Electrochemistry Logic/Intuition

[sshah92]

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Hey guys,

These are a couple of questions from the past few practice exams I took where I either guessed the answer correctly or just blindly guessed. Since we're not given reductive potentials on the MCAT, I was wondering how to answer these correctly. Thank you!

The standard potential for the reaction
Zn2+ + 2e- -> Zn
equals -0.763 V. When a strip of Zn is placed in a 0.1 M solution of HCl (aq), does the zinc strip begin to dissolve?

When a strip of Cu is placed into H20(l), no change is observed. However, when a strip of Cu is placed into a solution of HNO3+, a gas evolves. What is the most likely identity of the gas?

 

[Cmdr_Shepard]

One of the most important electrochemistry ideas you must know is that, in most cases, a metal plus acid will generate a metal salt plus hydrogen gas.

For the Zn question, if the reduction potential is negative, then the reverse (conversion of Zn metal to Zn ion) will have a positive potential, and therefore will be spontaneous in acid. This is because the reduction of acid to hydrogen gas (defined as having an E= 0 V) can be driven by any reaction that has a negative reduction potential. In this case, +0.763 V is more than enough to generate the gas.

For the second, you can tell the reaction is a redox reaction since there's no reaction in water, but there is a gas formed in a strong acid (tons of H+ in solution). And since the dissolution of Cu must be accompanied by a reduction, it's going to be the acid being reduced to gas.

 

[sshah92]

Beautiful. Thank you!!