TOPIC 4 ANSWERS & MARK SCHEMES QUESTIONSHEET 1 IONIC BONDING

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Augustus Greene
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1 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 1 IONIC BONDING a) A bond formed by the complete transfer of one or more electrons from one atom to another to form ions of opposite charge which attract each other electrostatically b) 1 mark for the correct electronic configuration (complete shell acceptable for the metal ion) (i) Ba + Ba 2+ 2 (ii) Ba + O Ba 2+ O 2 (iii) K + K + (iii) K + O K + 2 O 2 c) (i) In aqueous solution, ions are able to form ion-dipole interactions with water releasing sufficient energy to compensate for the energy required to break up the lattice They cannot interact with most organic solvents (ii) In the solid the ions are fixed, and cannot move In liquid or solution the ions are free to move So can carry charge when a potential difference is applied

2 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 2 COVALENT BONDING a) (i) A bond formed by the sharing of a pair of electrons between two atoms, one electron per atom (ii) A bond involving the sharing of a pair of electrons between two atoms / a special kind of covalent bond both electrons being donated by one atom b) (i) (ii) Si N + (iii) C N 2 marks each: deduct 1 mark for lack of charge, failure to show distinction between electrons, incorrect number of electrons c) B 3 / B 3 / Al 3 / Be 2 all electron-deficient Or SO 2 / SO 3 / P 5 / S 6 exceed octet d) Al 3 has a vacant orbital / has an incomplete octet and can accept a lone pair of electrons from the chlorine

3 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 3 σ AND π BONDING a) The double bond consists of a σ-bond and a π-bond whereas a single bond consists of a σ-bond only Electrons in a π-bond lie above and below the inter-nuclear axis making less contribution to the reduction of inter-nuclear repulsion / so that the bond is weaker / has a bond enthalpy of only ( ) = 264 kj mol -1 b) π σ σ C σ C σ σ π C- σ-bonds C-C σ-bond C-C π -bond c) In most reactions, only the π-bond is broken This is weaker than the σ-bond in ethane and the electrons, since they lie beyond the inter-nuclear axis, are more accessible to (electrophilic) attack d) (i) Because the bond enthalpy is intermediate between that of a single bond and that of a double bond the bond length will also be intermediate (ii) The six σ-bonds between the carbon atoms are all the same length so that the six singly-occupied p orbitals are the same distance apart and overlap to the same extent The six electrons are thus delocalised Maximum 3 marks

4 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 4 SAPES O MOLECULES AND IONS I a) (i) Pairs of electrons repel one another to achieve the lowest energy arrangement The order of repulsion is lone pair:lone pair > lone pair:bonding pair > bonding pair:bonding pair (ii) Only valence shell electrons are considered Effects of the nuclei are ignored Multiple bonds are treated as single bonds Maximum 2 marks b) (i) B Group 3 3 valence electrons; 3 atoms 3 valence electrons. Total 6 electrons 3 pairs. Lowest energy arrangement for 3 pairs is trigonal planar. 3 bonding pairs, no lone pairs, so shape is also trigonal planar: B (ii) P Group 5 5 valence electrons; 3 atoms 3 valence electrons. Total 8 electrons 4 pairs. Lowest energy arrangement for 4 pairs is tetrahedral. 3 bonding pairs, 1 lone pair, so shape is trigonal pyramidal: P < (iii) S Group 6 6 valence electrons; 2 atoms 2 valence electrons. Total 8 electrons 4 pairs. Lowest energy arrangement is tetrahedral. 2 bonding pairs, 2 lone pairs, so shape is bent / V-shaped / angular: S marks each: for reasoning and name of shape; for diagram - diagrams must show bond angles c) (i) Species which have the same number of electrons have the same shape (ii) SO 3 2- has = 42 electrons P 3 has = 42 electrons P 3 is trigonal pyramidal (cf P 3 above) so by the isoelectronic principle SO 3 2- is also trigonal pyramidal

