Q124P - Chemistry: Molecular Nature Of Matter And Change

Question

In the sepia "loning" of a black-and-white photograph, the image is converted to a rich brownish violet by placing the finished photograph in a solution of gold (III) ions, in which metallic gold replaces the metallic silver. Use Appendix D to explain the chemistry of this process.

Step-by-Step Solution

Answer

In the photograph, Ag will be the anode and produce $A{g}^{+}$ while Au (II) will be reduced in the cathode, and replace the silver metal in the photograph. This occurs readily because the $E_{\text{cell }}^o$  of the reaction is positive, indicating that it occurs spontaneously.

Step 1: Given information

The given information is when gold (III) ions are mixed with silver metal, the gold replaces the silver - that is, the silver metals are oxidized and turn into silver (I) ions

Step 2: Definition of Coordination Compound

A coordination complex consists of a central atom or ion, which is typically metallic and is termed the coordination centre, and a surrounding array of bound molecules or ions, that are successively referred to as ligands or complexing agents.

Step 3: Solve the given statements

When gold (III) ions are mixed with silver metal, the gold replaces the silver - that is, the silver metals are oxidized and turn into silver (I) ions, while the gold metals are oxidized, and turn from  $A{u}^{3+}$ ions to solid gold metal.

This happens because the  $E_{\text{half }-\text{ cell }}^o$ of $A{u}^{3+}$  is more positive than that of $A{g}^{+}$ .

Value $E_{\text{half - oell }}^o=1.50V$  for Au (III) to Au and value $E_{\text{half - eell }}^o=0.80V$  for Ag (l) to Ag gives :

 $E_{\text{eell }}^o=1.50-0.80=0.70\text{V}$

Since the   potential is less than that of  , it will be oxidized and the reaction will be reversed. In the photograph, Ag will be the anode and produce ${\text{Ag}}^{+}$ while Au(III) will be reduced in the cathode, and replace the silver metal in the photograph.

This occurs readily because the  $E_{\text{cell }}^a$ of the reaction is positive, indicating that it occurs spontaneously.