Chemistry HL

1. 1.

   A hypothetical chemistry-physics team compares energy-storage routes for delivering 12.0 kWh of useful electricity. Route H uses 30.0 kWh input electricity; route B uses 15.0 kWh. Calculate both overall efficiencies. Explain why a comparison using 1 kg of each storage material instead could be misleading, and name one further chemistry-related factor the team should investigate before choosing a route.

   [3 marks]

   Worked explanation

   1. Use useful output divided by input for each complete route; 40.0% and 80.0% describe the supplied hypothetical boundaries only.
   2. Agree the functional unit with the physics team before combining efficiencies with chemical storage, safety and material evidence. Efficiency alone does not measure lifecycle environmental impact.

   Common mistake: Ranking all environmental impacts solely by efficiency treats a single performance measure as a complete lifecycle assessment.

   Marking points

   • H efficiency = 12.0/30.0 x 100 = 40.0%, and B efficiency = 12.0/15.0 x 100 = 80.0%; both required for this point.
   • A common delivered-energy functional unit compares the same service, whereas equal masses need not deliver equal useful energy.
   • A relevant additional factor with a route-specific link, such as hydrogen leakage/storage hazards or battery-material extraction and end-of-life recovery.

   Examiner tip: Ranking all environmental impacts solely by efficiency treats a single performance measure as a complete lifecycle assessment.

2. 2.

   In a hypothetical shared water study, the chemistry team reports a pH probe reading of 6.0 while a second team reports 7.0 for an apparently identical sample. Propose a joint calibration check, identify two pieces of metadata needed to establish whether the measurements are comparable, and explain why simply averaging to pH 6.5 does not resolve the discrepancy.

   [3 marks] · no calculator

   Worked explanation

   1. Use common standards to distinguish a sensor-scale problem from a real difference between samples, preserving the original records rather than overwriting them.
   2. Compare traceable sample identities and conditions. A midpoint is not a diagnosis and is not necessarily the pH of mixed water because pH is logarithmic and chemical buffering also matters.

   Common mistake: Assuming a numerical compromise validates both measurements confuses agreement by negotiation with agreement supported by calibration.

   Marking points

   • Both teams measure the same appropriate standard buffers and a shared sample under agreed conditions, recording results before recalibration.
   • Two relevant metadata items, such as sample ID, sampling time, temperature, probe identity or calibration record; both required for this point.
   • An average can conceal a systematic difference or mismatched samples; identify the source and measurement validity before pooling results.

   Examiner tip: Assuming a numerical compromise validates both measurements confuses agreement by negotiation with agreement supported by calibration.

3. 3.

   A hypothetical project group favours a hydrogen bus proposal because it has no direct CO2 exhaust. A chemistry member raises hydrogen-production emissions and a physics member raises conversion losses. Propose a negotiated evidence-gathering plan, state one ethical boundary for reporting a provisional recommendation, and give one concrete reflection question about interdisciplinary cooperation.

   [3 marks] · no calculator

   Worked explanation

   1. Agree what service is compared and which production, use and disposal stages count before splitting research responsibilities. Record assumptions so the two disciplines' results can be reconciled.
   2. Communicate a conditional recommendation that distinguishes evidence from unresolved assumptions. Reflect on whether the team revised its preferred proposal when another discipline supplied conflicting evidence; this remains a written rehearsal, not evidence of actual collaboration.

   Common mistake: Excluding production emissions after favouring hydrogen changes the comparison boundary to protect a preferred conclusion.

   Marking points

   • Agree a common passenger-distance service and lifecycle boundary, assigning chemistry to production/material evidence and physics to energy-loss evidence with a shared comparison log.
   • Label the recommendation provisional, disclose sources and uncertainties, and avoid claiming zero lifecycle emissions from zero direct exhaust.
   • A specific reflection question examines how disciplinary evidence or dissent changed the decision, rather than merely asking whether members enjoyed the task.

   Examiner tip: Excluding production emissions after favouring hydrogen changes the comparison boundary to protect a preferred conclusion.

4. 4.