5 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 5 SAPES O MOLECULES AND IONS II a) (1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 ) 5s 2 5p 6 core valence shell - b) To form compounds, xenon has to expand its octet This requires energy to promote electrons to a vacant orbital of higher energy so that they may pair up with electrons from the bonding atoms Only xenon has such orbitals which are sufficiently close for this energy to be recovered in forming bonds and only then with very electronegative atoms such as fluorine and oxygen Maximum 4 marks c) In each case, xenon contributes 8 valence electrons: (i) Explanation and diagram 8 electrons + 2 from fluorine - total 10 electrons 5 pairs. 2 bonding pairs, 3 lone pairs; lone pairs enter equatorial plane Xe Description of shape Linear (ii) Explanation and diagram 8 electrons + 6 from oxygen total 14 electrons. Oxygen is divalent, but Xe-O double bonds are treated as single bonds. Effectively 3 bonding pairs, 1 lone pair O Xe O O Description of shape Trigonal pyramidal (iii) Explanation and diagram 8 electrons + 6 from fluorine - total 14 electrons 7 pairs. 6 bonding pairs, 1 lone pair (which enters equatorial plane) Xe 71 0 Description of shape Modified octahedron / pentagonal bipyramidal

6 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 6 IONIC BONDING TENDING TOWARDS COVALENCY a) Polarising power of a cation Its ability to polarise / distort an anion so that electrons are attracted to the cation causing the bond to assume some degree of covalent character It is proportional to charge : radius ratio / charge density on the surface of the cation Polarisability of an anion Its tendency to become distorted by a cation so that electrons move towards the cation again causing the bond to assume some degree of covalent character It is proportional to charge radius / charge size b) (i) Al 3+ is small and highly charged and therefore very polarising It can polarise the chloride ion to form a covalent bond but not the fluoride ion, which is much smaller and not polarisable (ii) The chloride ion is moderately polarisable Be 2+ would be a very small and hence very polarising cation, and would polarise - to form covalent bonds Mg 2+ is larger, and not sufficiently polarising to do this [Note: there is still some covalent character in Mg 2. Also, Be 2+ is very unlikely to be formed, since the sum of the ionisation energies for such a small atom would be very high.] c) The lithium ion is very small, and even though not highly charged is quite polarising The iodide ion is large, and reasonably polarisable Therefore expect a large measure of covalent character in an ionic bond d) The magnesium cation is much smaller than the barium cation, and is therefore more polarising The peroxide anion is too polarisable to exist in a stable lattice with magnesium (but can form such a lattice with barium)

7 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 7 COVALENT BONDING TENDING TOWARDS IONIC BONDING a) A measure of the attraction of a covalently bonded atom of that element for the electrons in the covalent bond b) (i) Na ( ) / 3.3 = 64% (ii) S ( ) / 3.3 = 15% (iii) Al ( ) / 3.3 = 45% c) (i) Chemical decomposition caused by water / a substitution reaction caused by water (ii) Na is not hydrolysed It is essentially ionic Al 3 is hydrolysed It is essentially covalent d) (i) A bond dipole is formed when a bond is polarised as the result of an electronegativity difference so that there is a partial charge on each atom i.e. X δ+ Y δ- (ii) Where a species has an overall resolved dipole the species will experience a turning force in an electric field e) There are differences in electronegativity between chlorine, and both phosphorus and aluminium therefore both P- and Al- bonds are polarised to give dipoles owever, the P 3 molecule is trigonal pyramidal and has a resolved dipole whilst Al 3 is trigonal planar and has no overall dipole / the bond polarisations cancel out Maximum 4 marks

8 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 8 METALLIC BONDING a) Solid metals consist of a regular lattice of cations formed by the loss of one or more valence electrons which form a delocalised cloud throughout the lattice The bonding consists of electrostatic attraction of the cations for these delocalised electrons b) (i) The electrostatic attraction of cations for their delocalised valence electrons is quite strong and requires considerable (thermal) energy to overcome it (ii) The delocalised electrons in the solid and the electrons and cations in the liquid are free to move under an applied potential difference (iii) eat is conducted in a metal by the increased kinetic energy of the randomly moving electrons and by increased vibrations of the regularly-packed cations c) Sodium contributes only one electron per atom to its delocalised electron cloud The cation formed is relatively large / has a low charge density which reduces the electrostatic attraction in the very open body-centred cubic lattice Magnesium cations are smaller and more highly charged and exist in a denser, hexagonal close-packed lattice giving a stronger metallic bond Maximum 5 marks

