Non-covalent bonding

Transcription

1 on-covalent bonding Mark Scheme Level Subject Exam Board Topic Booklet Pre U Chemistry Cambridge International Examinations on-covalent bonding-chemical forces Mark Scheme Time Allowed: 44 minutes Score: /37 Percentage: /100 Grade Boundaries:

2 1. (a (i) VSEPR or Valence Shell Electron Pair Repulsion (theory) Shape is trigonal planar (1) BF 3. Bond angle = 120 (1) [2] (b) The number of valence or outer-shell electrons (on the central atom) exceeds 8. R octet is expanded. Structure named as pentagonal bipyramidal. (1) Bond angles of 72 (ignore multiples of 72 ) (1) and 90 (ignore 180 ). (1) [3] (iii) There isn t space around the bromine atom for seven bonds to fluorine atoms / steric hindrance / bromine atom is too small. Allow suggestions that fluorine isn t oxidising enough R the 6 th and 7 th ionisation energies of bromine are too high R it would spontaneously dissociate to give BrF 5 and F 2. [1 (c) (Trigonal) pyramidal 4 bonding pairs (1) 2 lone pairs (1) [2]

3 (d) Line connecting hydrogen on adenine and upper on thymine (1) Line connecting adenine right-hand heterocyclic and hydrogen on thymine. (1) Thymine sugar Adenine sugar [2] Janus wedge between the two bases with a valid -bond connecting the wedge to each base. (1) All correct -bonds shown between Janus wedge and cytosine. (1) All correct -bonds between Janus wedge and guanine. (1) R R R [3] [Total: 16]

4 2. (a (i) Point plotted corrected (must be within the correct small square in the grid) Bonding is intermediate-covalent-ionic-metallic (b) 2 + 3e ½ R e mark for correct number of electrons on the left hand side 1 mark for the rest of the balanced half equation (ignoring charge) [2 3C C [1 (iii) Enzyme catalysis (c) xidation state = {(2 112) 8} / 36 = (+)6 [Mo 9 28 ( 2 ) 4 ] 2 R [Mo ] 2 [Total: 8]

5 3 (a) CsF (b) clear that a Cs electron is in the F outer shell (1) charges shown on the ions (1) allow a Cs ion with 8 electrons shown in the outer shell no marks awarded if there is sharing of electrons [2] (c) largest possible difference in electronegativity between its two constituent elements owtte (d) high boiling point / low vapour pressure / does not evaporate easily (e) wide separation of charges / large ions AD low charges both needed for mark (f) high surface tension (1) (relatively) high melting and/or boiling points or liquid at rtp (compared to analogous compounds) or high specific thermal capacity (1) less dense as a solid than as a liquid (owtte) or greatest density at 4 C (1) [max 2] (g) bond drawn between hydrogen on one molecule and oxygen on another (1) oxygen lone pair drawn at the start of one of the bonds and both ends of a dipole shown on each molecule (1) bond angle of 180 around an -bonding hydrogen shown explicitly or looks like 180 intended (1) [3] (h) two bonds shown between two amine groups on melamine and the carbonyl oxygens on PTCDI, and an bond shown between the group on the PTCDI and the = atom on melamine that is between the two -bonding amine groups. (2) for both marks the -bonds from the amine groups must be from one of the two hydrogens of each amine in particular, rather than just from 2 otherwise just 1 mark is given ecf from (g) if the notation for an -bond is incorrect no marks if all three bonds aren t correctly identified small errors in copying down the structures should not be penalised as long as they don t affect the -bonding interaction [2] [Total: 13]