(a)

Many commercial copper and brass polishes contain ammonia. The tarnish that forms on the surface of copper is often copper sulfide, CuS . In the presence of $\mathrm{O}_{2}$ from the air, $\mathrm{NH}_{3}$ can combine with this copper sulfide to produce the soluble cuprammonium sulfate, $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4}$ .

[ 1 ]

(i)

State the colour of cuprammonium sulfate solution.

[ 1 ]

(a)

When aqueous solutions of KI and $\mathrm{K}_{2} \mathrm{~S}_{2} \mathrm{O}_{8}$ are mixed almost no reaction occurs, but when a few drops of $\mathrm{Fe}^{2+}(\mathrm{aq})$ or $\mathrm{Fe}^{3+}(\mathrm{aq})$ are added, iodine, $\mathrm{I}_{2}(\mathrm{aq})$ , is produced at a steady rate.

[ 3 ]

(i)

State the precise role of the iron ions during this reaction.

(ii)

By means of equations or otherwise, explain why the presence of either $\mathrm{Fe}^{2+}$ or $\mathrm{Fe}^{3+}$ is able to speed up the reaction.

[ 3 ]

(a)

(i)

State how the melting point and density of iron compare to those of calcium.
melting point of iron:
density of iron:

(ii)

Explain why these differences occur.
melting point:
density:

[ 4 ]

[Maximum number: 1]

Transition elements are important metals because of their characteristic properties.

(a)

Define what is meant by a transition element.

[ 1 ]

(a)

Copper(I) and silver(I) salts are colourless.

Suggest why.

[ 2 ]

[Maximum number: 1]

Iron is a transition element in the fourth period. Iron forms compounds containing the ions $\mathrm{Fe}^{2+}$ and $\mathrm{Fe}^{3+}$ .

(a)

(i)

Define the term transition element.

[ 1 ]

[Maximum number: 1]

Iodine is found naturally in compounds in many different oxidation states.

(a)

The concentration of $\mathrm{Cu}^{2+}(\mathrm{aq})$ in a solution can be determined by the reaction of $\mathrm{Cu}^{2+}$ ions with $\mathrm{I}^{-}$ ions.

reaction $12 \mathrm{Cu}^{2+}+4 \mathrm{I}^{-} \rightarrow 2 \mathrm{CuI}+\mathrm{I}_{2}$

The $\mathrm{I}_{2}$ produced in reaction 1 is titrated against a solution containing thiosulfate ions, $\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}$ , using a suitable indicator.

\text { reaction } 2 \quad 2 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}+\mathrm{I}_{2} \rightarrow \mathrm{~S}_{4} \mathrm{O}_{6}^{2-}+2 \mathrm{I}^{-}

[ 1 ]

(i)

Copper(I) and copper(II) both contain electrons in all five 3d orbitals.

Sketch the shape of a $3 \mathrm{~d}_{\mathrm{xy}}$ orbital on the axes provided.

[ 1 ]

(a)

(i)

Explain why transition elements can form complex ions.

[ 1 ]

(a)

Describe three characteristic chemical properties of transition elements that are not shown by Group II elements.

[ 3 ]

(a)

Cobalt is a transition element and forms compounds with oxidation numbers +2 and +3 .
Complete the electronic structures for
a cobalt atom,
$1 s^{2} 2 s^{2} 2 p^{6}$
cobalt in the +3 oxidation state.
$1 s^{2} 2 s^{2} 2 p^{6}$

[ 2 ]

(b)

(i)

State two chemical properties of cobalt, other than the formation of complexes, that are not shown by a typical s-block element.

[ 2 ]