TOPIC 11 ANSWERS & MARK SCHEMES QUESTIONSHEET 1 PHYSICAL PROPERTIES OF THE HALOGENS

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Lucy Stokes
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1 QUESTIONSHEET 1 PHYSICAL PROPERTIES OF THE HALOGENS a) (i) F 2 gas (½) yellow (½) Cl 2 gas (½) green / yellow-green (½) Br 2 liquid (½) dark red / brown (½) I 2 solid (½) violet-black / black (½) (ii) Boiling points increase down the group as van der Waals forces / instantaneous dipole-dipole forces of attraction increase because the number of electrons per atom increases b) (i) Electron affinities become less negative / less exothermic as Group 7 is descended Down Group 7, nuclear charge increases but the effect is outweighed by increased atomic radius and inner shell shielding an incoming electron experiences less force of nuclear attraction Maximum 4 marks (ii) Its value is of low magnitude / similar to that for iodine The F atom is very small / fluorine has a very low atomic radius Seven outer shell electrons are close together and repel an incoming electron strongly less energy is released when the electron enters the outer shell / overall the process is less exothermic Maximum 3 marks c) Fluorine has a very low atomic radius (½) and little screening (½) so it has a high effective nuclear charge electrons are strongly attracted to F Or fluorine is the most electronegative element of all any bond involving F will be polarised so that electrons are nearer to F than the other atom

2 QUESTIONSHEET 2 REACTIVITY OF THE HALOGENS a) (i) Bond strength decreases as Group 7 is descended Halogen atoms increase in size down the group bond length increases down the group and bond strength decreases as bond length increases Maximum 3 marks (ii) H for bond breaking: Cl Cl +242 C H kj mol -1 H for bond formation: H Cl -431 C Cl kj mol -1 H θ r = = -115 kj mol-1 (iii) H θ values become less negative / less exothermic down the group r Or H θ r for CH 4 + Br 2 = -37 kj mol-1 (½) H θ r for CH 4 + I 2 = +26 kj mol-1 (½) Breaking halogen halogen bonds becomes less endothermic down the group but formation of C halogen and H halogen bonds becomes a lot less exothermic (Deduct 1 mark unless it is clear that the latter is dominant) (iv) (F 2 >) Cl 2 > Br 2 > I 2 b) (i) H 2 (g) + Cl 2 (g) 2HCl(g) (Examiners may accept equation without state symbols) (ii) Free radical (½) substitution (½) reaction In such a reaction, the rate determining / slowest step is Cl 2 2Cl. Increasing light intensity would increase the rate of this step and hence the rate of the overall reaction (iii) Br 2 : C (½) Pt catalyst (½) I 2 : C (½) Pt catalyst (½) Mention of reversible or incomplete reaction with I 2 / chemical equilibrium

3 QUESTIONSHEET 3 REACTIONS OF THE HALOGENS WITH WATER AND ALKALIS a) (i) Cl 2 (aq) + H 2 O(l) ¾ HCl(aq) + HOCl(aq) (ii) The odour of chlorine increases By Le Chatelier s principle / the system responds to counteract the increased [H + ] by shifting the position of equilibrium to the left hand side / to favour the production of chlorine (iii) Equation 2Cl 2 (aq) + 2H 2 O(l) 4HCl(aq) + O 2 (g) Reason Oxygen (atoms) in water undergo oxidation due to the oxidising power of Cl 2 but Br 2 is a weaker oxidising agent than chlorine b) (i) Cl 2 (aq) + 2NaOH(aq) NaCl(aq) + NaClO(aq) + H 2 O(l) (ii) Cl 2 is undergoing simultaneous oxidation and reduction Its oxidation number changes from 0 to +1 in ClO - and to 1 in Cl - (iii) 3ClO - (aq) 2Cl - (aq) + ClO 3- (aq) (2) (1 mark for species, 1 for balance) (iv) Litmus paper turns red because chlorine with water forms acids / HCl / HOCl It is bleached by the oxidising action of HOCl Examiners may accept equations without state symbols.

