Chemistry 9701/41 — May/June 2023

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

questions

100

marks

120

minutes

Topics Introduction to A Level Organic Chemistry · Transition Elements · Nitrogen Compounds · Analytical Techniques · Chemical Energetics · Equilibria · +5 more

Q1Group 2Transition ElementsFree sample

(a)

Group 2 nitrates decompose when heated.

Describe how the thermal stability of Group 2 nitrates changes with increasing proton number.

Explain your answer.

(b)

Copper(II) nitrate decomposes in a similar manner to Group 2 nitrates.

Write an equation for the decomposition of $\text{Cu(NO}_3)_2$ .

(c)

$\text{Cu(NO}_3)_2$ is added to water to form solution A.
Fig. 1.1 shows some reactions of solution A.

Fig. 1.1

Complete Table 1.1 to show the formula and colour of each of the copper-containing species present in A, B, C and D.

Table 1.1

	formula of copper-containing species formed	colour of copper-containing species formed
A
B
C
D

(d)

$\text{EDTA}^{4-}$ is a polydentate ligand.

(i)

Explain what is meant by a polydentate ligand.

(ii)

Group 2 metal ions can form complexes similar to those of transition elements.

A solution of $\text{EDTA}^{4-}$ is added to water containing $[\text{Ca}(\text{H}_2\text{O})_6]^{2+}$ to form a new complex, $[\text{CaEDTA}]^{2-}$ , as shown.

\text{equilibrium 1} \quad [\text{Ca}(\text{H}_2\text{O})_6]^{2+} + \text{EDTA}^{4-} \rightleftharpoons [\text{CaEDTA}]^{2-} + 6\text{H}_2\text{O}

Circle on the structure of $\text{EDTA}^{4-}$ in Fig. 1.2 the six atoms that form bonds with the metal ion.

Fig. 1.2

(iii)

The calcium ions in $[\text{Ca}(\text{H}_2\text{O})_6]^{2+}$ and $[\text{CaEDTA}]^{2-}$ have a coordination number of 6.

Explain what is meant by coordination number.

(iv)

The complex $[\text{CaEDTA}]^{2-}$ can be used to remove toxic metals from the body.

Table 1.2 shows the numerical values for the stability constants, $K_{\text{stab}}$ , for some metal ions with $\text{EDTA}^{4-}$ .

Table 1.2

complex	$K_{\text{stab}}$
$[\text{CaEDTA}]^{2-}$	$5.0 \times 10^{10}$
$[\text{CrEDTA}]^{-}$	$2.5 \times 10^{23}$
$[\text{FeEDTA}]^{-}$	$1.3 \times 10^{25}$
$[\text{PbEDTA}]^{2-}$	$1.1 \times 10^{18}$

An aqueous solution containing $[\text{CaEDTA}]^{2-}$ is added to a solution containing equal concentrations of $\text{Cr}^{3+}\text{(aq)}$ , $\text{Fe}^{3+}\text{(aq)}$ and $\text{Pb}^{2+}\text{(aq)}$ . The resulting mixture is left to reach a state of equilibrium.

State the type of reaction when $[\text{CaEDTA}]^{2-}$ reacts with $\text{Cr}^{3+}\text{(aq)}$ , $\text{Fe}^{3+}\text{(aq)}$ and $\text{Pb}^{2+}\text{(aq)}$ .

(v)

Deduce the relative concentrations of $[\text{CrEDTA}]^{-}$ , $[\text{FeEDTA}]^{-}$ and $[\text{PbEDTA}]^{2-}$ present in the resulting mixture.

Explain your answer.

\text{highest concentration} > \text{\quad\quad\quad\quad\quad\quad} > \text{lowest concentration}

(e)

The number of moles of water of crystallisation in a hydrated ionic salt can be determined by titration using aqueous $\text{EDTA}^{4-}$ ions with a suitable indicator.

0.255 g of hydrated chromium(III) sulfate, $\text{Cr}_2(\text{SO}_4)_3 \cdot n\text{H}_2\text{O}$ , is dissolved in water and made up to $100\text{ cm}^3$ in a volumetric flask.
$25.0\text{ cm}^3$ of this solution requires $26.2\text{ cm}^3$ of $0.00800\text{ mol dm}^{-3}$ aqueous $\text{EDTA}^{4-}$ ions to reach the end-point.

The reaction occurs as shown.

[\text{Cr}(\text{H}_2\text{O})_6]^{3+} + \text{EDTA}^{4-} \rightarrow [\text{CrEDTA}]^{-} + 6\text{H}_2\text{O}

Use the data to calculate the value of $n$ in the formula of $\text{Cr}_2(\text{SO}_4)_3 \cdot n\text{H}_2\text{O}$ .

Show your working.

(f)

A solution of $\text{Cr}^{3+}\text{(aq)}$ and a solution of $\text{Fe}^{3+}\text{(aq)}$ have different colours.

Explain why the two complexes have different colours.

The rest of this paper

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Q8Chemical Energetics9M

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