Cracking the Sat Chemistry Subject Test 2013-2014 Edition by The Princeton Review

Author:The Princeton Review [Review, The Princeton]
Language: eng
Format: epub
Tags: Itzy, Kickass.to
ISBN: 9780307945761
Amazon: B00C4BA5US
Publisher: Princeton Review
Published: 2013-04-16T04:00:00+00:00

Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)

Here are the half-reactions and E0 values.

Oxidation: Zn(s) → Zn2+(aq) + 2e−; E0ox = 0.76 V

Reduction: Cu2+ (aq) + 2e− → Cu(s); E0red = 0.34 V

For the above reaction, E0cell is 0.76 V + 0.34 V = 1.10 V. Since E0cell > 0, this redox reaction is spontaneous, which means that we can turn the energy that this reaction releases into electrical energy. But how?

If we simply drop some zinc metal into an aqueous solution of Cu2+ ion (say, a water solution of Cu(NO3)2, this spontaneous redox reaction will happen. Electrons will leave the zinc metal as it is oxidized and be captured by the Cu2+ ions, which will be reduced. All of this will happen, yet we will get no electrical energy out of the reaction. Why not? Because the electrons will immediately travel from the zinc atoms to the Cu2+ ions. Electrical energy involves the flow of electrons, so if we want to derive electrical energy from this reaction, we must force the electrons that leave zinc to flow through a wire in order to reach Cu2+. This is precisely what the electrochemical cell does. In an electrochemical cell, the oxidation and reduction half-reactions occur in separate vessels. Each vessel contains an aqueous solution containing the ions participating (as reactants or products) in the reaction. For example, we can use Zn(NO3)2(aq) and Cu(NO3)2(aq) for the redox reaction shown on the previous page. A strip of metal called an electrode is placed in each solution; each electrode is made out of a metal that also participates in the reaction. The electrode in the vessel in which oxidation occurs is called the anode. The cathode is the electrode in the vessel in which reduction takes place. Each electrode is made out of a metal that also participates in the reaction. In our electrochemical cell, the anode is a strip of zinc metal, and the cathode is a strip of copper metal.

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