Race to the same color.

Learning goal

Students compare three fresh-mixture endpoint times at each of three temperatures, keep the endpoint and other conditions fixed, and support an explanation with numerical evidence. Introduce reactants, collision theory, and activation energy before beginning. Allow about 15–25 minutes plus discussion as a planning estimate.

Higher temperature shifts the energy distribution so a greater fraction of particles can overcome an activation barrier. Warming does not remove that barrier. The student explanation should connect this idea to measured time comparisons; it should not claim a universal doubling of rate per 10°C.

The generic color-change model and its parameters are invented for this activity. It is not a recipe or prediction for a named chemical reaction. Mixing and endpoint detection are automatic supported steps rather than tests of student reaction time.

Procedure and controls

Predict how temperature affects time to a fixed endpoint. Assemble water bath, temperature probe, reaction flask, prepared A/B aliquots, and automatic endpoint timer.
Prepare fresh 5.0 mL aliquots of solution A and initiator B. Equilibrate both aliquots and the flask in the bath; verify temperature.
Transfer A to the flask. Add B to initiate the reaction and start the timer on contact.
Observe a compressed color-change animation. The timer stops at the same color threshold, corresponding to 80% conversion in this model. This does not mean the reaction is complete.
Discard the spent mixture and repeat three fresh trials at 20°C, then 30°C, then 40°C. The bath holds temperature after mixing.
Compare mean times and ranges, write an evidence-based explanation, and name a limitation. Optionally calculate 1/time for the first trial.

Keep solution formulations, aliquot volumes, flask, mixing, temperature equilibration, and endpoint rule constant. Only the bath target changes between groups. Each measured temperature includes explicit probe variation and is recorded separately from its target.

Interpretation and assessment

With no numerical variation, endpoint times are approximately 77.3 s at 20°C, 45.0 s at 30°C, and 27.1 s at 40°C. Small prescribed temperature, rate, and timing variations change displayed values. Assess numerical support and the scientific explanation, not exact matches to those predictions.

1/time in s⁻¹ is a relative endpoint-rate index under the same formulation and threshold. It is not a concentration-change rate in mol/L/s or an independently measured rate constant. Group mean indices average the individual 1/time values; they are not computed as the reciprocal of the group mean time.

Claim: identifies the shortest-time temperature among tested conditions.
Evidence: compares at least two means in seconds with their temperatures; uses repeats/ranges accurately.
Reasoning: explains energy and activation barrier without claiming temperature removes the barrier or automatically changes concentration.
Limit: names an unmodeled effect or explains why another reaction may respond differently.

The automatic check evaluates choices and the optional calculation. A teacher reviews scientific writing using the shared checklist on the overview.

Discussion prompts

Why must both aliquots reach the bath temperature before mixing?
Why would timing to different shades make the comparison unfair?
Does reaching the endpoint mean every reactant particle has reacted?
Why is a consistent timing delay especially important in a fast reaction?
Would the same numerical temperature multiplier apply to every chemical reaction?
Which new experiment could separate temperature effects from concentration effects?

Explicit teaching model

Conversion is p = 1 − exp(−kt), an effective first-order approximation, with a fixed endpoint p = 0.80. The prescribed Arrhenius relationship is k(T) = k(30°C) exp[−40000/8.314 × (1/(T + 273.15) − 1/303.15)], where k(30°C) = ln(5)/45 s⁻¹. The 40 kJ/mol parameter and formulation are chosen instructional values, not experimentally measured chemical properties. This effective law does not establish the molecular reaction mechanism or reaction orders of the two solutions.

Actual bath temperature is target ±0.2°C; probe noise ±0.1°C, rounded to 0.1°C. Each fresh trial’s effective rate factor varies ±2%; endpoint time receives an offset within ±0.2 s and rounds to 0.1 s. The threshold is detected exactly in conversion space before the timing offset. Perfect mixing and thermal control are assumed. Heat release, transfer lag, evaporation, chemical side reactions, probe lag, and subjective endpoint detection are omitted. Observed minimum–maximum ranges are not confidence intervals.

The three-second animation compresses the full modeled endpoint time. Students with reduced motion see a static apparatus while timing completes. Progress saves in the browser; download or print the report before changing devices or restarting.

Background: OpenStax Chemistry 2e — Collision Theory

Original scenario, procedure, numerical parameters, and assessment. The background source supports general kinetic theory, not these synthetic trial values. Real heating, glassware, and chemicals require teacher supervision and approved classroom procedures.