Edexcel Jun 2012 Paper 1 Q26 (with explained solutions) |

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Edexcel Jun 2012 Paper 1 Q26

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26 (a) Explain how the atoms are held together by the covalent bond in a molecule of hydrogen. (1)(b) Draw the dot and cross diagrams for (i) methane, CH4 (ii) ethene, CH2 CH2 (iii) nitrogen, N2 (iv) the ammonium ion, NH4 (1) (1) (1) (1) 20 *P39306A02024*<br />
(c) Silicon exists in a giant covalent lattice. (i) The electrical conductivity of pure silicon is very low. Explain why this is so in terms of the bonding. (2)(ii) Explain the high melting temperature of silicon in terms of the bonding. (2)(Total for Question 26 = 9 marks) TOTAL FOR SECTION B = 60 MARKS TOTAL FOR PAPER = 80 MARKS *P39306A02124* 21<br />

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Question Number 26(a) Acceptable Answers Reject (Electrostatic) attraction between (bonding) electrons and nuclei/protons Just a shared pair of electrons Mark IGNORE ANY INNER SHELL ELECTRONS DRAWNONLY THE TOTAL NUMBERS OF ELECTRONS IN OUTER SHELLS ARE BEING ASSESSEDALLOW ELECTRONS TO BE ALL DOTS OR ALL CROSSES OR BOTH Reject Reject Reject Mark Mark Mark Reject Mark Acceptable Answers Acceptable Answers Acceptable Answers NOTE: The lone pair of electrons on each N atom do not have to be shown as a pair Acceptable Answers The + sign can be shown anywhere Ignore missing brackets Ignore if the + is missing Question Number 26(b)(i) Question Number 26(b)(ii) Question Number 26(b)(iii) Question Number 26(b)(iv)<br />
Mark Mark Acceptable Answers Reject IGNORE any references to molecules in this part only First mark: Location of silicons electrons Silicons (outer) electrons are fixed (in covalent bonds)/ silicons (outer) electrons are in fixed positions (in covalent bonds)/ silicons (outer) electrons are involved in bonding (1) Second mark: Lack of mobility of silicons electrons (therefore) silicons electrons are not free (to move)/ silicon has no free electrons/ there are no mobile electrons in silicon/ silicon has no delocalized electrons/ silicons electrons cannot flow (1) IGNORE references to lack of ions Silicon is ionic scores (0) for the question silicons ions are not free to move scores (0) for the question Acceptable Answers Reject (The covalent) bonds are strong (throughout the lattice) (1) (therefore) a lot of energy is required to break the bonds / a lot of energy is needed to overcome the attractions (1) IGNORE any references to giant molecular (simple) molecular silicon (0) /molecules of silicon (0) /silicon has ions (0) /intermolecular forces / van der Waals forces/ London forces (0) ALL THE ABOVE SCORE (0) OVERALL TOTAL FOR SECTION B = 60 MARKS Question Number 26(c)(i) Question Number 26(c)(ii)<br />

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