Periodic Trends: Electronegativity and Reactivity

Q1 — Sample Band 6 response (8 marks), annotated with marking criteria

NaCl (Na–Cl bond): |Δχ| = 3.2 − 0.9 = 2.3 ≥ 1.7 → ionic bond [1 mark — correct calculation and classification]. Na effectively transfers its valence electron to Cl; Na carries a full + charge (Na⁺) and Cl a full − charge (Cl⁻). Cl carries δ− because it has a much higher electronegativity (3.2 vs 0.9) and strongly attracts electrons away from Na [1 mark — correct δ− identification with electronegativity reasoning].

NH₃ (N–H bond): |Δχ| = 3.0 − 2.2 = 0.8, which lies between 0.4 and 1.7 → polar covalent bond. N is more electronegative → δ− on N, δ+ on H. The N–H bond polarity gives ammonia a significant molecular dipole (net uneven charge distribution across the molecule). A real-world consequence is that NH₃ dissolves readily in water because the polar N–H bonds allow strong attractions with the polar water molecules (“like dissolves like”); the resulting solution is alkaline because the δ− N atom accepts a proton from water [1 mark — real-world consequence stated using polarity/solubility or reactivity with water, based on the electronegativity difference and bond polarity taught in this lesson].

HF (H–F bond): |Δχ| = 4.0 − 2.2 = 1.8 ≥ 1.7 → classified as ionic by the threshold rule [1 mark — correct calculation]. However, HF is a discrete molecular compound in the gas phase with a real covalent H–F bond that has very high polarity (F carries δ−, H carries δ+). This illustrates that the |Δχ| = 1.7 threshold is an approximate guide, not a sharp boundary; HF is better described as a highly polar covalent bond sitting at the ionic/covalent boundary. Bond character is a continuum, not a discrete classification [1 mark — evaluates the threshold as approximate with HF as evidence].

OF₂ (O–F bond): |Δχ| = 4.0 − 3.4 = 0.6, which lies between 0.4 and 1.7 → polar covalent bond. F (χ = 4.0) is more electronegative than O (χ = 3.4) → δ− on F, δ+ on O [1 mark — correct identification including the unusual δ+ on O]. This is notable because in virtually all other compounds (H₂O, alcohols, carbonyl groups), oxygen carries δ−. In OF₂, F’s extreme electronegativity reverses the usual charge distribution, making O partially positive [1 mark — explains the unusual nature of O being δ+].

Threshold evaluation (summary): The 0.4 and 1.7 thresholds are approximate guidelines because bond polarity is a continuum. HF with |Δχ| = 1.8 behaves as a polar covalent compound (it is a molecular gas that dissolves to give an acid), despite being technically classified as ionic by the threshold. The classification is practically useful but should always be accompanied by the qualifier “approximately” [1 mark — clear evaluative conclusion integrating threshold evidence, total 8 marks].

Marking criteria summary (8 marks): 1 = NaCl: correct |Δχ| calculation and ionic classification. 1 = correct δ− identification and electronegativity reasoning for at least two bonds. 1 = NH₃: correct polar covalent classification for N–H. 1 = Real-world consequence of N–H polarity stated using vocabulary from this lesson (e.g. molecular dipole creating attractions, high water solubility, or alkaline reaction with water — all derivable from polarity and electronegativity concepts in L18). 1 = HF: correct |Δχ| calculation. 1 = Evaluation of threshold as approximate guide using HF as evidence. 1 = OF₂: correct identification of δ+ on O and explanation referencing F’s extreme electronegativity. 1 = Clear evaluative statement that bond polarity is a continuum, not a sharp classification.

Q2 — Sample Band 6 response (7 marks), annotated with marking criteria

Assessment of student’s claim: The claim is invalid — the colour of the halogen solution is a physical property unrelated to its oxidising power (reactivity). Br₂(aq) is orange-brown and I₂(aq) is pale yellow-brown; darker colour does not indicate greater reactivity. Reactivity must be tested by whether a halogen can displace a less reactive halide ion.

Hypothesis: If Br₂ is more reactive than I₂ as an oxidising agent, then adding Br₂(aq) to KI(aq) will produce a colour change (purple/brown I₂ forms), but adding I₂(aq) to KBr(aq) will produce no colour change. Independent variable: identity of halogen added (Br₂ or I₂). Dependent variable: colour change in the test tube (presence or absence of displacement product). Controlled variables: volume and concentration of halide salt solution (e.g. 2 mL of 0.1 mol/L), temperature (room temperature), halogen solution concentration. [1 mark — testable hypothesis with IV and DV identified]

Procedure: (1) Label four clean test tubes: A (Br₂ + KI), B (I₂ + KBr), C (Br₂ + KBr, control), D (I₂ + KI, control). (2) Add 2 mL of KI(aq) to test tube A and 2 mL of Br₂(aq). Observe and record colour. (3) Add 2 mL of KBr(aq) to test tube B and 2 mL of I₂(aq). Observe and record colour. (4) Repeat steps 2–3 twice more for reliability. Record and compare colour in each test tube against the colour chart. [1 mark — four clear steps including controls and colour comparison]

Expected results: Tube A should turn orange-brown/purple (Br₂ displaces I⁻, producing I₂). Tube B should show no significant colour change (I₂ cannot displace Br⁻). Controls C and D should show the original halogen colour with no displacement.

Falsification: If I₂(aq) added to KBr(aq) produced an orange-brown colour (Br₂), this would falsify the hypothesis that Br₂ is more reactive than I₂. Alternatively, if Br₂(aq) added to KI(aq) produced no colour change, the hypothesis would also be falsified [1 mark].

Limitations: (1) The presence of multiple dissolved species in solution means colour changes may be subtle or ambiguous — a colour chart or colorimeter would improve accuracy [1 mark]. (2) Br₂(aq) already has an orange-brown colour, which may mask a slight colour change in tube A from a different reaction [1 mark].

Improvement: Use a colorimeter to quantitatively measure absorbance at a specific wavelength (e.g. 520 nm for I₂) to reduce subjectivity in colour comparison; repeat the experiment three times and average results to improve reliability [1 mark].

Assessment of student’s claim: The claim is incorrect because colour of a solution is determined by wavelength of light absorbed (a physical property) and is not a measure of chemical reactivity. The correct way to assess relative reactivity is displacement reactions; colour change from displacement product (not the original halogen colour) is the valid evidence [1 mark — explicit evaluative judgement].

Marking criteria summary (7 marks): 1 = testable hypothesis with IV and DV. 1 = four clear steps with controls. 1 = correct falsification condition. 1 = first valid limitation. 1 = second valid limitation. 1 = specific improvement. 1 = explicit correct evaluation of student’s claim (colour is not a measure of reactivity).