HSC Exam Practice

Sample response. An oxoacid is an acid that contains hydrogen, oxygen and at least one other element; it is named from the polyatomic ion it contains (e.g. H&sub2;SO&sub4; = sulfuric acid, from sulfate SO&sub4;²−). A binary acid contains only hydrogen and one other non-metal with no oxygen; it is named using the hydro-/ic pattern (e.g. HCl = hydrochloric acid).

Marking notes. 1 mark for oxoacid definition (H, O and another element; named from polyatomic ion). 1 mark for binary acid definition (H + one non-metal, no oxygen; hydro-/ic naming). 1 mark for one correct example of each.

Sample response. (a) HCl = hydrochloric acid; binary acid; strong. (b) H&sub2;SO&sub4; = sulfuric acid; oxoacid (from sulfate); strong (first ionisation complete). (c) HNO&sub3; = nitric acid; oxoacid (from nitrate); strong. (d) H&sub3;PO&sub4; = phosphoric acid; oxoacid (from phosphate); weak.

Marking notes. 1 mark per compound — must include correct name AND correct strong/weak classification. Partial credit: name only or classification only = 0.5, but round to whole marks per HSC convention.

Sample response. HF is a weak acid because the H–F bond is unusually short and strong: fluorine has the smallest atomic radius of the halogens, so the bond length is very short and the bond energy is high, making proton donation energetically difficult [1]. Despite fluorine’s very high electronegativity, which would normally favour proton donation by stabilising the fluoride ion, the bond strength effect dominates and HF does not fully ionise [1]. By contrast, HCl, HBr and HI all have longer, weaker H–X bonds (atomic radius increases down the group), allowing complete ionisation and strong acid behaviour [1].

Marking notes. 1 mark for identifying the unusually short/strong H–F bond (fluorine’s small atomic radius). 1 mark for explaining this inhibits proton donation. 1 mark for contrasting with HCl/HBr/HI (longer, weaker bonds → complete ionisation). Do not accept “fluorine is less electronegative” as this is factually incorrect.

Sample response.

(a) Acid + carbonate (Pattern 2). 2HCl + CaCO&sub3; → CaCl&sub2; + H&sub2;O + CO&sub2;. [2 marks: 1 for correct products, 1 for balanced equation with all three products.]

(b) Acid + base (Pattern 1). H&sub2;SO&sub4; + 2KOH → K&sub2;SO&sub4; + 2H&sub2;O. [2 marks: 1 for correct salt K&sub2;SO&sub4;, 1 for balanced (2KOH, 2H&sub2;O).]

(c) Acid + reactive metal (Pattern 3). Zn is above H in activity series. Zn + 2HNO&sub3; → Zn(NO&sub3;)&sub2; + H&sub2;. [2 marks: 1 for correct salt Zn(NO&sub3;)&sub2; with H&sub2; product, 1 for balanced equation.]

Marking notes. Do not award marks for unbalanced equations. Gas products must be labelled with correct formula: CO&sub2; not CO; H&sub2; not H.

Sample response. An indicator’s transition pH range is approximately pKa ± 1 pH units — spanning two pH units centred on the indicator’s own pKa value [1]. Methyl orange has pKa ≈ 3.5, so its transition range is approximately pH 2.5–4.5 (actual: 3.1–4.4) [1]. Phenolphthalein has pKa ≈ 9.2, so its transition range is approximately pH 8.2–10.2 (actual: 8.3–10.0) [1]. Within the transition range, both the acid (HIn) and base (Inˉ) forms coexist in measurable concentrations and an intermediate colour is observed.

Marking notes. 1 mark for pKa ± 1 relationship. 1 mark for correct MO application with numerical range. 1 mark for correct PP application with numerical range.

Sample response. NaOH is an Arrhenius base (releases OHˉ ions directly on complete dissociation in water: NaOH → Na¹+ + OHˉ); it is a strong base [1]. NH&sub3; is a Brønsted-Lowry base only (accepts a proton from water: NH&sub3; + H&sub2;O ⇌ NH&sub4;¹+ + OHˉ); it is a weak base because this equilibrium lies far to the left [1]. The key distinction: NaOH is classified by both Arrhenius and BL models (releases OHˉ and accepts protons); NH&sub3; is BL only (no OHˉ released directly — OHˉ comes from water) [1].

Marking notes. Do not award marks for writing NH&sub4;OH → NH&sub4;¹+ + OHˉ as the equation for ammonia in water; NH&sub4;OH is not a significant species. 1 mark each for correct classification, strength and equation per base.

Sample response. Equivalence point volume: approximately 25 mL (accept 24–26 mL) [1]. pH at equivalence point: approximately 7.0 (accept 6.5–7.5) [1]. This is expected for a strong acid–strong base titration where the salt produced (Na&sub2;SO&sub4;) does not hydrolyse.

