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Chemistry IA Exemplar: Acid Catalysts and Iodine Clock Reaction | RevisionDojo
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IB Chemistry HL Internal Assessment Example
How does the use of different acid catalysts (H2SO4, HCl, H3PO4, CH3COOH, p-TSA) affect the rate of reaction between potassium iodide (KI), sodium thiosulfate (Na2S2O3) and hydrogen peroxide (H2O2) also known as the iodine clock reaction.
6
Official IB Result
17/24
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Criteria A: Research Design
5/6
0
3
6
Criteria Strands
Excellent
Research question context
Good
Methodological considerations
Good
Methodology description
Criteria Feedback
Research question situated in a specific industrial and kinetic theory context
Comprehensive variable table with apparatus and uncertainties
Clear chemical equations and Maxwell–Boltzmann diagram enhancing theoretical rationale
Mismatch between acids listed in title and research question
Inaccurate pKₐ value for sulfuric acid needs correction
Temperature‐control procedure is not described or quantified
Assumptions about uncontrolled variables lack mitigation or measurement strategies
1.1·Weakness
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Mismatch between acids listed in the title (includes HNO₃) and those in the research question (includes p-TSA) suggests inconsistent experimental focus. Ensure acid selection is consistent throughout to maintain clarity.
1.2·Strength
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The introduction provides a clear rationale linking the iodine clock reaction’s sensitivity to catalysts with industrial applications, effectively justifying the study’s relevance.
1.3·Weakness
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Temperature control is mentioned as a factor but details on how fluctuations will be managed (e.g., thermostated bath) are lacking, limiting reproducibility of controlled conditions.
1.4·Strength
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The research question is well-focused within the context of acid-catalyzed iodine clock reaction, clearly listing catalysts and reactants to investigate their effect on reaction rate.
1.5·Strength
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Chemical equations for both the slow oxidation and fast reduction steps are correctly presented, enhancing conceptual clarity of the reaction mechanism.
1.6·Strength
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Including a Maxwell–Boltzmann diagram to illustrate catalyst effects aptly visualizes how activation energy lowering increases reaction rates, reinforcing theoretical background.
1.7·Weakness
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The pKₐ value table lists sulfuric acid’s pKₐ as −10, which appears inaccurate; verify and source correct dissociation constants to ensure validity of theoretical predictions.
1.8·Strength
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The variables table comprehensively identifies independent, dependent, and controlled variables with apparatus and uncertainties, demonstrating thorough experimental planning.
1.9·Weakness
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Control of starch concentration is noted but concentration uncertainty is not quantified; specifying mass and volume uncertainty would enhance rigor of this control variable.
1.10·Strength
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Materials and equipment are clearly listed with uncertainties, facilitating reproducibility through transparent specification of analytical tools and their precision.
1.11·Weakness
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While assumptions list uncontrolled variables (temperature, impurities), no strategies to quantify or mitigate these uncertainties (e.g., calibration logs, impurity assays) are provided.
1.12·Strength
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The step-by-step methodology is detailed with volumes, concentrations, stirring speed, and repeat trials, allowing another researcher to replicate the procedure with minimal ambiguity.
1.13·Weakness
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Preliminary trials are described qualitatively but lack supporting data (e.g., time vs. concentration readings) to justify final reagent concentrations quantitatively.
Criteria B: Data Analysis
4/6
0
3
6
Criteria Strands
Good
Communication of data recording and processing
Good
Consideration of uncertainties
Moderate
Data processing quality
Criteria Feedback
Well-organized quantitative data tables with consistent significant figures and uncertainties
Sample calculations for averages and uncertainties clearly shown
Clear presentation of data-recording procedures and LaTeX-formatted equations
Graph axes labeled with numeric codes rather than catalyst names
Propagation of uncertainty formula misapplies the concentration term
Uncertainties for KI and H₂O₂ concentrations are omitted
2.1·Strength
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Quantitative data table is well-organized with trials, consistent significant figures, and uncertainties, enabling clear interpretation of raw measurements.
2.2·Strength
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The qualitative data figure clearly captures the end-point colour change setup, reinforcing the observational methodology of the iodine clock reaction.
2.3·Weakness
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Total time uncertainty calculation includes human and stopwatch errors but does not account for possible systematic reaction-time lag; consider measuring instrument response delay.
2.4·Strength
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Sample calculations for average time and propagated uncertainties include LaTeX-formatted equations, enhancing clarity of data-processing methodology.
2.5·Weakness
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The mole calculation for Na₂S₂O₃ is correct, but units (mol) are missing; always include units in intermediate steps for unambiguous data processing.
2.6·Weakness
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The uncertainty propagation formula omits the concentration uncertainty of I₂ and misapplies terms; revisit the error propagation for Δ[I₂]/[I₂] to ensure accurate ΔRate calculation.
2.7·Weakness
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The commentary on the graph’s error bars highlights HCl deviation but lacks quantitative analysis (e.g., R² or residuals) that would strengthen discussion of data precision.
2.8·Weakness
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X-axis labels use numeric codes rather than catalyst names, making the graph ambiguous; relabel axes directly with acid abbreviations for clear data communication.
Criteria C: Conclusion
4/6
0
3
6
Criteria Strands
Good
Conclusion relevance and support
Moderate
Scientific context comparison
Criteria Feedback
Conclusion logically correlates observed reaction rates with pKₐ values and kinetic theory
Discussion of error bars and controlled‐variable lapses shows consistency with analysis
Comparison to literature is qualitative only, lacking quantitative metrics
Limited critical discussion of discrepancies between own data and published studies
3.1·Weakness
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Comparison to literature is limited to qualitative agreement; incorporating quantitative metrics or discussing discrepancies would deepen the scientific context evaluation.
3.2·Strength
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The conclusion succinctly correlates reaction rates to pKₐ values and kinetic theory, justifying results within the context of activation-energy reduction by acid catalysts.
Criteria D: Evaluation
4/6
0
3
6
Criteria Strands
Moderate
Methodological weaknesses
Moderate
Suggested improvements
Criteria Feedback
Specific methodological weaknesses (calibration, impurities, detection method) are identified
Relevant and realistic improvements (UV-Vis detection, reagent purity, environmental control) are described
No prioritization of which errors dominate overall uncertainty
Proposed improvements lack detailed implementation plans and quantified impact
4.1·Suggestion
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Ensure calibration protocols for pipettes and cylinders are detailed, including frequency and traceability, to validate improved equipment accuracy.
4.2·Suggestion
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Discussing calibration frequency and error reduction magnitude for stopwatch improvements would strengthen this evaluation of systematic errors.
4.3·Suggestion
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The proposed UV–Vis spectrophotometer improvement is relevant; consider detailing instrument parameters (λ selection, calibration) to demonstrate practical implementation and cost–benefit analysis.
4.4·Weakness
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Evaluation tables list generic weaknesses but do not assess their relative impact on overall uncertainty; prioritizing errors by magnitude would improve critical evaluation.
