OCR A Jan 2010 Paper 1 Q3 (with explained solutions) |

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OCR A Jan 2010 Paper 1 Q3

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This question is about different models of bonding and molecular shapes. (a) Magnesium sulfide shows ionic bonding. (i) What is meant by the term ionic bonding?[1] (ii) Draw a dot-and-crossdiagram to show the bonding in magnesium sulfide. Show outer electron shells only. (b) Dot-and-crossdiagrams can be used to predict the shape of covalent molecules. Fluorine has a covalent oxide called difluorine oxide, F2O. The oxygen atom is covalently bonded to each fluorine atom. (i) Draw a dot-and-crossdiagram of a molecule of F2O. Show outer electron shells only. [2] [2]OCR 2010<br />
(ii) Predict the bond angle in an F2O molecule. Explain your answer.[3] (c) Liquid ammonia, NH3, and water, H2O, both show hydrogen bonding. (i) Draw a labelled diagram to show hydrogen bonding between two molecules of liquid ammonia. [3] (ii) Water has several anomalous properties as a result of its hydrogen bonding. Describe and explain one anomalous property of water which results from hydrogen bonding.[2]OCR 2010 [Total: 13] Turn over<br />

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F321 Question 3 (i) (a) Expected Answers Mark Scheme January 2010 Additional Guidance IGNORE force IGNORE references to transfer of electrons MUST be ions, not particles Mark charges on ions and electrons independently For first mark, if 8 electrons are shown around the Mg then extra electrons around S must match the symbol chosen for electrons around Mg Shell circles not required IGNORE inner shell electrons Brackets are not required Must be dot-and-cross circles for outer shells NOT needed IGNORE inner shells Non-bonding electrons of O do not need to be shown as pairs Non-bonding electrons of F do not need to be shown as pairs ALLOW 103105O (103O is the actual bond angle) ALLOW responses equivalent to second marking point. e.g. There are 4 pairs of electrons and 2 of these are lone pairs ALLOW bonds for bonded pairs DO NOT ALLOW atoms repel DO NOT ALLOW electrons repel ALLOW LP for lone pair ALLOW BP for bonded pair ALLOW LP repel more if bonded pairs have already been mentioned Marks (Electrostatic) attraction between oppositely charged ions. (cid:57) (ii) Mg shown with either 8 of 0 electrons AND S shown with 8 electrons with 2 crosses and 6 dots (or vice versa) (cid:57) Correct charges on both ions (cid:57) 2+ Mg 2- (i) Electron pairs in covalent bonds shown correctly using dots and crosses in a molecule of the F2O (cid:57) Lone pairs correct on O and both F atoms (cid:57) (ii) Predicted bond angle 104105O. (cid:57) There are 2 bonded pairs and 2 lone pairs (cid:57) Lone pairs repel more than bonded pairs (cid:57) (b)<br />
F321 Question (i) (c) Expected Answers (At least) two NH3 molecules with correct dipole shown with at least one H with (cid:71)+ and one N with (cid:71) (cid:57) (Only) one hydrogen bond from N atom on one molecule to a H atom on another molecule (cid:57) Lone pair shown on the N atom and hydrogen bond must hit the lone pair (cid:57) Hydrogen bond (cid:71)- N (cid:71)+ H (cid:71)+ (cid:71)+ (cid:71)+ (cid:71)+ H (cid:71)- (cid:71)+ Mark Scheme January 2010 Marks Additional Guidance DO NOT ALLOW first mark for ammonia molecules with incorrect lone pairs DO NOT ALLOW first mark if H2O, NH2 or NH is shown ALLOW hydrogen bond need not be labelled as long as it clear the bond type is different from the covalent NH bond ALLOW a line (i.e. looks like a covalent bond) as long as it is labelled hydrogen bond) ALLOW 2-D diagrams ALLOW two marks if water molecules are used. One awarded for a correct hydrogen bond and one for the involvement of lone pair ORA ALLOW ice floats for first mark ALLOW higher melting OR boiling point than expected DO NOT ALLOW H2O has a high melting / boiling point ALLOW other properties caused by hydrogen bonding not mentioned within the specification E.g. high surface tensionstrong hydrogen bonds on the surface 13 (ii) Liquid H2O is denser than solid (cid:57) In solid state H2O molecules are held apart by hydrogen bonds OR ice has an open lattice (cid:57) OR H2O has a relatively high boiling point OR melting point (cid:57) (relatively strong) hydrogen bonds need to be broken OR a lot of energy is needed to overcome hydrogen bonds OR hydrogen bonds are strong (cid:57) Total<br />

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