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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

Cobalt forms the transition metal complex [Co(NH3)4 (H2O)Cl]Br.

Trends in physical and chemical properties are useful to chemists.

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.

[3]
b.

State the shape of the complex ion.

[1]
d.i.

Deduce the charge on the complex ion and the oxidation state of cobalt.

[2]
d.ii.

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.

[2]
e.

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]

b.

«distorted» octahedral

Accept “square bipyramid”.

d.i.

Charge on complex ion: 1+/+
Oxidation state of cobalt: +2

d.ii.

Lewis «acid-base reaction»

H2O: electron/e pair donor

OR

Co2+: electron/e pair acceptor

e.

Examiners report

[N/A]
b.
[N/A]
d.i.
[N/A]
d.ii.
[N/A]
e.

Syllabus sections

Additional higher level (AHL) » Topic 18: Acids and bases » 18.1 Lewis acids and bases
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Additional higher level (AHL) » Topic 13: The periodic table—the transition metals » 13.1 First-row d-block elements
Additional higher level (AHL) » Topic 13: The periodic table—the transition metals
Additional higher level (AHL) » Topic 18: Acids and bases
Additional higher level (AHL)

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