A student is designing rate experiments for three unrelated reactions at 25 °C. The only instruments available are a visible-range spectrophotometer (400–700 nm), a gas-collection apparatus calibrated to measure volume at atmospheric pressure, and a conductivity probe.
Reaction 1: Zn2+(aq)+2I−(aq)→Zn(s)+I2(aq) (reaction mixture turns brown as I2 forms)
Reaction 2: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) (hydrogen gas evolves steadily)
Reaction 3: NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq) (ionic precipitate forms rapidly)
Assume all reactions are carried out in well-stirred, constant-temperature conditions at atmospheric pressure.
(a) For each reaction, identify the single most appropriate instrument from the three provided to monitor the progress of the reaction in real time. Justify each choice by clearly connecting an observable property measured by the instrument to the changing concentration of a reactant or product.
(b) For Reaction 1, derive an algebraic relationship that expresses the instantaneous reaction rate in terms of the time derivative of the measurable quantity identified in part (a). State any assumptions you make.
(c) For Reaction 2, describe how the ideal-gas relationship can be used to convert the measured quantity from the chosen instrument into molar concentration of H2(g) inside the collection apparatus.
(d) For Reaction 3, the conductivity probe registers a nearly instantaneous drop in conductivity when the reagents are mixed. Explain why this technique is unsuitable for obtaining an accurate rate law for Reaction 3, and propose one modification to the experimental approach that would allow meaningful kinetic data to be collected.