bonding
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Draw the shape of a molecule of krypton difluoride (KrF2). Include in your answer any lone pairs of electrons that influence the shape. Name the shape produced by the atoms in a KrF2 molecule and suggest a bond angle.
Allow diagram with 2 bonds and 3 lone pairs Linear 180°
There are two lone pairs of electrons on the oxygen atom in a molecule of oxygen difluoride (OF2). Explain how the lone pairs of electrons on the oxygen atom influence the bond angle in oxygen difluoride.
Lone pairs repel more than bond pairs Allow idea of reducing bond angle bond angle will be lower (than regular tetrahedral angle) / bond angle of 103-106°
Silicon tetrafluoride (SiF4) is a tetrahedral molecule. Deduce the type of intermolecular forces in SiF4 Explain how this type of intermolecular force arises and why no other type of intermolecular force exists in a sample of SiF4 Intermolecular forces in SiF4 ____________________________________________ Explanation _
Van der Waals forces (Uneven distribution of electrons in) one molecule induces dipole in neighbouring/another/nearby molecule symmetrical molecule / dipoles cancel OR no hydrogens bonded to F (N or O), therefore no hydrogen bonding
which type of bonding does ammonium chloride have ? covalent dative covalent Ionic ?
covalent dative covalent Ionic
State the general formula of an alkane containing n carbon atoms. Deduce an expression for the relative molecular mass (Mr) of an alkane in terms of n.
CnH2n+2 14.0n + 2.0 or 14n + 2
Explain why nonane has a higher boiling point than 2,4-dimethylheptane.
nonane has stronger / greater / more van der Waals’ forces between molecules nonane molecules pack closer together / more (surface) contact
Give an equation for the complete combustion of nonane
C9H20 + 14O2 → 9CO2 + 10H2O
Nonane is often found in fuel for jet engines. Combustion in jet engines produces pollutants including nitrogen monoxide (NO). Explain how this nitrogen monoxide is formed.
M1 nitrogen and oxygen from air react M2 at high temperature
Nonane can be cracked to form large quantities of propene. Name the type of cracking used.
thermal cracking
Which molecule does not have a permanent dipole? A CH3Br B CH2Br2 C CHBr3 D CBr4
d when the permanent dipoles are spread out evenly in a molecule they cancel out
Which compound has the highest boiling point? A butanal B butan-2-ol C but-2-ene D 1-fluorobutane
B
Use your knowledge of structure and bonding to explain why sodium bromide has a melting point that is higher than that of sodium, and higher than that of sodium iodide.
Stage 1 - Na 1a) Na has metallic bonding 1b) there is attraction/ bonding between the positive nucleus/ ion and the delocalised electrons in Na 1c) Na has a giant/lattice structure Stage 2 – NaBr or NaI 2a) Ionic bonding in NaBr and/or NaI 2b) There is attraction/ bonding between the + and – ions in NaBr and/or NaI 2c) NaBr and/or NaI have a giant/lattice structure Stage 2 – comparison of bonding 3a) The ionic bonds are stronger (or wtte) than the metallic bonds 3b) there is stronger attraction (or wtte) between the + and – ions in NaBr than in NaI 3c) since the Br– ion is smaller than the I– ion
Sodium reacts with ammonia to form the compound NaNH2 that contains the NH2– ion. Draw the shape of the NH2– ion. Include any lone pairs of electrons that influence the shape. Predict the bond angle. Justify your prediction.
M1 shape- the shape of water but with N and a negative bracket M2 104.5° Allow 104-106 1 M3 (4) electron pairs repel to be as far apart as possible 1 M4 lp/lp repulsion> lp/bp repulsion (> bp/bp repulsion) For M4 allow lone pairs repel more than bonding pairs
Give the IUPAC name for CH2(OH)CH(CH3)CH2Br
3-bromo-(2)-methylpropan-1-ol ONLY
Reaction 1 occurs via a nucleophilic substitution mechanism. Explain why the halogenoalkane is attacked by the nucleophile in this reaction.
Bromine is more electronegative than carbon Allow difference in electronegativity if polarity of bond shown M1 C is partially positive / electron deficient M2 and M3 can be awarded from diagram that shows nucleophilic attack M2 Lone/electron pair (on the nucleophile) donated to the partially positive carbon
Compound Boiling point / °C propan-1-ol 97 propanal 49 propanoic acid 141 In a preparation of propanal, propan-1-ol is added dropwise to the oxidising agent and the aldehyde is separated from the reaction mixture by distillation. (a) Explain, with reference to intermolecular forces, why distillation allows propanal to be separated from the other organic compounds in this reaction mixture.
Aldehyde/propanal has dipole-dipole forces (between molecules) If any ‘covalent bonds broken’ CE=0 for clip. Ignore Van der Waal forces M1 Alcohol/propan-1-ol AND Carboxylic acid/ propanoic acid have hydrogen bonding (between molecules). Ignore reference to energy M2 The forces between the molecules in aldehyde are weaker (than those in alcohol and acid so it will evaporate first.)
Give two ways of maximising the yield of propanal obtained by distillation of the reaction mixture.
Keep the temperature of the reaction mixture below the boiling point of propan-1-ol/below 97 °C Allow temperature in range 49-96 inclusive Allow description of cooling the vessel M1 Cool the distillate / collecting vessel
Describe how you would carry out a simple test-tube reaction to confirm that the sample of propanal obtained by distillation does not contain any propanoic acid.
Add named carbonate/hydrogencarbonate OR magnesium to a sample of the distillate. Effervescence/fizz/bubbles would confirm presence of acid or converse
Explain how this type of stereoisomerism arises. EZ
C=C bond cannot rotate and Each carbon in the double bond has (2) different groups attached.
Explain how these hydrogen bonds form
electron deficient H (Which attracts) lone pair/electron pair on O,N,F
Which species has a shape that is influenced by the presence of one or more lone pairs of electrons around the central atom? A AlCl3 B ClF3 C IF6+ D PCl6–
B
Carbocation D can undergo a type of reaction called a rearrangement to form carbocation E. In this reaction, a hydrogen atom and its bonding pair of electrons move from carbon a to carbon b as shown in the diagram below. Use your knowledge of carbocations to explain why this rearrangement takes place.
M1 more stable (carbocation formed) M2 changes from secondary to tertiary (carbocation)
Cyclohexene is prepared by the dehydration of cyclohexanol using concentrated phosphoric acid as a catalyst. The structure of concentrated phosphoric acid is shown. Identify the factors that influence the boiling points of each of the compounds in this reaction mixture. State how and explain why cyclohexene can be separated from the reaction mixture.
M1 cyclohexene : van der Waals’ forces (between molecules) 1 M2 cyclohexanol : hydrogen bonds (between molecules) 1 M3 phosphoric acid: hydrogen bonds (between molecules) 1 M4 idea that cyclohexene has weakest forces 1 M5 separated by (simple / fractional) distillation 1 M6 cyclohexene has lowest boiling point / boils off first
Chloroethene can be polymerised to form poly(chloroethene), Chloroethene has a melting point of −154 °C All types of PVC melt at temperatures over 100 °C Explain why PVC melts at a higher temperature than chloroethene.
M1 it / PVC is bigger/longer molecule / has more electrons / has bigger surface area / greater Mr 1 M2 it / PVC has stronger (van der Waals’ / dipole-dipole) forces between molecules / intermolecular forces