Edexcel IAL Chemistry Unit 5 Transition Metals and Organic Nitrogen Chemistry

Use this Unit 5 bank to revise redox cells, transition-metal chemistry, aromatic reactions, amines, amides, polymers and multi-step organic synthesis.

Covered syllabus points

Topic 16: Redox Equilibria Topic 17: Transition Metals and their Chemistry Topic 18: Organic Chemistry – Arenes Topic 19: Organic Nitrogen Compounds: Amines, Amides, Amino Acids and Proteins Topic 20: Organic Synthesis

[Maximum number: 12]

This question is about mercury, Hg, and its compounds.
Mercury is a liquid element in the same group of the Periodic Table as zinc.
The electronic configuration of mercury is $[\mathrm{Xe}] 4 \mathrm{f}^{14} 5 \mathrm{~d}^{10} 6 \mathrm{~s}^{2}$ .

(a)

Mercury forms compounds in either the +1 or +2 oxidation states.

Explain why mercury is classified as a d-block element but is not a transition element.
(3)

[ 3 ]

An explanation that makes reference to the following points:

- $\left(\mathrm{Hg}^{+}\right.$ is $\left.[\mathrm{Xe}] 4 \mathrm{f}^{14}\right) 5 \mathrm{~d}^{10} 6 \mathrm{~s}^{1}$

and $\left(\mathrm{Hg}^{2+}\right.$ is $\left.[\mathrm{Xe}] 4 \mathrm{f}^{14}\right) 5 \mathrm{~d}^{10}\left(6 \mathrm{~s}^{0}\right)$

(1)

- (d-block element as last) electron goes into a (5)d-orbital(s) (when the electronic configuration is written according to the Aufbau principle)

(1)

- (not transition element as) $\mathrm{Hg}^{+}$ and $\mathrm{Hg}^{2+}$ /(stable) ions do not have incompletely filled (5)d-orbital(s)

(1)

Accept use of d-subshell for d-orbital(s) Allow use of d-shell for d-subshell Penalise use of just d-block for d-shell once only Penalise use of 3d/4d for 5d once only

Allow Hg loses (only) its 6s electrons (when forming ions/compounds)

Do not award answer in terms of the electronic configuration of an ion of mercury

Allow $\mathrm{Hg}^{+}$ and $\mathrm{Hg}^{2+}$ /(stable) ions have completely full (5)d-orbital(s) Ignore any reference to d-d transitions / other transition element properties

Do not award answer in terms of the electronic configuration of the element / an Hg atom

(3)

Expert

(b)

Mercury reacts with nitric acid to form an aqueous solution of $\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}$ and nitrogen monoxide gas.

The unbalanced equation is shown.

\mathrm{Hg}(\mathrm{l})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{NO}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

[ 2 ]

(i)

Explain, using oxidation numbers, why this is a redox reaction.
(2)

[ 2 ]

An explanation that makes reference to the following
points:

Allow oxidation numbers from annotation to the equation
Ignore any reference to electron loss/gain

(2)

Expert

- Hg/mercury oxidised and
from 0 (in Hg ) to +2 (in $\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}$ )

(1)

Do not award reference to oxidation of any other element

- N / nitrogen is reduced
and
from +5 (in $\mathrm{HNO}_{3}$ ) to +2 (in NO)

(1)

Do not award $\mathrm{HNO}_{3}$ is reduced
Do not award reference to reduction of any other element

If no other mark awarded,
Hg/mercury oxidised and N/nitrogen reduced
OR
Hg / mercury from 0 to +2 and N / nitrogen from +5 to +2
scores (1)

(c)

Mercury(I) chloride, $\mathrm{Hg}_{2} \mathrm{Cl}_{2}$ , is also known as calomel.

A saturated calomel electrode may be used as an alternative to the standard hydrogen electrode.

Figure for Question (c) — Edexcel A-Level Chemistry A2

The half-equation for the calomel electrode is

\mathrm{Hg}_{2} \mathrm{Cl}_{2}(\mathrm{~s})+2 \mathrm{e}^{-} \rightleftharpoons 2 \mathrm{Hg}(\mathrm{l})+2 \mathrm{Cl}^{-}(\mathrm{aq})

The standard electrode potential of the calomel electrode is $E^{\ominus}=+0.24 \mathrm{~V}$ .

[ 7 ]

(i)

Suggest why KCl crystals are needed in the outer tube of the electrode.

