Investigation 11: Kinetics and Rate Laws

Crystal violet reacts with hydroxide ion and loses its purple color as the reaction proceeds. That color change is the whole point: a colorimeter measures absorbance every few seconds, and because absorbance tracks concentration directly, you get a continuous record of concentration dropping over time without ever pulling a sample out of the flask.

Beer's law says absorbance equals a constant times concentration (A = ebc), so absorbance is just concentration wearing a different unit. That means you can test the integrated rate laws directly on absorbance data instead of converting back to molarity first. Plot ln(A) versus time: a straight line means first order in crystal violet, and the slope of that line equals -k. Plot 1/A versus time instead: a straight line there would mean second order. Whichever plot comes out linear tells you the order, and its slope hands you the rate constant. A slope of -0.0140 per second means k = 0.0140 per second, since the slope of a first-order ln(concentration)-vs-time plot is defined as -k.

The hydroxide ion is deliberately loaded in large excess, so its concentration barely budges during the run even though it's a reactant. That is not sloppy design, it is pseudo-first-order technique: with hydroxide essentially constant, any change in rate traces back only to crystal violet, so its order can be isolated cleanly. This investigation connects directly to the broader unit skills of graphing for linearity, reading slopes as rate constants, and propagating error through a derived quantity like k.