4 QUESTIONSHEET 4 DISPLACEMENT REACTIONS a) (i) Colour of mixed solution Red / orange / brown Equation Cl 2 (aq) + 2Br - (aq) Br 2 (aq) + 2Cl - (aq) (2) (ii) Colour of mixed solution Dark brown Equation No reaction (iii) Colour of mixed solution Dark brown Equation Br 2 (aq) + 2I - (aq) I 2 (aq) + 2Br - (aq) (2) (iv) Colour of mixed solution Red / orange / brown Equation No reaction For each equation, award for correct species and for balance and state symbols. b) Add starch solution A blue-black colour confirms I 2 Or Add CHCl 3 / other suitable solvent I 2 dissolves to give a violet / purple / pink colouration in the organic layer c) Raise the temperature Increase concentrations of reactants

5 QUESTIONSHEET 5 EXTRACTION OF BROMINE FROM SEA WATER a) Cl 2 (aq) + 2e - 2Cl - (aq) 2Br - (aq) Br 2 (aq) + 2e - Cl 2 (aq) + 2Br - (aq) Br 2 (aq) + 2Cl - (aq) b) Bromine molecules contain non-polar covalent bonds The only forces between Br 2 and H 2 O molecules are van der Waals forces / temporary dipole-dipole forces These forces are weak / hence the molecules are easily separated c) (i) H 2 SO 3 (aq) + H 2 O(l) (aq) + 4H + (aq) + 2e - (ii) Br 2 (aq) + 2e - 2Br - (aq) (iii) H 2 SO 3 (aq) + H 2 O(l) + Br 2 (aq) (aq) + 2Br - (aq) + 4H + (aq) d) A physical state / solid, liquid or gas / part of a system which is homogeneous within itself e) Concentration of Br - in sea water is very low Sea water in a confined area would soon become exhausted of Br - On the coast of Anglesey the current brings in fresh sea water directly from the Atlantic (which is a huge water mass) / in the Severn estuary this would not happen

6 QUESTIONSHEET 6 HYDROGEN HALIDES a) (i) Hydrogen fluoride / HF (ii) HF undergoes hydrogen bonding between its molecules This is stronger than the permanent dipole-dipole forces in HCl, HBr and HI b) (i) Hydrogen chloride / HCl (ii) The van der Waals forces / intermolecular forces increase in strength from HCl(l) HI(l) because the number of electrons per molecule increases down the group c) (i) An acid which is only slightly dissociated / ionised in water to give a low concentration of hydrogen ions / oxonium ions (ii) HF < HCl < HBr < HI (iii) Only hydrogen fluoride / hydrofluoric acid is a weak acid because the H F bond is strong / has a high bond enthalpy But HF is a stronger acid in concentrated solution because HF combines with F - to give HF 2 - and the equilibrium HF + H 2 O ž F - + H 3 O + is displaced to the RHS Maximum 4 marks d) HF is a relatively weak acid It can etch glass not because it is a strong acid but because it acts as a source of F - ions to react with silica

7 QUESTIONSHEET 7 SOLID IONIC HALIDES WITH CONCENTRATED SULFURIC ACID a) HCl is a gas / is volatile (½) and escapes from the system (½) It cannot take part in the reverse reaction / equilibrium is destroyed / position of equilibrium shifts to favour production of more HCl until all the NaCl has reacted b) (i) H 2 + 2H + + 2e - SO 2 + 2H 2 O (2) (1 for species, 1 for balance) (ii) 2Br - Br 2 + 2e - (iii) Full equation H 2 + 2HBr Br 2 + SO 2 + 2H 2 O (2) (1 for species, 1 for balance) Observations Misty white fumes (of HBr) (½) brown fumes (of Br 2 ) (½) white residue (of NaH ) (½) choking fumes (of SO 2 ) (½) c) (i) H 2 + 8H + + 8e - H 2 S + 4H 2 O (2) (1 for species, 1 for balance) (ii) 8I - 4I 2 + 8e - H 2 + 8HI H 2 S + 4I 2 + 4H 2 O (iii) Violet / purple fumes (of I 2 ) Bad egg smell (of H 2 S) (iv) Br - / HBr and I - / HI are easily oxidised / good reductants They do not react because H 3 PO 4 is a weaker oxidising agent than H 2

8 QUESTIONSHEET 8 SILVER HALIDES a) (i) Silver chloride White Silver bromide Cream / off-white Silver iodide Yellow (ii) AgF is soluble (iii) To destroy CO 3 ions / other anions Or to prevent precipitation of Ag 2 CO 3 / other insoluble silver compounds which would interfere with the test b) AgCl dissolves in dilute aqueous ammonia AgBr dissolves only in conc. aqueous ammonia AgI is insoluble in aqueous ammonia, regardless of concentration c) (i) H 3 N: Ag + :NH 3 (2) Award for arrows and for lone pairs (ii) Ag + contributes the +ve charge NH 3 molecules are electrically neutral (iii) Linear

