Question

Suppose you measured reactant concentration when increasing the volume of water used in each cup. A friend chose to measure reactant concentration by dropping more than one antacid tablet into equal quantities of water. Will your graph look the same as your friend’s graph?

Answer 1

Short answer: In theory, yes — if two experiments produce the same reactant concentration (moles per liter) and everything else is identical, the rate vs concentration plot should have the same shape and values regardless of whether you got that concentration by adding more water or by adding more tablet. In practice, no — the graphs often differ because the two ways of changing concentration introduce different secondary effects.

Why they might be the same
- Chemical kinetics depend on concentration (and temperature, catalysts, etc.). If the dissolved active ingredient has the same molarity in both sets of runs, a given rate law predicts the same rate.

Why they might differ in a real lab
- Dissolution/availability: Whole tablets take time to dissolve. Adding extra tablets changes surface area and dissolution kinetics, so the measured reaction rate may be limited by how fast the active ingredient dissolves, not by the intrinsic rate law.
- Tablet composition: Tablets contain binders, fillers, coatings, and possibly different amounts of active ingredient per tablet; adding more tablets can change solution properties (viscosity, pH, ionic strength) and affect rate.
- Gas evolution or mixing: Antacid reactions often produce CO2; more tablets can change bubbling, mixing, and measured rate (bubbles can carry reactant/product away or impede contact).
- Volume and concentration errors: Adding tablet solids can change effective volume; your “equal volume” assumption may be off. The two methods give different ranges and resolution of concentration (discrete jumps with whole tablets).
- Secondary equilibria/saturation: If any species reaches solubility limits or if other reagents (acid, catalyst) are in limited supply, the dependence on concentration can change.
- Experimental measurement issues: Timing, temperature changes, and stirring can differ when adding different numbers of tablets, producing different apparent kinetics.

Practical recommendation
- If you want comparable graphs, make the concentrations directly comparable: determine the moles of active ingredient per tablet, dissolve/crush tablets to ensure consistent dissolution, keep volume constant or correct for added solids, control temperature and stirring, and measure initial rates. Under those controlled conditions you should get matching rate vs concentration behavior.

Conclusion: The ideal kinetic plots would match, but real experimental differences usually make the two graphs look different unless you control for dissolution, tablet composition, mixing, and other secondary effects.