrogen Bond & Id–id (x) (NH4)2SO4 tetrahedral about N and S Ionic bond O S O OO 109.5 o 2- H N H HH 109.5 o + 2

National Junior College SH1 H1 Chemistry 5 6 A stable molecule containing atoms of the elements, X, Y and Z has the following structure. Which option is a possible combination of the elements? X Y Z A N P Cl B O S Cl C B N H D P O F • Y formed 5 covalent bonds→ Group 15, Period 3 and beyond • X formed 3 covalent bonds. Could be Group 13 or 15. • Z formed 1 covalent bond → Either hydrogen or Group 17 7 MTBE is a constituent of petrol. What are the values of angle P and angle Q in a molecule of MTBE? angle P angle Q A 90o 105o B 90o 180o C 109o 105o D 109o 180o Answer C • Central atom, C, has 4 bond pairs and no lone pair→ Tetrahedral in shape with bond angle of 109.5o • Central atom, O, has 2 bond pairs and 2 lone pair→ Bent in shape with bond angle of 105o 8 Using VSEPR theory, predict which compound in each of the following pair of molecules has a larger bond angle. (a) BCl3 and NCl3 (b) H2S and PH3 (a) BCl3 has 3 electron regions (3 bond pairs) whereas NC l3 has 4 electron regions (3 bond pairs and 1 lone pair). By VSEPR theory, the electron regions around the central atom arrange themselves to be as far apart as possible to minimise mutual repulsion. Therefore, BCl3 has a trigonal planar shape and the bond angle is 120 o whereas NCl3 has a trigonal pyramidal shape and the bond angle is 107 o. BCl3 has a larger bond angle. (b) H2S and PH3 has 4 electron regions each. H2S has 2 bond pairs and 2 lone pairs whereas PH 3 has 3 bond pairs and 1 lone pair. By VSEPR theory, lone pair-lone pair repulsion is stronger than lone pair -bond pair repulsion which is in turn stronger than bond pair-bond pair repulsion. In H2S, the stronger lone pair -lone pair repulsion forces the bond pairs to be closer together, decreasing the bond angle to 105o as compared to 107o in PH3. Therefore, PH3 has a larger bond angle. X Y X Y X Y Z Z Z Z Z Z C CH3 H3C O CH3 CH3 angle Q MTBE angle P

National Junior College SH1 H1 Chemistry 6 9 [N10/III/4c] (a) Draw ‘dot-and-cross’ diagrams to show the bonding in the molecules of NO2, O3 and BF3. In the molecule NO 2, the central atom is nitrogen. In each case you should distinguish carefully between electrons originating from the central atom and those from the two outermost atoms. Include all lone pairs in your diagrams.