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Which two substances react spontaneously?

(A) $\ce{Ag}$ and $\ce{Cu}$

(B) $\ce{Ag+}$ and $\ce{Cu}$

(C) $\ce{Ag}$ and $\ce{Cu^2+}$

(D) $\ce{Ag+}$ and $\ce{Cu^2+}$

$\tiny{\text{Taken from 2012 Chem Olympiad}}$

The given answer is B, yet I am unclear as to why. I believe it has something to do with reduction potentials/redox or the gibbs free energy. Can someone please explain?

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  • $\begingroup$ Do you have data regarding the reduction potentials of these species and elementary idea about what these values suggest? This can only be solved with known reduction potential data. $\endgroup$ – Satwik Pasani Mar 12 '14 at 4:55
  • $\begingroup$ This was from the 2012 chem olympiad and they do not provide a table of reduction potentials (and you obviously can't bring your own :P). So in the absence of that info how would you solve it? $\endgroup$ – 1110101001 Mar 12 '14 at 6:01
  • $\begingroup$ You then need an idea about their reduction potentials, or equivalently, their relative position in the electrochemical/activity series. $\endgroup$ – Satwik Pasani Mar 12 '14 at 9:48
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From the faint outline of electrochemical series I remember $Cu$ is above $Ag$. The substance which is above is able to displace the substance below from its aqueous solution(obviously lower substance is in ionic form).

Hence $(B)$ is true.

We were made to learn this outline in lower class :

K Na Ca Mg Al Zn Fe Pb H Cu Hg Ag Au Pt

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  • $\begingroup$ By displace you mean act as a reducing agent? $\endgroup$ – 1110101001 Mar 12 '14 at 18:16
  • $\begingroup$ Yep...btw kids are taught in the way of simple displacement reaction. In higher classes, they introduce the electrode potentials and other stuff. Like : $Cu + AgNO_3$ ... $\endgroup$ – evil999man Mar 13 '14 at 4:28

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