Rate Laws: Crystal Violet

The question behind the experiment

A fading violet solution provides a concentration-linked signal that can be followed through time. This investigation uses absorbance measurements to infer a rate relationship, rather than deciding reaction order from a color impression. Start by identifying the mixed concentrations and measurement wavelength. The hydroxide can be present in large excess, making its concentration nearly constant during a run. That simplifies the time trace, but it does not mean hydroxide has no role in determining the observed reaction rate.

Plan a useful comparison

Collect an absorbance-time record under a fixed preparation and temperature. Compare plots of absorbance, logarithmic absorbance, and reciprocal absorbance against time. The useful straight-line transformation reveals a model relationship across the selected data interval. Inspect the complete record rather than choosing a transformation because a few neighboring points look linear. Keep excluded or unreliable measurements visible in your notes, and explain why an interval is suitable before using its slope to estimate a kinetic quantity from the trial.

Connect the observation to the model

A linear logarithmic-absorbance trace supports a first-order dependence on the remaining crystal violet under the specified conditions. Its negative slope gives the observed rate constant for that run. The raw fading slows as the remaining dye decreases, even while the transformed graph can retain a steady slope. Compare full-dye and half-dye preparations to investigate the half-life relationship. A smaller starting absorbance does not necessarily imply a different fractional disappearance time when the other reaction conditions remain matched.

Interpret the result carefully

Compare runs at different hydroxide concentrations while keeping the dye preparation and temperature fixed. If the observed rate constant scales with hydroxide concentration in this investigation, use that evidence to build the full rate law. Distinguish the run-specific pseudo-first-order constant from the constant in the complete concentration relationship. Their units and meaning differ. A graph that explains one time trace is therefore only part of the evidence needed to establish how another reactant affects the rate across preparations.

Build an explanation from your evidence

Check whether optical preparation or kinetic preparation could account for a mismatch. Blank choice, mixing, dilution, and temperature enter different places in the measurement chain. An absorbance error can distort the inferred concentration record, while a temperature change alters the chemical rate under the model. Preserve these possibilities separately in the explanation. The optional temperature extension examines a further dependence; the central learning question remains how transformed time data and controlled concentration comparisons jointly support the proposed reaction orders.

A reference case from this course

For the specified 0.010 M hydroxide reference, the pseudo-first-order constant is 1.6 × 10⁻³ s⁻¹ and the idealized half-life is 433.2169878 s. The displayed fitted half-life can differ through rounded absorbance measurements. Keep the preparation and temperature conditions attached; this is not a universal crystal-violet fading time.

Keep the investigation reproducible

Before changing the setup, write a prediction that names the measured quantity and the expected direction of change. Retain the original trial as a reference. Record the apparatus conditions, units, and whether you used Ideal or Real lab mode. If the selected activity includes seeded variation, keep the seed and run identity with the result so another comparison does not silently mix different conditions.

Separate the observation from its explanation. A reading or a counted object belongs in the evidence; the relationship you use to interpret it belongs in the reasoning. When a result differs from the reference case, check the settings and measurement procedure before deciding that the model relationship has failed. Describe a specific source of uncertainty and where it enters your calculation. Finish with a claim supported by the recorded comparison, and identify an additional trial that could distinguish your explanation from an alternative.

Continue the comparison

Review the concentration calibration in Beer’s Law, then compare the gas-volume method in Reaction Rates. Identify how each measurement relates to reactant or product amount before comparing its time-curve slope.