In an investigation of the reaction between sodium thiosulfate and hydrochloric acid

CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics

Problem

In an investigation of the reaction between sodium thiosulfate and hydrochloric acid, a student adds Cu

^{2+}

(aq) and observes a dramatic increase in reaction rate. Both Na

^+

and Cu

^{2+}

ions are present in the reaction mixture, yet only Cu

^{2+}

is described as a catalyst. Which statement best accounts for this difference? (a) Cu

^{2+}

is present in a higher concentration than Na

^{+}

, making its effect on the reaction rate more pronounced. (b) Cu

^{2+}

accelerates the reaction by shifting the equilibrium toward products in accordance with Le Châtelier’s principle, something Na

^{+}

cannot do. (c) Cu

^{2+}

participates in the reaction steps and is regenerated, whereas Na

^{+}

does not participate chemically and remains unchanged throughout. (d) Cu

^{2+}

is reduced during the reaction and therefore consumed, whereas Na

^{+}

is not.