RI 2025 H2 Chemical Bonding I Tutorial Answers

10 Answers to Practice Questions (Long) 10. (a) (b) H3O+ and Cl– ions formed Species Bond angle No. of e – pairs No of bond pairs No. of lone pairs Shape H2O 105 4 2 2 bent H3O+ 107 4 3 1 trigonal pyramidal Lone pair – lone pair repulsion > lone pair – bond pair repulsion > bond pair – bond pair repulsion H2O  H3O+ One lone pair becomes a bond pair through co-ordinate bonding  No lone pair – lone pair repulsion in H3O+. this results in a greater bond angle. Dot-and-cross Structural formula H2O H3O+ O H H H 1070 (c) BeF2 ∙ 2NH3 adduct N Be N H H H H H H F F 109o 109o 109o 109o 109o tetrahedral wrt to both Be and N in the product 11. (a) (b) Both molecules have three regions of electron density and thus their electron-pair geometry is trigonal planar.  Electron pairs exert greater repulsion than an unpaired electron. As a result, the bond pair – bond pair repulsion > bond pair – unpai red electron repulsion in NO 2. Thus, the bond angle in NO2 would be greater than 120 o, e.g. 130 o (The actual bond angle is 134 o. Acceptable answers are any value from 120o to 170o).  Lone pair – bond pair repulsion > bond pair – bond pair repulsion. For O 3, the lone pair of electrons on the central O exert greater r epulsion than the bond pairs of electrons in O-O  x O OO xxx x     x  x N OO xxx x    

11 bond. Thus, the bond angle in O 3 would be less than 120 o, e.g. 118o (The actual bond angle is 117o. Acceptable answers are from 110o to 120o). Teachers, pls use this question as a teaching point: A lone pair or a bond pair exerts greater repulsion than a single electron as the repulsion exerted by 2 electrons is greater than 1 electron. (c) NO 2 has an odd number of electrons i.e. there is an unpaired electron in N. Dimerisation only involves the formation of N–N covalent bond which releases energy to the surrou ndings. The energy of the products is less than that of the reactants, hence the reaction is feasible. (Please note that there are other considerations that will determine the feasibility of reactions. They will be discussed in Lecture 5 – Energetics.) (d) NO2+ NO 2– Dot-and-cross diagram No. of electron densities around N: 2 3 To minimise electrostatic repulsion, the electron–pair geometries are: linear trigonal planar Number of lone pairs 0 1 Molecular shape is linear bent 12 (a) (i) Phosgene, Cl2C=O, has 3 bond pairs and 0 lone pairs of electrons around the central C atom. To minimize electronic repulsion between the bond pairs, the shape of the phosgene molecule is trigonal planar. (ii)  (sigma) bond p p head-on overlap of p orbitals (show head-on overlap of either s/p orbitals)  (pi) bond