9 QUESTIONSHEET 9 CHLORIC(I)ACID AND SODIUM CHLORATE(I) a) Oxidation number Name ClO - +1 (½) chlorate(i) (½) - ClO 3 +5 (½) chlorate(v) (½) - IO 4 +7 (½) iodate(vii) (½) b) (i) HOCl (2) (1 for pairs, 1 for lone pairs. Ignore inner shells) (ii) 2HOCl(aq) 2HCl(aq) + O 2 (g) State symbols are not essential. c) (i) NaClO solutions always contain some NaCl The suspension X is due to AgCl (ii) On warming ClO - disproportionates into Cl - - and ClO 3 Cl - reacts with Ag + to give the precipitate Y of AgCl (iii) Redox / oxidation-reduction Not just oxidation or reduction. d) (i) Acidify test solution with dil. H 2 (½) Add CHCl 3 or other suitable organic non-polar solvent (½) to form a separate phase (½) Add NaClO (aq) dropwise (½) with shaking (½) Organic layer colourless Cl - (½) red brown Br - (½) purple / violet I - (½) (ii) NaClO / ClO - oxidises Br - to Br 2 (½) and I - to I 2 (½) but cannot oxidise Cl - Br 2 and I 2 are transferred to the organic solvent because they have simple molecular structures and are therefore more soluble in the non-polar solvent than in water Maximum 3 marks

10 QUESTIONSHEET 10 TEST QUESTION I (Reaction with concentrated Sulfuric Acid) a) Halogens form salts / halides which are found in the Earth s crust b) Z is hydrogen chloride / HCl(g) c) White smoke with NH 3 (g) Moist blue litmus paper turns red Milkiness with AgNO 3 (aq) on a rod (Any 2) d) NaCl(s) + H 2 (l) HCl(g) + NaH (s) (Examiners may accept equation without state symbols) e) HCl(g) + H 2 O(l) ž H 3 O + (aq) + Cl - (aq) (2) (1 mark for formulae, 1 mark for state symbols) f) Because dilute sulfuric acid is not an oxidising agent g) (i) Anion is CO 3 2Ag + (aq) + CO 3 (aq) Ag 2 CO 3 (s) (2) (1 for formulae, 1 for state symbols and balance) Ag 2 CO 3 (s) + 2NH 3 (aq) 2[Ag(NH 3 ) 2 ] + (aq) + CO 3 (aq) (2) (1 for formulae, 1 for state symbols and balance) (ii) Because H + (aq) ions reacted with CO 3 (aq) ions to give CO 2 and H 2 O (Both marks are obtainable from an equation)

11 QUESTIONSHEET 11 TEST QUESTION II a) (i) Cl 2 (aq) + H 2 O(l) ¾ HCl(aq) + HOCl(aq) Name... hydrochloric acid... chloric(i) acid Oxidation no. of chlorine Award marks out of 3, deducting 1 for each error. (ii) A dynamic equilibrium exists / reaction does not proceed to completion Hence free chlorine is present which is yellow-green (½) and very volatile (½) b) Type of reaction Redox / oxidation-reduction / oxidation Compound responsible HOCl c) (i) HOCl is an oxidising agent Bacteria cell walls are destroyed by a redox reaction (ii) Chlorine gas is contained in high pressure cylinders which are heavy / difficult to handle Chlorine is toxic / causes health problems if a leakage occurs Chlorine is corrosive Solid granules are safer to handle Maximum 3 marks. d) (i) Effect [HOCl] increases (½) NCl 3 is formed, which stings the eyes (½) Explanation By Le Chatelier s principle, equilibrium is disturbed to LHS (ii) Effect [HOCl] becomes low (½) disinfecting power is reduced (½) Explanation By Le Chatelier s principle, equilibrium is disturbed to RHS

12 QUESTIONSHEET 12 a) (i) Anode reaction 2F - F 2 + 2e - Cathode reaction 2H + + 2e - H 2 TEST QUESTION III (Including equilibria) (ii) Reason Hydrogen and fluorine react together explosively at room temperature Equation H 2 (g) + F 2 (g) 2HF(g) b) (i) Cl 2 (aq) + H 2 O(l) ž HCl(aq) + HOCl(aq) (ii) 2F 2 (aq) + 2H 2 O(l) 4HF(aq) + O 2 (g) (iii) Chlorine with water Disproportionation Fluorine with water Redox / oxidation-reduction (iv) Fluorine is a more powerful oxidising agent than chlorine c) (i) Observation Misty white fumes Equation NaF(s) + H 2 (l) HF(g) + NaH (s) (ii) Observation Black solid Equations NaAt(s) + H 2 (l) HAt(g) + NaH (s) H 2 (l) + 8HAt(g) 4At 2 (s) + H 2 S(g) + 4H 2 O(l) Examiners may accept equations without state symbols.

FACTFILE: GCE CHEMISTRY

FACTFILE: GCE CHEMISTRY HALOGENS Halogens Students should be able to: 1.8.1 recall the colours of the elements and explain the trends within the Group, limited to physical state at room temperature, melting