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Sodium and its Compounds
2018 Paper One
Question Eight
This question has a 16 mark value.
1. wrong terminology, for example wrong bonding and/or intermolecular forces in all/some of the substances, and showed contradictions in answers.
2. Â Equation was often seen with sodium oxide as the product rather than sodium hydroxide. Many students did not use the 2:1 ratio from the equation.
3. Â Most common error was to multiply the amount, in moles, by volume rather than dividing by volume.
4. Â The structure often had the wrong number of lone pairs and/or the wrong bond angle.
Indicative chemistry content.
Contradictions (eg molecules, IMFs, covalent bonding,) negate statements.
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 3 - 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
Full marks are awarded only if all three Stages are covered
Make sure you are familiar with chemical terminology
moles of Na = 0.250/23.0 = 0.0109
moles of hydrogen = 0.0109/2 = 0.00545
temperature = 25 + 273 = 298 K
101 kPa = 101 000 Pa
Use PV = nRT (remember volume is in cubic metres)
V = (nRT)/P = (0.00545 x 8.31 x 298)/101 000 = 0.000134 cubic metres
Volume is 134 cubic centimetres
Concentration of sodium ions = 0.0109/0.5 = 0.0218 (mol dm-3)
104.5°
The 4 electron pairs repel so are as far apart as possible
Lone pairs repel more than bonding pairs: lp/lp repulsion> lp/bp repulsion > bp/bp repulsion.
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