| Date | November 2017 | Marks available | 2 | Reference code | 17N.2.hl.TZ0.3 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Describe | Question number | 3 | Adapted from | N/A |
Question
Trends in physical and chemical properties are useful to chemists.
Cobalt forms the transition metal complex [Co(NH3)4 (H2O)Cl]Br.
Explain why the melting points of the group 1 metals (Li → Cs) decrease down the group whereas the melting points of the group 17 elements (F → I) increase down the group.
State the shape of the complex ion.
Deduce the charge on the complex ion and the oxidation state of cobalt.
Describe, in terms of acid-base theories, the type of reaction that takes place between the cobalt ion and water to form the complex ion.
Markscheme
Any three of:
Group 1:
atomic/ionic radius increases
smaller charge density
OR
force of attraction between metal ions and delocalised electrons decreases
Do not accept discussion of attraction between valence electrons and nucleus for M2.
Accept “weaker metallic bonds” for M2.
Group 17:
number of electrons/surface area/molar mass increase
London/dispersion/van der Waals’/vdw forces increase
Accept “atomic mass” for “molar mass”.
[Max 3 Marks]
«distorted» octahedral
Accept “square bipyramid”.
Charge on complex ion: 1+/+
Oxidation state of cobalt: +2
Lewis «acid-base reaction»
H2O: electron/e– pair donor
OR
Co2+: electron/e– pair acceptor
