Problem 1

To determine the solubility product of copper(II) iodate,

\ce{Cu(IO3)2}

, a saturated aqueous solution of

\ce{Cu(IO3)2}

at 25 °C is analysed by iodometric titration in acidic solution: an excess of iodide is added and the liberated iodine is titrated with sodium thiosulphate.

30.00\ \mathrm{cm^3}

of a 0.100 M sodium thiosulphate solution are needed for

20.00\ \mathrm{cm^3}

of the saturated solution. (1.1) Write the sequence of balanced equations for the reactions described. (1.2) Calculate the initial concentration of

\ce{Cu^{2+}}

and the solubility product of copper(II) iodate. Activity coefficients can be neglected.

Step 1 of 3: Balanced equations

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Intuition

Copper(II) is not just an observer: it is itself an oxidant that liberates extra iodine.

2\ce{Cu^{2+}} + 6\ce{IO3^-} + 24\ce{I^-} + 24\ce{H+} -> 2\ce{CuI} + 13\ce{I2} + 12\ce{H2O},\qquad \ce{I2} + 2\ce{S2O3^{2-}} -> 2\ce{I^-} + \ce{S4O6^{2-}}

Analysis

In acidic solution $\ce{IO3^-}$ oxidises $\ce{I^-}$ : $\ce{IO3^-} + 8\ce{I^-} + 6\ce{H+} -> 3\ce{I2} + 3\ce{H2O}$ , and $\ce{Cu^{2+}}$ oxidises $\ce{I^-}$ : $2\ce{Cu^{2+}} + 4\ce{I^-} -> 2\ce{CuI} + \ce{I2}$ . Summing gives the first equation; titration of $\ce{I2}$ with thiosulphate gives the second.