Q) Using a dot and cross diagram to show the bonding in an ammonia, NH3, molecule. (2009, Hl, 7 marks)
Join N with three H atoms.
N (group 5) > valency of 3 i.e. needs 3 electrons from other atom(s) for it to become stable.
H (group 1) > valency of 1 i.e. needs one more electron to be stable. NOTE THE RELEVANCE OF VALANCY IN FUTURE
N has an Atomic Number of 7 and 7 electrons in ground state
Electron pattern 2,5
N has 5 e- on outer shell
3 of these 5 are involved in bonding The NH3 has a pyramidal shape.
Exam Revision: 2009
- Define oxidation in terms of electron transfer. (4)
- Oxidation is the loss of electrons
The electrolysis, using inert electrodes, of aqueous potassium iodide, KI, to which a few drops of phenolphthalein indicator have been added, is shown in the diagram.
- Name a suitable material for the electrodes. (3)
- platinum / carbon (graphite)
- Write balanced half equations for the reactions that take place at the electrodes. (12)
- anode: I¯ + ½ I2 + e– / 2I¯ + I2 + 2e– FORMULAS: (3) BALANCING: (3)
- [Electrons may be shown subtracted on the left. Neg. charge on e need not be shown.]
- cathode: 2H2O + 2e– + H2 + 2OH¯ / H2O + e– + ½ H2 + OH¯
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- FORMULAE: (3) BALANCING: (3)
- [Electrons may be shown subtracted on the right. Neg. charge on e need not be shown.]
- Explain the colour change observed at the positive electrode (anode). (6)
- colour of the iodine produced
- [Allow 3 for “correct colour : red, orange, yellow, brown or combinations of them” if iodine not stated]
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