   For a hypothetical chemistry-physics team rehearsal, route A uses 10 kWh of electricity and route B uses 16 kWh for the same delivered transport service. Non-electric lifecycle emissions within an agreed boundary are 5.0 kg CO2e for A and 2.0 kg CO2e for B. Let grid intensity be g kg CO2e per kWh, identical for both. Form total-emission expressions, find the crossover g, and compare routes at g=0.20. Explain how the team should qualify a recommendation if future g is unknown.

   [5 marks]

   Syllabus placement evidence: Chemistry subject brief, first assessment 2025, II Curriculum model overview p.2 collaborative sciences project paragraph; p.3 Inquiry process, Concluding and evaluating. Combines chemistry-related lifecycle accounting and physics-related electric demand through an explicit functional unit and conditional decision. All factors are invented supplied inputs; this is HL interdisciplinary preparation, not actual emissions research or completed collaboration.

   Worked explanation

   1. Add electric-use emissions to the separately supplied non-electric amounts without counting either twice. The same service is the denominator for both totals.
   2. Subtract E_B from E_A to get 3-6g. Its sign changes at 0.50, explaining why the energy-efficiency advantage and the non-electric-emission advantage can favour different routes. Shared scenario assumptions must accompany any recommendation.

   Common mistake: Comparing only electricity consumption ignores the stated non-electric emissions and can reverse the ranking at a low grid intensity.

   Marking points

   • E_A=5.0+10g and E_B=2.0+16g, in kg CO2e per stated service.
   • Equality gives 3.0=6g, so crossover g=0.50 kg CO2e kWh^-1.
   • At g=0.20, E_A=7.0 and E_B=5.2 kg CO2e; B is lower by 1.8 kg CO2e.
   • B is lower below 0.50, A lower above 0.50, within the supplied common boundary.
   • Present a conditional recommendation with grid-intensity scenarios and disclose boundary/input uncertainty rather than claiming either route is universally cleaner.

   Examiner tip: Comparing only electricity consumption ignores the stated non-electric emissions and can reverse the ranking at a low grid intensity.

5. 5.

   In a hypothetical shared energy-storage review, a chemistry member finds '90% material recovery' for a battery process, while a physics member has '60% energy efficiency' for the same process. No definitions or denominators accompany either number. A draft multiplies 0.90 by 0.60 to claim '54% sustainable'. Explain two distinct denominator questions the team must resolve, propose a traceable joint revision of the draft, and identify an environmental issue neither percentage necessarily captures. No actual study is being cited.

   [4 marks] · no calculator

   Syllabus placement evidence: Chemistry subject brief, first assessment 2025, I Aims 8-9; II Curriculum model overview p.2 collaborative sciences project paragraph. Rehearses evidence reconciliation and environmental evaluation in interdisciplinary communication. The percentages are hypothetical and lack definitions deliberately; no real study, negotiated team performance, assessed project or universal sustainability criterion is claimed.

   Worked explanation

   1. Percentages can be multiplied as sequential fractions only when the numerators and denominators form a justified chain. A mass-recovery fraction and an energy-efficiency fraction do not establish a universal sustainability scale.
   2. The revision should show what each discipline measures and what is still missing, not replace 54% with another unsupported score. A separate issue such as effluent toxicity illustrates why one resource metric cannot stand for every environmental impact.

   Common mistake: Two dimensionless percentages are not automatically commensurable or sufficient to define sustainability.

   Marking points

   • Ask which input mass/material and recovered output define 90%, including composition/purity and whether collection losses are included.
   • Ask which useful energy output and total energy inputs/stages define 60%, and whether it describes recycling or a different storage cycle.
   • Withdraw the undefined composite, retain separate sourced/defined metrics in a common boundary table, assign members to verify definitions and record unresolved assumptions before revising the recommendation.
   • Neither metric alone necessarily captures toxicity, water use or extraction impacts; identify one with a plausible route-specific link such as contaminated recycling effluent.

   Examiner tip: Two dimensionless percentages are not automatically commensurable or sufficient to define sustainability.

Original, unofficial practice questions from SubjectScout — not official past-paper questions or complete syllabus coverage. Added English teaching explanations are AI-authored; independent teacher review is pending.