Hydrogen peroxide decomposes in aqueous solution according to the overall reaction 2 H2O2(aq) → 2 H2O(l) + O2(g). In the presence of a small amount of iodide ion, I−(aq), the reaction is dramatically accelerated.
A student prepares a reaction mixture that is 0.500 M in H2O2 and 0.020 M in I− at time t = 0. The temperature is held constant and the solution is stirred continuously. No additional species capable of reacting with I− are introduced.
Assume the reaction volume remains effectively constant as O2 gas escapes.
Answer the following questions about the progress of the reaction.
(a) On clearly labeled axes, sketch qualitative concentration–time curves for [H2O2], [O2] (expressed as moles of O2 per liter of original solution), and [I−]. Rank the three curves in order of decreasing absolute slope magnitude at t = 0 and justify your ranking.
(b) Explain, at the particulate level of the mechanism, why the net concentration of I− remains essentially unchanged throughout the reaction even though the iodide ion participates in elementary steps.
(c) Persulfate ion, S2O82−(aq), if accidentally present in trace amounts, slowly oxidizes I− to I2(aq). Describe qualitatively how the concentration–time curve for I− and the overall rate of H2O2 decomposition would differ from the curves you sketched in part (a). Justify your predictions.