DP Chemistry (first assessment 2025)

Question 22M.2.HL.TZ1.6

Date	May 2022	Marks available	[Maximum mark: 11]	Reference code	22M.2.HL.TZ1.6
Level	HL	Paper	2	Time zone	TZ1
Command term	Deduce, Draw, Explain, Show, State	Question number	6	Adapted from	N/A

[Maximum mark: 11]

22M.2.HL.TZ1.6

Nitric acid is usually produced by the oxidation of ammonia.

A mixture of nitric acid and sulfuric acid can be used to convert benzene to nitrobenzene, C₆H₅NO₂.

(a(i))

Draw arrows in the boxes to represent the electron configuration of a nitrogen atom.

[1]

Markscheme

Accept all 2p electrons pointing downwards.

Accept half arrows instead of full arrows.

Examiners report

Drawing arrows in the boxes to represent the electron configuration of a nitrogen atom was done extremely well.

(a(ii))

Deduce a Lewis (electron dot) structure of the nitric acid molecule, HNO₃, that obeys the octet rule, showing any non-zero formal charges on the atoms.

[2]

Markscheme

bonds and non-bonding pairs correct ✔

formal charges correct ✔

Accept dots, crosses or lines to represent electron pairs.

Do not accept resonance structures with delocalised bonds/electrons.

Accept + and – sign respectively.

Do not accept a bond between nitrogen and hydrogen.

For an incorrect Lewis structure, allow ECF for non-zero formal charges.

Examiners report

Drawing the Lewis structure of HNO₃ was performed extremely poorly with structures that included H bonded to N, no double bond or a combination of single, double and even a triple bond or incorrect structures with dotted lines to reflect resonance. Many did not calculate non-zero formal charges.

(a(iii))

Explain the relative lengths of the three bonds between N and O in nitric acid.

[3]

Markscheme

Any three of:

two N-O same length/order ✔
delocalization/resonance ✔

N-OH longer «than N-O»
OR
N-OH bond order 1 AND N-O bond order 1½ ✔

Award [2 max] if bond strength, rather than bond length discussed.

Accept N-O between single and double bond AND N-OH single bond.

Examiners report

Poorly done; some explained relative bond strengths between N and O in HNO₃, not relative lengths; others included generic answers such as triple bond is shortest, double bond is longer, single longest.

(a(iv))

State a technique used to determine the length of the bonds between N and O in solid HNO₃.

[1]

Markscheme

X-ray crystallography ✔

Examiners report

A majority could not state the technique to determine length of bonds; answers included NMR, IR, and such instead of X-ray crystallography.

(b(i))

Write an equation for the reaction between the acids to produce the electrophile, NO₂⁺.

[1]

Markscheme

HNO₃ + 2H₂SO₄ $⇌$ NO₂⁺ + H₃O⁺ + 2HSO₄^- ✔

Accept “HNO₃ + H₂SO₄ $⇌$ NO₂⁺ + H₂O + HSO₄^-”.

Accept “HNO₃ + H₂SO₄ $⇌$ H₂NO₃⁺ + HSO₄^-” AND “H₂NO₃⁺ $⇌$ NO₂⁺ + H₂O”.

Accept single arrows instead of equilibrium signs.

Examiners report

Many had difficulties writing the balanced equation(s) for the formation of the nitronium ion.

(b(ii))

Draw the structural formula of the carbocation intermediate produced when this electrophile attacks benzene.

[1]

Markscheme

Accept any of the five structures.

Do not accept structures missing the positive charge.

Examiners report

Again, many had difficulty drawing the structural formula of the carbocation intermediate produced in the reaction.

(b(iii))

Deduce the number of signals that you would expect in the ¹H NMR spectrum of nitrobenzene and the relative areas of these.

[2]

Markscheme

Number of signals: three/3 ✔

Relative areas: 2 : 2 : 1 ✔

Examiners report

Deducing the number of signals in the 1H NMR spectrum of nitrobenzene, which depend on the number of different hydrogen environments, was done poorly. Also, instead of relative areas, the common answer included chemical shift (ppm) values.

Syllabus sections

Structure 1. Models of the particulate nature of matter » Structure 1.3—Electron configurations » Structure 1.3.5—Each orbital has a defined energy state for a given electron configuration and chemical environment, and can hold two electrons of opposite spin. Sublevels contain a fixed number of orbitals, regions of space where there is a high probability of finding an electron. Apply the Aufbau principle, Hund’s rule and the Pauli exclusion principle to deduce electron configurations for atoms and ions up to Z = 36.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.14—Formal charge values can be calculated for each atom in a species and used todetermine which of several possible Lewis formulas is preferred. Apply formal charge to determine a preferred Lewis formula from different Lewis formulas for a species.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.4—Electron-pair sharing reactions » Reactivity 3.4.2—In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond, as another bond breaks producing a leaving group. Deduce equations with descriptions and explanations of the movement of electron pairs in nucleophilic substitution reactions.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.4—Electron-pair sharing reactions » Reactivity 3.4.12—The relative stability of carbocations in the addition reactions between hydrogen halides and unsymmetrical alkenes can be used to explain the reaction mechanism. Predict and explain the major product of a reaction between an unsymmetrical alkene and a hydrogen halide or water.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.4—Electron-pair sharing reactions » Reactivity 3.4.13—Electrophilic substitution reactions include the reactions of benzene with electrophiles. Describe and explain the mechanism of the reaction between benzene and a charged electrophile, E+.

Structure 3. Classification of matter » Structure 3.2—Functional groups: Classification of organic compounds » Structure 3.2.10—Proton nuclear magnetic resonance spectroscopy (1H NMR) gives information on the different chemical environments of hydrogen atoms in a molecule. Interpret 1H NMR spectra to deduce the structures of organic molecules from the number of signals, the chemical shifts, and the relative areas under signals (integration traces).

Question 22M.2.HL.TZ1.6

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