[ 1 ]

An answer that makes reference to the following point:

- (to provide a) constant concentration (of $\mathrm{Cl}^{-}$ )

Allow KCl for $\mathrm{Cl}^{-}$ throughout

Allow to keep the solution / $\mathrm{Cl}^{-}$ saturated

Allow to replace $\mathrm{Cl}^{-}$

Ignore just to provide $\mathrm{Cl}^{-}$ Ignore stated concentrations

Do not award salt bridge / to complete the circuit

Do not award catalyst

(1)

Expert

(ii)

A calomel electrode was used to measure the standard electrode potential of the $\mathrm{Sn}^{2+}(\mathrm{aq}) \mid \mathrm{Sn}(\mathrm{s})$ half-cell.

Figure for Question (ii) — Edexcel A-Level Chemistry A2

The reading on the voltmeter in this cell was +0.37 V .
Deduce the standard electrode potential for the $S n^{2+}(a q) \mid S n(s)$ half-cell.

[ 1 ]

An answer that makes reference to the following point:

- $(0.24-0.37=)-0.13(\mathrm{~V})$

Ignore working, even if incorrect

(1)

Expert

(iii)

Write the overall equation for the cell reaction.

[ 1 ]

An answer that makes reference to the following point:
$\text { - } \mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn} \rightarrow 2 \mathrm{Hg}+\mathrm{Sn}^{2+}+2 \mathrm{Cl}^{-}$

Allow $\mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn} \rightarrow 2 \mathrm{Hg}+\mathrm{SnCl}_{2}$ Allow multiples

Allow ⇌ for →

Ignore state symbols even if incorrect Ignore half-equations even if incorrect Ignore use of cell diagrams

Do not award uncancelled electrons

Do not award $2 \mathrm{Hg}^{+}\left(+2 \mathrm{Cl}^{-}\right)$ for $\mathrm{Hg}_{2} \mathrm{Cl}_{2}$

If answer to (d)(ii) is $+0.61(\mathrm{~V}) /+0.37(\mathrm{~V}) /$ greater than $+0.24(\mathrm{~V})$ , equation must be reversed:

$2 \mathrm{Hg}+\mathrm{Sn}^{2+}+2 \mathrm{Cl}^{-} \rightarrow \mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn}$ OR

$2 \mathrm{Hg}+\mathrm{SnCl}_{2} \rightarrow \mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn}$

(1)

Expert

(iv)

Add labels to complete the diagram of a standard hydrogen electrode. Include details of any essential conditions.
(3)

Figure for Question (iv) — Edexcel A-Level Chemistry A2

[ 3 ]

(v)

Suggest one advantage of using a calomel electrode, in place of a standard hydrogen electrode, when measuring a cell potential.

[ 1 ]

[Maximum number: 12]

This question is about benzene, C_6 H_6, a colourless liquid first isolated in 1825, and some related compounds.

Three C_6 H_6 structures proposed in the 1860 s are shown.

Table for Question Unit 5: Transition Metals and Organic Nitrogen Chemistry question 2 — Edexcel A-Level Chemistry A2

The delocalised model for the structure of benzene has been accepted since the 1930s following the study of its X-ray diffraction pattern and the understanding of electron delocalisation in bonding theory.

The Dewar and Ladenburg structures have since been isolated as stable compounds but there is no compound with the Kekulé structure.

Figure for Question Unit 5: Transition Metals and Organic Nitrogen Chemistry question 2 — Edexcel A-Level Chemistry A2

(a)

Describe a chemical test, including the result, that could distinguish the Dewar structure from benzene.

[ 2 ]

suitable test: bromine water $/ \mathrm{Br}_{2}(\mathrm{aq})$ (1)

- result of test: decolourises (from orange) with Dewar structure (and no change with benzene) (1)

Allow bromine $/ \mathrm{Br}_{2}((1)) / \mathrm{Br}_{2}$ in organic solvent

Do not award $\mathrm{Br} / \mathrm{Br}^{-}$

Accept potassium manganate((VII))/ $\mathrm{KMnO}_{4}$ and acidified/ named $\operatorname{acid} / \mathrm{H}^{+}$

Do not award hydrogenation

Do not award combustion

Allow does not decolourise with benzene

Allow brown/orange/yellow for colour of bromine water

Allow red/brown/orange for colour of bromine

Allow pink/purple for colour of potassium manganate((VII))

Ignore colour fades for decolourises

Ignore reference to addition/substitution

Do not award any reference to decolourisation/reaction with benzene

(b)

Explain how X-ray diffraction shows that benzene has a delocalised structure and not a Kekulé structure.

[ 2 ]

(c)

The Ladenburg and Dewar structures both isomerise to benzene.