Sample response. Bromothymol blue (BTB; pH 6.0–7.6) is the most appropriate indicator [1]. The equivalence point for H&sub2;SO&sub4; + NaOH (strong acid + strong base) is pH 7.0, which lies within BTB’s transition range [1]. At the equivalence point, the sharp rise in pH passes through 6.0–7.6, causing the indicator equilibrium HIn ⇌ H¹+ + Inˉ to shift from left (HIn dominant, yellow in acid) to right (Inˉ dominant, blue in base) as [H¹+] drops dramatically [1]. By Le Chatelier’s Principle, the sudden decrease in [H¹+] past the equivalence point shifts the equilibrium right toward Inˉ, giving a sharp yellow-to-blue colour change [1]. Methyl orange (ends at pH 4.4) and phenolphthalein (starts at pH 8.3) both miss the equivalence point at pH 7.

Marking notes. 1 mark for identifying BTB. 1 mark for linking BTB range to EP pH using data. 1 mark for correct indicator equilibrium (HIn ⇌ H¹+ + Inˉ) with direction of shift. 1 mark for Le Chatelier reasoning connecting [H¹+] change to colour change.

Sample response. Between 0 and 20 mL, there is a large excess of H&sub2;SO&sub4; in the flask and the added NaOH is simply being consumed by the excess acid, making only a small fractional change to the total [H¹+]; pH changes slowly [1]. Between 24 and 26 mL, the system is very close to the equivalence point where the moles of acid and base are nearly equal — even a tiny additional volume of NaOH removes the last traces of H¹+ and immediately creates an excess of OHˉ, causing a large, abrupt change in pH across many units [1].

Sample response. The naming conventions for inorganic acids are not arbitrary labels but reflect the acid’s composition and structure. Binary acids (HX, no oxygen) are named hydro-[element]-ic acid because the “hydro-” prefix signals the absence of oxygen and the direct H–X bond; HCl = hydrochloric acid is the classic example. Oxoacids contain oxygen and are named from their parent polyatomic ion: the –ate suffix becomes –ic acid and –ite becomes –ous acid. This systematic rule means H&sub2;SO&sub4; is sulfuric acid (from sulfate), HNO&sub3; is nitric acid (from nitrate) and H&sub3;PO&sub4; is phosphoric acid (from phosphate). In the context of Orica’s Kooragang Island facility, both HNO&sub3; (nitric acid, the precursor) and the ammonium nitrate product can be correctly named from this system. The naming principle also captures an important chemical distinction: HF is a weak acid (unusually strong H–F bond due to fluorine’s small atomic radius) despite being a binary acid like HCl, HBr and HI (all strong). The “hydro-” prefix indicates composition, not strength.

Indicator behaviour reflects the general principle of chemical equilibrium and Le Chatelier’s Principle. Every indicator is a weak acid (HIn ⇌ H¹+ + Inˉ) where the two forms have different colours. In the Mildura citrus-processing context, a technician dipping methyl orange into fresh-squeezed juice (pH ~2.4) sees red because the high [H¹+] shifts the equilibrium left, favouring HIn. As the juice is diluted and processed to pH ~6.8, [H¹+] falls, the equilibrium shifts right and MO turns yellow. This is not a unique property of the dye — it is a direct application of Le Chatelier’s Principle to a weak-acid equilibrium. The same principle explains indicator selection in lead-acid battery quality control: H&sub2;SO&sub4; + NaOH has an equivalence point at pH 7.0, so bromothymol blue (range 6.0–7.6) is the only indicator whose transition range includes the equivalence point.

Acid reaction patterns follow from the nature of the reactants. Pattern 1 (acid + base → salt + water) occurs whenever a proton donor encounters a proton acceptor or OHˉ source — whether the base is NaOH, KOH, Ca(OH)&sub2; (all strong) or NH&sub3; (weak BL base). Orica’s ammonium nitrate reaction (HNO&sub3; + NH&sub3; → NH&sub4;NO&sub3;) is Pattern 1. Pattern 2 (acid + carbonate/hydrogen carbonate → salt + water + CO&sub2;) occurs because carbonic acid (H&sub2;CO&sub3;), the intermediate product, is thermodynamically unstable and immediately decomposes. Limestone caves dissolving in rainwater (CO&sub2; dissolved to form H&sub2;CO&sub3;, which then reacts with CaCO&sub3;) is an Australian geochemical example. Pattern 3 (acid + active metal → salt + H&sub2;) occurs only when the metal lies above hydrogen in the activity series — the metal must be a strong enough reducing agent to reduce H¹+ to H&sub2;. Iron, zinc and magnesium react; copper and silver do not.

Marking notes. 1 mark — binary/oxoacid naming distinction with correct example (e.g. HCl vs H&sub2;SO&sub4;). 1 mark — oxoacid naming rule applied correctly (–ate → –ic). 1 mark — named Australian industrial/natural example for naming (Orica, citrus, batteries). 1 mark — indicator equilibrium HIn ⇌ H¹+ + Inˉ stated correctly with Le Chatelier link. 1 mark — indicator colour change explained in terms of [H¹+] shift for a specific indicator + pH context. 1 mark — named Australian example for indicator (Mildura citrus pH monitoring, Orica neutralisation endpoint, battery titration). 1 mark — Pattern 1 identified and applied to a named example. 1 mark — Pattern 2 with CO&sub2; mechanism (H&sub2;CO&sub3; decomposition) explained. 1 mark — Pattern 3 with activity series condition stated; correct identification of which metals react and which do not. High-band responses will integrate all three sub-topics cohesively rather than treating them as separate lists.