Consider the following standard electrode potentials.
Zn2+(aq) + 2e– --> Zn(s) (E= –0.76 V)
Cl2(g) + 2e– --> 2Cl–(aq) (E = +1.36 V)
Mg2+(aq) + 2e– --> Mg(s) (E= –2.37 V)
What will happen when zinc powder is added to an aqueous solution of magnesium chloride?
A. No reaction will take place.
B. Chlorine gas will be produced.
C. Magnesium metal will form.
D. Zinc chloride will form.
I thought the answer was B since the standard electrode potential of chlorine is higher than zinc, meaning chlorine is reduced to form chlorine gas. However, the answer is A, can anyone please clarify this? Thanks.
Trouble with Understanding Redox Chemistry?
2018-03-08 6:02 pm
回答 (1)
2018-03-08 9:39 pm
The solution of magnesium chloride contains magnesium ions and chloride ions.
Cl⁻ is a reducing agent, and thus no redox reaction occurs when zinc powder is added because zinc is also a reducing agent.
Consider the reaction between zinc powder and magnesium ions :
Zn(s) + Mg²⁺(aq) → Zn²⁺(aq) + Mg(s) …. E°(cell) = (-2.37) - (-0.76) = -1.61 V < 0 V
As E°(cell) < 0, the above reaction is non-spontaneous and thus does not occur.
Hence, there is no reaction.
The answer: A. No reaction will take place.
Cl⁻ is a reducing agent, and thus no redox reaction occurs when zinc powder is added because zinc is also a reducing agent.
Consider the reaction between zinc powder and magnesium ions :
Zn(s) + Mg²⁺(aq) → Zn²⁺(aq) + Mg(s) …. E°(cell) = (-2.37) - (-0.76) = -1.61 V < 0 V
As E°(cell) < 0, the above reaction is non-spontaneous and thus does not occur.
Hence, there is no reaction.
The answer: A. No reaction will take place.
2018-03-08 11:09 pm
Redox reactions....
How about we set this up another way, where there hypothetical oxidation and reduction half-reactions. For an oxidation half-reaction, the sign of the reduction potential is reversed.
Zn(s) --> Zn2+ + 2e- .............. E = +0.76V
Mg2+ + 2e- --> Mg(s) ............. E = -2.37V
----------------- ------------------ -------------------------
Zn + Mg2+ --> Zn2+ + Mg ...... E = -1.61V
A negative cell potential indicates the reaction as written is not spontaneous (occurs without outside intervention), but the reverse reaction is spontaneous.
The format above is a little easier to remember than "Is it cathode minus anode, or is it anode minus cathode?" Reduction occurs at the cathode, and oxidation occurs at the anode. (A red cat ate an ox.) By the way, it is "E(cathode) - E(anode)".
The result is the same, the cell potential is negative and there is no reaction, but there is less chance for error with writing an explicit oxidation half-reaction and a reduction half-reaction.
How about we set this up another way, where there hypothetical oxidation and reduction half-reactions. For an oxidation half-reaction, the sign of the reduction potential is reversed.
Zn(s) --> Zn2+ + 2e- .............. E = +0.76V
Mg2+ + 2e- --> Mg(s) ............. E = -2.37V
----------------- ------------------ -------------------------
Zn + Mg2+ --> Zn2+ + Mg ...... E = -1.61V
A negative cell potential indicates the reaction as written is not spontaneous (occurs without outside intervention), but the reverse reaction is spontaneous.
The format above is a little easier to remember than "Is it cathode minus anode, or is it anode minus cathode?" Reduction occurs at the cathode, and oxidation occurs at the anode. (A red cat ate an ox.) By the way, it is "E(cathode) - E(anode)".
The result is the same, the cell potential is negative and there is no reaction, but there is less chance for error with writing an explicit oxidation half-reaction and a reduction half-reaction.
收錄日期: 2021-05-01 22:26:31
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20180308100214AAqZwDV