Edexcel Jun 2014 Paper 2 Q22 (with explained solutions) |

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Edexcel Jun 2014 Paper 2 Q22

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Answer ALL the questions. Write your answers in the spaces provided. SECTION C Spontaneous combustion is often a subject of fantasy in movies, but it does actually happen with some chemical compounds. One such compound is silane, SiH4, which is analogous to methane, CH4. Methane is the main gas that is used in school and college laboratories with Bunsen burners, but it requires a spark or a lighted splint to ignite. Silane does not require any such ignition, and at room temperature is spontaneously flammable. This can make an interesting chemical demonstration. One method of making silane is by mixing together two solids, silicon dioxide and magnesium, with the magnesium being in excess. The two chemicals are thoroughly mixed together. This mixture is then heated to red-heat and initially produces powdered silicon. The silicon then reacts further with the excess magnesium powder and forms magnesium silicide, Mg2Si. The two reactions which occur are shown in the equations below. SiO2 + 2Mg Si + 2Mg 2MgO + Si Mg2Si The magnesium silicide formed is then reacted with hydrochloric acid to form silane, SiH4. (a) Complete and balance the equation for this reaction. State symbols are not required. (2) 22 Mg2Si + HCl+(b) Bubbles of silane rise to the surface in the reaction mixture and spontaneously combust with oxygen in the air. Suggest the names or formulae of the products of the reaction between silane and oxygen. (2)(c) Predict the molecular shape of silane, SiH4, and suggest the bond angle. (2) ShapeBond angle18 *P42971A01824*<br />
(d) Explain why the SiH bond is longer than the CH bond. (2)*(e) Identify the intermolecular forces present in pure samples of both silane and methane. Explain why silane has a higher boiling temperature than methane and why both are gases at room temperature. (4)(f) (i) Define the term electronegativity. (2)*P42971A01924* 19 Turn over<br />
*(ii) Some Pauling electronegativity values for selected elements are given below. H 2.1 Li 1.0 Na 0.9 Be 1.5 Mg 1.2 B 2.0 Al 1.5 C 2.5 Si 1.8 N 3.0 P 2.1 O 3.5 S 2.5 F 4.0 Cl 3.0 Using the values in the table above, compare the polarity of the bonds in a molecule of methane with that found in a molecule of silane. Comment on the significance of any difference. (3)(iii) Using the table in (f)(ii), choose an element which, when covalently bonded to hydrogen, forms a molecule containing bonds that are more polar than those in silane or methane. Give the formula of the hydride of your chosen element and state the electronegativity difference. (2)(iv) Explain why it is possible for the bonds within a molecule to be polar, but for the molecule itself to be non-polar. Give an example of such a molecule. (2)(Total for Question 22 = 21 marks) TOTAL FOR SECTION C = 21 MARKS TOTAL FOR PAPER = 80 MARKS 20 *P42971A02024*<br />

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Section C Question Number 22 (a) Question Number 22 (b) Question Number 22 (c) Mark Mark Acceptable Answers Reject Mg2Si + 4HCl 2MgCl2 + SiH4 Correct formulae of products (1) Balancing of equation (1) Second mark dependent on first Ignore state symbols even if incorrect Allow one mark for the following equation Mg2Si + 4HCl Mg2Cl4 + SiH4 Acceptable Answers Silicon dioxide/ SiO2 /Silica (1) Water/H2O (1) Allow names or formulae or both together but they both must be correct. Allow incorrect spellings if unambiguous Allow products to be given in an equation which does not have to be balanced. Reject Silicon oxide/ SiO CO2 Silicone/Silicane dioxide H2 Acceptable Answers Reject Tetrahedral (1) 109.5 (o) (1) 109 Mark 6CH0201 1406<br />
Question Number 22 (d) Acceptable Answers Silicon (atom) is larger OR Because it has more shells of electrons (1) Weaker attraction/bond OR Greater shielding in silicon (1) Reverse argument applies in both marking points Stand-alone marks Ignore references to electronegativity/bond polarity Mark Reject Ions/ionic radius Charge density Molecule Just more electrons Reference to intermolecular forces 6CH0201 1406<br />
Question Number 22 (e) Mark Acceptable Answers Marking Point 1 London forces/Dispersion forces Temporary/instantaneous dipole induced dipole forces (1) ALLOW van der Waals forces Marking Point 2 Silane/silicon has more electrons (1) Marking Point 3 Silane has stronger/more London forces (1) Marking Point 4 London forces are weak OR Little energy is required to break London forces (hence both are gases) (1) Ignore references to surface area Reject Dipole-dipole Induced dipole-induced dipole Hydrogen bonds Larger molar mass/ Great density of electrons/ larger electron cloud Break Si-H or C-H bonds 6CH0201 1406<br />
Question Number 22 (f)(i) Reject Mark Acceptable Answers The ability (of an atom) to attract/to pull/to draw the electrons (1) in/of a covalent bond Allow shared electron pair (1) Allow species/element for atom Acceptable Answers Question Number 22 (f)(ii) Marking Point 1 In methane difference is 0.4 and silane difference is 0.3/Methane has a difference of 0.1 more than silane (1) Marking Point 2 Hydrogen is more electronegative than silicon but less electronegative than carbon OR In methane, hydrogen will have a + charge but in silane hydrogen will have a - charge OR The C-H bonds in methane are more polar (than the Si-H bonds in silane) (1) Marking Point 3 The difference is small and so not significant, or has a minor effect (1) Ignore references to bond strength Mark Reject 6CH0201 1406<br />
Acceptable Answers Question Number 22 (f)(iii) Any suitable example and electronegativity difference scores 2 marks NH3/H2O/HF/HCl (1) 0.9/1.4/1.9/0.9 (1) Second mark consequential on the first, but if the formula is incorrect, e.g. HF2, but the difference is correct of 1.9, then allow second mark. Allow one mark for an ionically-bonded hydride with a correct electronegativity difference greater than 0.4, e.g. NaH and 1.2 (1) Allow H2S and the difference of 0.4 for 1 mark. Acceptable Answers Question Number 22 (f)(iv) Bond polarities cancel in a symmetrical molecule/ Mark Reject BH3 / B2H6 / PH3 scores 0 Mark Reject H2 centres of charge coincide in a symmetrical molecule OR Linking bond polarities cancelling due to the molecular shape which needs to be stated/drawn (1) Allow polar bonds for bond polarities Any suitable example, e.g. CCl4/CO2 / BF3 / SF6 (1) Allow CH4 / SiH4 Standalone marks TOTAL FOR SECTION C (Question 22) = 21 MARKS TOTAL FOR PAPER = 80 Marks 6CH0201 1406<br />

Question 17 18 19 20 21 22

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