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Avis Harrington
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1 Covalent bonding Mark Scheme Level Subject Exam Board Topic Booklet Pre U Chemistry Cambridge International Examinations Covalent bonding-chemical forces Mark Scheme Time Allowed: 72 minutes Score: /60 Percentage: /100 Grade Boundaries:

2 1. (a Different forms of an element with a different structure / arrangement of atoms (b) (i) Both shared electrons originating from the SAME atom in the bond 1 double bond and 1 dative bond from central oxygen atom 8 electrons in the outer shell of each oxygen atom [2] (c) The energy (required) to break a mole of bonds in the gas phase Energy change = ((2 144) 498) kj mol 1 = 210 kj mol 1 1 mark for multiplying O O bond energy by 2 1 mark for correct answer with correct sign [2] (d) (Trigonal) pyramidal 3 bonding pairs and 1 lone pair [2] Ammonia / NH 3 (e) (i) Hydrogen of hydronium cation connected with a labelled hydrogen-bond to the oxygen of a water molecule 180 angle indicated around the H-bonded hydrogen Lone pair indicated on the oxygen atom in the hydrogen-bond Both ends of a dipole shown on the water molecule [4] Hydronium cation inside crown ether with its three hydrogen atoms pointing towards ether oxygens [Total: 17]

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

4 Question Number Answer Max marks 3. (a) Mass of HCl = mol 36.5 g mol 1 = 507 g No sig figs or units penalties (b) Amount of NaOH = dm mol dm 3 = mol Amount of HCl in volumetric flask = mol = mol [HCl] = mol / dm 3 = 12.4 mol dm 3 Final answer to 3 sig figs [4] (c) (i) H 2 SO 4 + NaCl HCl + NaHSO 4 Ignore state symbols Allow H 2 SO 4 + 2NaCl 2HCl + Na 2 SO 4 H 2 SO 4 + 2HBr Br 2 + SO 2 + 2H 2 O Ignore state symbols Allow H 2 SO 4 + 2HBr Br 2 + H 2 SO 3 + H 2 O (iii) Sulfuric acid is the oxidising agent No credit for S being the oxidising agent The oxidation number of bromine increases (from 1 to 0) OR the oxidation number of sulfur decreases (from +6 to +4) (d) (i) Bond strength decreases because the bonds gets longer OR because there is greater shielding of the bonding electrons from the halogen nucleus due to the additional inner shells of electrons No credit for answers based on electronegativity or ionic radii Acidic strength increases because the H Hal bond gets weaker (iii) Increasing boiling point for HCl HBr HI due to increasing van der Waals / instantaneous dipole induced dipole forces HF boiling point higher than HCl due to hydrogen bonding [2] [Total: 13]

5 4. (a) (i) Dot-cross diagram where electrons from each atom are distinguishable; three shared pairs and one lone pair on each atom; no inner shell electrons shown. (iii) (iv) Different or alternative structure for the same element (in the same physical state). Breaking bonds: 12 + (6 198 kj mol 1 ) = 1200 kjmol 1 Making bonds = kj mol 1 = 970 kj mol 1 Positive sign with correct answer 1 Enthalpy change = +230 kj mol earns all 3 marks. Bond order in Bond order in P 2+ 2 P + 2 = 2 (accept double ) = 2.5 or 5/2 ( 1) [2] ( b) (i) (iii) Energy (required) to remove one electron ( 1) from (every atom in) a mole of atoms ( 1) in the gas phase Allow mole of electrons from a mole of atoms. Do not allow molecules or compound. Outer electrons in silicon are further from the nucleus. OR Si is a larger atom OR Si has larger atomic radius. ( 1) There is greater shielding of the outer electrons in silicon compared to carbon. (Despite the additional protons in silicon) the outer electrons experience a reduced attraction to the nucleus OR the outer electrons are at higher energy. ( 1) NB The statement effective nuclear charge decreases down a group is false: it increases. Single bonds: σ or sigma Additional bonds in multiple bonds: π or pi Both required for the mark. (iv) Dividing mass percentages by molar masses C: 41.3 / 12.0 = 3.44 H: 10.3 / 1.0 = 10.3 Si: 48.4 / 28.1 = 1.72 Correct empirical formula = C2H6Si Possible structure must contain a Si=Si bond, with each silicon bonded to hydrogens or alkyl groups that satisfy the molecular formula of C 4 H 1 2Si 2. [ 3] [Total: 17]

Non-covalent bonding

Non-covalent bonding 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: