OCR A Jun 2014 Paper 1 Q4 (with explained solutions) |

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OCR A Jun 2014 Paper 1 Q4

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PMT (i) Complete the diagram below to show hydrogen bonding between the H2O molecule 4 Oxides can have different types of bonding. (a) H2O has hydrogen bonding. shown and one other H2O molecule. Include relevant dipoles and lone pairs. Label the hydrogen bond. [2] (ii) State and explain two anomalous properties of ice caused by hydrogen bonding. 12[4]OCR 2014<br />
(b) Draw a dot-and-cross diagram to show the bonding in CO2. Show outer electrons only. PMT [1] (c) Silicon dioxide, SiO2, has the same structure and bonding as diamond. State the structure and bonding in SiO2.[1] (d) Describe and explain the electrical conductivity of sodium oxide, Na2O, and sodium in their solid and molten states. In your answer you should use appropriate technical terms, spelled correctly.[5] [Total: 13]OCR 2014 Turn over<br />

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F321 Mark Scheme June 2014 Question 4 (a) Answer (i) The Dipole Mark At least one H+ AND one O- shown correctly on each water molecule (see diagram) (cid:57) Hydrogen bondO+ H H + ++ The Hydrogen bonding Mark One Hydrogen bond between H in one water molecule and a lone pair of O in an adjacent water molecule (cid:57) Mark Guidance DO NOT ALLOW H- OR O+ IGNORE lone pairs for first marking point All Hydrogen bonds must hit a lone pair Hydrogen bond does NOT need to be labelled but it must be different from the covalent bond if it is not labelled ALLOW H-bond as label ALLOW only one lone pair on O atom ALLOW additional, correctly drawn Hydrogen bonded water molecules with correct dipoles DO NOT ALLOW more than two lone pairs on O atom<br />
F321 Mark Scheme June 2014 Question 4 (a) Answer (ii) Property 1 Mark Guidance ALLOW ice floats (on water) ALLOW ice contracts when it melts ALLOW ice (water) has a higher melting point than expected OR predicted ALLOW other expressions which convey that the melting point is anomalously high eg Ice has an unusually high melting point IGNORE boiling point IGNORE the following unqualified statements Ice has a higher melting point or Ice has a high melting point IGNORE references to surface tension as a property IGNORE explanations of surface tension ALLOW hydrogen bonds are the strongest intermolecular attraction or force DO NOT ALLOW hydrogen bonds are strong but ALLOW this as part of a qualified statement (eg hydrogen bonds are strong compared with weak van der Waals forces) Lone pairs on O must be seen Lone pairs may be seen as 4 individual electrons ALLOW correct use of three different symbols Ice is less dense than water (cid:57) Explanation 1 The molecules in ice are held apart by hydrogen bonds (cid:57) OR ice has an open lattice OR structure Property 2 Ice has a relatively high melting point (cid:57) Explanation 2 Hydrogen bonds are relatively strong OR Hydrogen bonds are stronger (than other intermolecular attractions or forces) OR More energy is needed to overcome hydrogen bonding dot-and-cross of CO2 (cid:57) (b)<br />
F321 Question 4 (c) (d) Answer Giant covalent (lattice) (cid:57) Conductivity of Na mark M1: Sodium conducts in the solid and molten states (cid:57) Reason for conductivity of Na mark M2: Sodium has delocalised electrons (in both solid and liquid state) (cid:57) Conductivity of Na2O mark M3: Na2O conducts when molten and not when solid (cid:57) Reason for conductivity of Na2O marks M4: Molten Na2O has ions which are mobile (cid:57) M5: Solid Na2O has ions which are fixed (in position) OR ions are held (in position) OR ions are not mobile AND in an (ionic) lattice OR structure (cid:57) Total 13 Mark Scheme June 2014 Mark Guidance ALLOW Giant lattice with covalent bonds ALLOW Giant covalent bonds IGNOREGiant molecular or macromolecular DO NOT ALLOW Covalent bonds between molecules Quality of written communication delocalis(z)ed spelled correctly once and used in context for second marking point ALLOW carries charge for conducts for M1 and M3 IGNORE charge carriers for electrons OR ions for M2, M4 and M5 DO NOT ALLOW M2 if incorrect bonding is seen for Na DO NOT ALLOW ions move for solid Na for M2 IGNORE ions move for molten Na for M2 ALLOW solid Na2O is a poor conductor for M3 IGNORE references to aqueous Na2O for M3 IGNORE references to aqueous Na2O for M4 IGNORE delocalised ions OR free ions for mobile ions for M4 DO NOT ALLOW M4 AND M5 if incorrect bonding is seen in Na2O DO NOT ALLOW any mention of electrons moving for M4 DO NOT ALLOW suggestion that it is only positive or only negative ions move for M4 IGNORE there are no delocalised electrons for M5 ALLOW first and second statements of M5 to be unlinked in separate sentences ALLOW ions fixed in position by ionic bonds for M5<br />

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