In lab, we took $\ce{CuCl_2(aq)}$ and poured it into a cup. Then we took some aluminum foil and put it inside and started to stir. I could see the aluminum getting eaten by the copper because it was a better reducing agent. However, when I put silver instead of aluminum, nothing happened. I know it's because the standard cell potential of silver is greater than that of copper, but why didn't the copper act as a reducing agent?
$\begingroup$
$\endgroup$
2
-
7$\begingroup$ What would the copper reduce? $\endgroup$– Buck Thorn ♦Feb 7 at 8:56
-
$\begingroup$ Cu++ from CuCl2 has already given its electrons to 2 Cl-. Aluminum metal is the reducing agent, giving them back to Cu++ --> Cu metal. Similarly, Copper metal would reduce Ag+. $\endgroup$– Robert DiGiovanniFeb 8 at 13:26
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
Copper is already oxidized as $\ce{Cu^2+(aq)}$ and silver reduced as $\ce{Ag(s)}$. Copper would act as a reducing agent in $\ce{Cu(s)}$ and $\ce{AgNO3(aq)}$ scenario.
Using $\ce{CuCl2}$ instead of $\ce{CuSO4}$ is a mistake, if $\ce{Ag}$ is involved, as forming insoluble $\ce{AgCl(s)}$ would complicate things. Eventually, $\ce{Cl-(aq)}$ may keep $\ce{Ag/Ag+}$ potential below or near $\ce{Cu/Cu^2+}$ potential:
$$E = E_{\ce{Ag/Ag+}}^\circ + 0.059 \log{[\ce{Ag+}]} = E_{\ce{Ag/Ag+}}^\circ + 0.059 \log{\frac{K_{\mathrm{s},\ce{AgCl}}}{[\ce{Cl-}]}}$$ (simplified replacing activities by concentrations)
But, forming $\ce{AgCl(s)}$ layer would freeze it kinetically.
$\begingroup$
$\endgroup$
The answer lies in the redox potential chart:
Reactions:
$\ce{2 Alumimum metal + 3 Cu++ -> 2 Al+++ + 3 Copper metal}$
$\ce{2 Silver metal + Cu++ -> 2 Ag+ + Copper metal no reaction}$
Drawing the half reactions:
$\ce{Al+++ -> Aluminum metal - 1.66 V}$
$\ce{Cu++ -> Copper metal + 0.34 V}$
$\ce{Ag+ -> Silver metal + 0.80 V}$
We can see why Copper ions react with Aluminum metal but not Silver metal. Silver metal has a higher voltage potential to keep its electrons than both Copper and Aluminum metals. Aluminum metal (and Sodium) are strong electron donors.
In this case Copper metal sits right between Silver and Aluminum metal. Copper metal will give electrons to Silver ions, and Copper ions will take electrons from Aluminum metal. Silver metal and Copper ions will not react, nor will Silver metal and Aluminum ions.
