The concentration of mercury, a toxic heavy metal pollutant, in aqueous solution depends in part on the redox properties of its compounds. Suppose you are studying the properties of mercury. You might need to construct an electrochemical cell and write the chemical equation for the cell reaction. Write the reaction for the cell Pt(s)|H₂(g)|HCl(aq)|Hg₂Cl₂(s)|Hg(l).

PLAN Follow the procedure set out in Toolbox 6L.1.

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Toolbox 6L.1 HOW TO WRITE A CELL REACTION CORRESPONDING TO A CELL DIAGRAM CONCEPTUAL BASIS A cell diagram corresponds to a specific cell reaction in which the right-hand electrode in the cell diagram is treated as the site of reduction and the left-hand electrode is treated as the site of oxidation. The sign of the cell potential then distinguishes whether the resulting reaction is spontaneous in the direction written (Ecell> 0) or whether the reverse reaction is spontaneous (Ecell 0) under the stated conditions. PROCEDURE Step 1 Write the half-reaction for the electrode on the right of the cell diagram as a reduction (remember: Right for Reduction). Step 2 Write the half-reaction for the electrode on the left of the cell diagram as an oxidation. Step 3 Multiply one or both equations by a factor if necessary to match the number of electrons in each half-reaction and then add the two equations. If the cell potential is positive, then the reaction is spontaneous as written. If the cell potential is negative, then the reverse reac- tion is spontaneous. This procedure is illustrated in Example 6L.2.