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Date	May 2019	Marks available	2	Reference code	19M.2.hl.TZ1.6
Level	HL	Paper	2	Time zone	TZ1
Command term	Calculate	Question number	6	Adapted from	N/A

Question

This question is about iron.

Deduce the full electron configuration of Fe²⁺.

[1]

Explain why, when ligands bond to the iron ion causing the d-orbitals to split, the complex is coloured.

[2]

State the nuclear symbol notation, ${}_{\text{Z}}^{\text{A}}{\text{X}}$ , for iron-54.

[1]

Mass spectrometry analysis of a sample of iron gave the following results:

Calculate the relative atomic mass, A_r, of this sample of iron to two decimal places.

[2]

An iron nail and a copper nail are inserted into a lemon.

Explain why a potential is detected when the nails are connected through a voltmeter.

[2]

Calculate the standard electrode potential, in V, when the Fe²⁺ (aq) | Fe (s) and Cu²⁺ (aq) | Cu (s) standard half-cells are connected at 298 K. Use section 24 of the data booklet.

[1]

f(i).

Calculate ΔG^θ, in kJ, for the spontaneous reaction in (f)(i), using sections 1 and 2 of the data booklet.

[1]

f(ii).

Calculate a value for the equilibrium constant, K_c, at 298 K, giving your answer to two significant figures. Use your answer to (f)(ii) and section 1 of the data booklet.

(If you did not obtain an answer to (f)(ii), use −140 kJ mol⁻¹, but this is not the correct value.)

[2]

f(iii).

Markscheme

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ [✔]

«frequency/wavelength of visible» light absorbed by electrons moving between d levels/orbitals [✔]

colour due to remaining frequencies
OR
complementary colour transmitted [✔]

${}_{{\text{26}}}^{{\text{54}}}{\text{Fe}}$ [✔]

«A_r =» 54 × 0.0584 + 56 × 0.9168 + 57 × 0.0217 + 58 × 0.0031

«A_r =» 55.9111 [✔]

«A_r =» 55.91 [✔]

Note: Award [2] for correct final answer

Do not accept data booklet value (55.85).

lemon juice is the electrolyte
OR
lemon juice allows flow of ions
OR
each nail/metal forms a half-cell with the lemon juice [✔]

Any one of:
iron is higher than copper in the activity series
OR
each half-cell/metal has a different redox/electrode potential [✔]

iron is oxidized
OR
Fe → Fe²⁺ + 2e⁻
OR
Fe → Fe³⁺ + 3e⁻
OR
iron is anode/negative electrode of cell [✔]

copper is cathode/positive electrode of cell
OR
reduction occurs at the cathode
OR
2H⁺ + 2e⁻ → H₂ [✔]

electrons flow from iron to copper [✔]

«E^θ = +0.34 V −(−0.45 V) = +»0.79 «V» [✔]

f(i).

«ΔG^θ = −nFE^θ = −2mol × 96 500 C mol⁻¹ × $\frac{{0.79{\text{ J }}{{\text{C}}^{ - 1}}}}{{1000}}$ =» −152 «kJ» [✔]

Note: Accept range 150−153 kJ.

f(ii).

«lnK_c = $- \frac{{\Delta {G^\theta }}}{{RT}} = - \frac{{ - 152 \times {{10}^3}{\text{ Jmo}}{{\text{l}}^{ - 1}}}}{{8.31{\text{J}}{{\text{K}}^{ - 1}}{\text{mo}}{{\text{l}}^{ - 1}} \times 298{\text{K}}}}$ =» 61.38 [✔]

K = 4.5 × 10²⁶ [✔]

Note: Accept answers in range 2.0 × 10²⁶ to 5.5 × 10²⁶.

Do not award M2 if answer not given to two significant figures.

If −140 kJmol⁻¹ used, answer is 3.6 × 10²⁴.

f(iii).

Examiners report

Done fairly well with common mistakes leaving in the 4s² electrons as part of Fe²⁺ electron configuration, or writing 4s¹ 3d⁵

This was poorly answered and showed a clear misconception and misunderstanding of the concepts. Most of the candidates failed to explain why the complex is coloured and based their answers on the emission of light energy when electrons fall back to ground state and not on light absorption by electrons moving between the split d-orbitals and complementary colour transmitted of certain frequencies.

Many candidates wrote the nuclear notation for iron as Z over A.

This question on average atomic mass was the best answered question on the exam. A few candidates did not write the answer to two decimal places as per instructions.

Very few candidates scored M1 regarding the lemon juice role as electrolyte. Some earned M2 but a lot of answers were too vague, such as ‘electrons move through the circuit’, etc.

Only 50 % of candidates earned this relatively easy mark on calculate EMF from 2 half-cell electrode potentials.

f(i).

Average performance; typical errors were using the incorrect value for n, the number of electrons, or not using consistent units and making a factor of 1000 error in the final answer.

f(ii).

This question was left blank by quite a few candidates. Common errors included not using correct units, or more often, calculation error in converting ln K_c into K_c value.

f(iii).

Syllabus sections

Core » Topic 2: Atomic structure » 2.1 The nuclear atom

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