# Which two substances react spontaneously?

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?

• 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. – Satwik Pasani Mar 12 '14 at 4:55
• 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? – 1110101001 Mar 12 '14 at 6:01
• You then need an idea about their reduction potentials, or equivalently, their relative position in the electrochemical/activity series. – Satwik Pasani Mar 12 '14 at 9:48

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.
• Yep...btw kids are taught in the way of simple displacement reaction. In higher classes, they introduce the electrode potentials and other stuff. Like : $Cu + AgNO_3$ ... – evil999man Mar 13 '14 at 4:28