9 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 9 INTERMOLECULAR ORCES a) Van der Waals forces The movement of electrons within the molecule results in the creation of transient dipoles so that weak attractions arise between molecules b) (i) A strong form of dipole-dipole attraction between molecules involving molecules in which is covalently bonded to a highly electronegative atom /, O or N A lone pair of electrons on the electronegative atom of one molecule is drawn towards the atom of another molecule (ii) δ+ δ- O δ+. δ- O δ+ δ+ c) (i) Since the boiling point is raised, there must be increased forces of attraction between the molecules The polarisation of the C bond in C 3 is increased by the presence of 3 atoms so that it forms a strong dipole-dipole interaction with the electronegative oxygen in propanone (ii) δ+ δ- δ+ C 3 C O C C 3 δ- δ- δ- d) ydrogen bonds and van der Waals forces between water molecules Ion-dipole attraction between ions and water molecules

10 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 10 PROPERTIES AND BONDING a) Titanium(IV) Chloride Bonding: covalent Structure: simple molecular Reasons: low melting point b) Silicon Dioxide Bonding: covalent Structure: giant Reasons: high melting point poor conductor of electricity c) Titanium Bonding: metallic Structure: giant Reasons: good electrical conductivity as a solid d) Cryolite Bonding: ionic Structure: giant Reasons: good conductor of electricity as a liquid, but not as a solid e) Bitumen Bonding: Covalent Structure: simple molecular Reasons: soluble in non-polar solvent/ low melting point f) Ethane-1,2-diol This is a liquid which boils at 197 C, and is used in antifreeze in car radiators. Its aqueous solution in water is a poor conductor of electricity. Bonding: covalent Structure: simple molecular Reasons: low melting point The fact that it is both soluble in water and has poor electrical conductivity tell means hydrogen bonding is present Quality of language: at least two sentences with correct spelling, punctuation and grammar, and are relevant.

11 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 11 CRYSTAL STRUCTURE a) Diamond Graphite 2 layers 2 marks each must show 3-D nature b) (i) Graphite has a layer lattice in which the layers are held together only by van der Waals forces / not held by covalent bonds and can slide over each other (ii) One electron from each atom is delocalised and able to move along the lattice when a potential difference is applied c) (i) The silicon-silicon bonds are longer and therefore weaker than in diamond because silicon atoms are larger than carbon atoms (ii) To melt diamond requires the breaking of strong covalent bonds but in ice only the much weaker hydrogen bonds need to be broken (iii) The lattice in ice is very open, with water molecules held in fixed positions by extensive hydrogen bonding In water, molecules are able to move closer together, because there is less hydrogen bonding Ice is therefore less dense than water

12 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 12 CANGES O STATE a) (i) Temperature/ o C e Ne Ar Kr Xe (ii) The general increase in boiling point occurs because, as relative atomic mass increases, the number of electrons in each atom increases and therefore van der Waals forces increase b) Argon is monatomic, with 18 electrons in each atom, but chlorine is diatomic and there are thus 34 electrons in each molecule As chlorine has more electrons the van der Waals forces in chlorine are considerably greater c) Although the relative molecular masses are similar, strontium chloride is ionic whilst carbon tetrachloride is covalent To melt strontium chloride, strong ionic bonds have to be broken but in carbon tetrachloride only the weak van der Waals forces between molecules must be overcome d) Solid Liquid Gas Motion Vibrational motion about a mean position Translational motion Translational motion with a relatively long mean free path Order ighly ordered ar less ordered / some short range order but no long range order Totally disordered / chaotic

