3.4 Questions and Answers Archive
- Conflicting opinion on inclusion of radioactivity
- Cu(II) complex ions
- Could you please confirm to me the bond angles for methane, nitrogen and water?
Q1. Just a wee query for you re: the new AS90780. There seem to be conflicting opinions as to whether or not radioactivity is still going to be assessed in this module. Please would you clarify this issue for me.
A. All aspects of radioactivity are definitely gone. However, it is not always possible for commercial products such as text books or even study guides to keep fully up to date with the changes. For example, I note that the sample of this year's Yr 13 Chemistry NCEA level 3 Study Pass booklet still includes radioactivity and students would need to be advised about this where appropriate.
Q2. Various questions have arisen about the correct formulae of the aqueous, chloro and ammine complexes of Cu(II). Although there are 6 ligand sites available around the Cu(II) cation but two of the ligands are always weakly held further from the cation than the other four. In fact for the ammine complex addition of the 5 th and 6 th ammonia ligands only occurs in liquid ammonia as water will occupy these sites in any aqueous solution.
Accordingly the best forms of the various complex ions are:
Cu (H2O)62+ but Cu(NH3)42+ and CuCl42-
However, as with all of these sort of picky details I'm sure any sensible formula would be accepted by the markers.
With the new combined AS of bonding and thermochem, i am a bit unsure about the transition metals component. The only properties that need to be investigated are colour and variable oxidation state. With colours, would you expect the colours of the complex ions of the named elements to be examined?
A. Special notes in the standard note that transition metals will be limited to iron, vanadium, chromium, manganese, copper and zinc.
There are lots of exceptions but as an overview I would choose from
- Zn(II) colourless
- Cu(I) brick red, Cu(II) blue (with some black compounds)
- Mn(VII) purple, Mn (VI) green, Mn(IV) brown, Mn(II) colourless/pale pink
- V(V) orange, V(IV) blue, V(III) green, V(II) violet
- Fe(II) pale green, Fe(III) orange
Q3. Sorry to ask again about this one but could you please confirm to me the bond angles for methane, nitrogen and water. We have problems teaching 109 for all and the students seem to grasp the concept for 109, 107, 105.
A. The actual angles are 109.5, 107.3 (for ammonia I presume you mean) and 104.5 respectively. However, I can't see any problems with students accepting these as all being approximately equal to the tetrahedral angle of 109. Surely good teaching practice at this level should firstly emphasise that essentially all these angles are based on a tetrahedral arrangement of the 4 pairs of electrons and therefore will be close to 109. The fact that a lone pair causes greater repulsion than a bonded pair (because it is held closer to the central atom) and therefore squeezes the bond angle in from the tetrahedral value is of less significance and is certainly not expected at level 3.