Chemistry 9701/42 — February/March 2021

Cambridge A-Level · A Level Structured Questions · worked solutions for every part, with the mark scheme

questions

100

marks

120

minutes

Topics Transition Elements · Electrochemistry · Introduction to A Level Organic Chemistry · Chemical Energetics · Reaction Kinetics · Equilibria · +6 more

Q1Transition ElementsElectrochemistryGroup 2Free sample

Answer all the questions in the spaces provided.

(a)

The most common oxidation states of cobalt are +2 and +3.

Complete the electronic configurations of the following free ions.

$\text{Co}^{2+}$ [Ar] ....................................................................................................................
$\text{Co}^{3+}$ [Ar] ....................................................................................................................

(b)

$\text{Co}^{2+}$ and $\text{Co}^{3+}$ both form complexes with $\text{edta}^{4-}$ .

half-equation	$E^\ominus / \text{V}$
$\text{Co}^{3+} + \text{e}^- \rightleftharpoons \text{Co}^{2+}$	+1.82
$\text{O}_2 + 4\text{H}^+ + 4\text{e}^- \rightleftharpoons 2\text{H}_2\text{O}$	+1.23
$[\text{Co}(\text{edta})]^{-} + \text{e}^- \rightleftharpoons [\text{Co}(\text{edta})]^{2-}$	+0.38
$\text{Co}^{2+} + 2\text{e}^- \rightleftharpoons \text{Co}$	-0.28

Use the data in the table to predict what happens, if anything, when separate aqueous solutions of $\text{Co}^{3+}$ and $[\text{Co}(\text{edta})]^{-}$ are left to stand in the air.

aqueous solution of $\text{Co}^{3+}$

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aqueous solution of $[\text{Co}(\text{edta})]^{-}$

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(c)

Hydrated cobalt(II) nitrate, $\text{Co}(\text{NO}_3)_2\cdot6\text{H}_2\text{O}$ , is a red solid that behaves like hydrated magnesium nitrate, $\text{Mg}(\text{NO}_3)_2\cdot6\text{H}_2\text{O}$ , when heated.

Describe in detail what you would expect to observe when crystals of $\text{Co}(\text{NO}_3)_2\cdot6\text{H}_2\text{O}$ are heated in a boiling tube, gently at first and then more strongly.

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(d)

Explain why the thermal stability of the Group 2 nitrates increases down the group.

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The rest of this paper

6 more questions

Q2Chemical Energetics · Transition Elements · Electrochemistry13M
Q3Halogen Compounds · Electrochemistry · Reaction Kinetics · Equilibria · Chemical Energetics18M
Q4Transition Elements11M
Q5Transition Elements · Hydrocarbons · Carboxylic Acids and Derivatives · Introduction to A Level Organic Chemistry12M
Q6Hydrocarbons · Polymerisation · Introduction to A Level Organic Chemistry · Reaction Kinetics10M
Q7Nitrogen Compounds · Carboxylic Acids and Derivatives · Introduction to A Level Organic Chemistry · Equilibria28M

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