13 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 13 BOILING POINTS O YDRIDES a) (i) Group 4 (ii) Group 6 Temperature/ o C Temperature/ o C C Si Ge Sn Pb O S Se Te Po b) The general increase in boiling point in both groups is due to an increase in van der Waals forces between the molecules as the relative molecular mass / number of electrons increases In Group 6, water / the hydride of oxygen has a significantly higher boiling point than expected because hydrogen bonding occurs in water and is not present in the other hydrides in this group ydrogen bonding is significantly stronger than van der Waals forces In Group 4, methane / the hydride of carbon does not exhibit hydrogen bonding Maximum 5 marks c) Both resemble the Group 6 graph Both N 3 and exhibit hydrogen bonding because of the high electronegativity of N and and the presence of a lone pair d) The C- bonds in methane are very strong, since they are very short The Pb- bonds in plumbane are much longer, since the Pb atom is much larger; they therefore break much more easily (when the hydride is heated) Plumbane vapour could be obtained by warming the liquid at reduced pressure

14 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 14 SOLUBILITY O IONIC COMPOUNDS a) Ions are surrounded by shells of water molecules held by electrostatic attraction as the result of polarisation / charge separation in the 2 O molecule caused by the electronegativity difference between O and Cation A + δ - O δ + δ + Anion δ + B - δ - O δ + Only one 2 O molecule in each case need be shown Maximum 5 marks b) (i) Transition metal cations are relatively small / can attract a lone pair of electrons from O A coordinate / dative covalent bond is formed (ii) The Na + ion is relatively large / has a low charge density A lone pair of electrons is not strongly attracted Or a strong coordinate bond cannot be formed c) (i) The heat released when 1 mole of gaseous ions becomes hydrated by a large volume of water at 298 K and 1 atm (ii) ydration enthalpies decrease in magnitude / become less exothermic because cations become larger Surface charge density / charge : radius ratio decreases hence the attraction for 2 O molecules decreases Maximum 3 marks d) (i) Decrease (ii) Solubility is also affected by the lattice enthalpy of the compound Lattice enthalpy / enthalpy required to separate the ions of a crystal decreases in magnitude down the group This factor tends to cause a solubility increase

15 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 15 TEST QUESTION I a) (i) N 3 (ii) B 3 N B b) N B ( 1 for, 1 for rest) c) (i) Trigonal pyramidal Repulsion between 3 bonding pairs and 1 lone pair (ii) Trigonal planar Repulsion between 3 bonding pairs only d) (i) Tetrahedral (approximately) (ii) In the addition compound, angle is governed by equal repulsion between four bond pairs This is weaker than repulsion between lone pair and bond pairs in ammonia e) Smaller angle / ~109 0 instead of because in B 3.N 3 geometry is governed by repulsion between four bond pairs

16 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 16 TEST QUESTION II a) (i) Trigonal bipyramidal (ii) P Group 5 5 valence electrons + 5 electrons from total 10 electrons i.e. 5 pairs Lowest energy arrangement for 5 pairs of electrons is trigonal bipyramidal. ive bonding pairs, no lone pairs, so molecular shape is also trigonal bipyramidal (iii) P Diagram Bond angles (iv) [P 4 ] + [P 6 ] - + _ P P all Tetrahedral [NB Charges & bond angles must be shown] Octahedral b) To form the pentachloride would require nitrogen to expand its octet, which it cannot do, since it has no empty orbitals of sufficiently low energy In the nitrate ion, one of the nitrogen oxygen bonds is a dative covalent bond utilising the lone pair on the nitrogen atom

17 TOPIC 4 ANSWERS & MARK SCEMES QUESTIONSEET 17 TEST QUESTION III a) (i) A = s orbital B = p orbital C = bonding (or hybrid) orbitals D = repulsion (4x1) (ii) diagram suggests trigonal planar bond angle = 120 o trigonal planar stated (iii) uniform bond arrangement cancels dipoles in space/vector terms b) (i) dative covalent (ii) value within o (iii) B-N bond polar giving molecular dipole

Definition: An Ionic bond is the electrostatic force of attraction between oppositely charged ions formed by electron transfer.

Definition: An Ionic bond is the electrostatic force of attraction between oppositely charged ions formed by electron transfer. 3 Bonding Definition An Ionic bond is the electrostatic force of attraction between oppositely charged ions formed by electron transfer. Metal atoms lose electrons to form +ve ions. on-metal atoms gain