The enthalpy changes are -376 kJ mol^-1 and -297 kJ mol^-1 respectively.

[ 2 ]

(i)

Draw a labelled enthalpy level diagram showing the relative thermodynamic stability of the Ladenburg structure, the Dewar structure and benzene.
Include the enthalpy change values in kJ mol^-1.
Your diagram does not need to be to scale.

\mathrm{H} / \mathrm{kJ} \mathrm{~mol}^{-1}

[ 2 ]

(d)

The enthalpy change of hydrogenation of hex-3-ene is -118 kJ mol^-1.

Figure for Question (d) — Edexcel A-Level Chemistry A2

The table shows the enthalpy changes of hydrogenation of two further alkenes containing six carbon atoms.

Table for Question (d) — Edexcel A-Level Chemistry A2

Use your knowledge of benzene thermochemistry to suggest explanations for both of these enthalpy changes of hydrogenation in relation to the value for hex-3-ene.

[ 2 ]

(f)(i)

- skeletal formula of any one isomer

- skeletal formulae of second and third isomers (1)

Example of correct skeletal formulae:
(1,2-isomer)
(1,3-isomer)
(1,4-isomer)

Allow Kekulé benzene ring

Allow structural/displayed $\mathrm{CH}_{3}$ and $\mathrm{CH}_{3} \mathrm{CO}$ groups

If no other mark awarded, 1,2-, 1,3- and 1,4-isomers with incorrect side chain/cyclohexane ring scores (1)

(e)

Methylbenzene, C_6 H_5 CH_3, reacts with ethanoyl chloride, CH_3 COCl, in the presence of the catalyst aluminium chloride, AlCl_3, to form a mixture of organic products with the formula CH_3 COC_6 H_4 CH_3.

\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}+\mathrm{CH}_{3} \mathrm{COCl} \rightarrow \mathrm{CH}_{3} \mathrm{COC}_{6} \mathrm{H}_{4} \mathrm{CH}_{3}+\mathrm{HCl}

[ 4 ]

(i)

Complete the diagram, including curly arrows, to show the mechanism for the formation of compound X in this reaction.
Include an equation for the regeneration of the catalyst.

\mathrm{CH}_{3} \mathrm{COCl}+\mathrm{AlCl}_{3} \rightarrow \mathrm{CH}_{3} \mathrm{CO}^{+}+\mathrm{AlCl}_{4}^{-}

Figure for Question (i) — Edexcel A-Level Chemistry A2

[ 4 ]

A mechanism including:

- curly arrow from on or within circle to $\mathrm{C}^{+}$ of $\mathrm{CH}_{3} \mathrm{CO}^{+}$

(1)

- structure of intermediate ion

(1)

- curly arrow from C-H bond to within ring and correct product

(1)

- balanced equation for regeneration of catalyst (1)

Example of mechanism:

Figure for Answer to Question (i) — Edexcel A-Level Chemistry A2

Allow Kekulé benzene ring

‘Horseshoe’ facing tetrahedral carbon and covering at least three carbons with some part of positive sign within 'horseshoe'

Allow methyl at 1,2- or 1,3-positions

Ignore missing methyl substituent

Do not award dotted/dashed C-H / C-C bonds unless 3D structure

Allow 1,2- or 1,3-product from corresponding intermediate Do not award missing methyl substituent

$\mathrm{AlCl}_{4}^{-}+\mathrm{H}^{+} \rightarrow \mathrm{AlCl}_{3}+\mathrm{HCl}$

[Maximum number: 2]

The amino acid serine has the structure shown.

Figure for Question Unit 5: Transition Metals and Organic Nitrogen Chemistry question 3 — Edexcel A-Level Chemistry A2

(a)

The IUPAC name for serine is

2-amino-2-carboxyethanol

2-amino-1-hydroxyethanoic acid

2-amino-1-hydroxypropanoic acid

2-amino-3-hydroxypropanoic acid

[ 1 ]

(b)

Which is the organic product of the reaction between serine and sodium hydroxide solution?
□ A

Figure for Question (b) — Edexcel A-Level Chemistry A2

□ B

□ C

□ D

[ 1 ]

All question bank results loaded

Covered syllabus points

Unit 5: Transition Metals and Organic Nitrogen Chemistry question 1

Question (a)

Question (b)

Question (i)

Question (c)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (v)

Unit 5: Transition Metals and Organic Nitrogen Chemistry question 2

Question (a)

Question (b)

Question (c)

Question (i)

Question (d)

Question (e)

Question (i)

Unit 5: Transition Metals and Organic Nitrogen Chemistry question 3

Question (a)

Question (b)