Standard reduction potential and half reactions

In summary, the question is asking whether a redox reaction will occur when lead metal (Pb) is added to a 1.0-M solution of NiCl2 at 25°C. The standard reduction potentials for Pb and Cl2 are given as -0.13V and +1.36V, respectively. The student used the equation Ecell = Ecathode - Eanode to determine that the reaction should occur, but the answer in the back of the book says "no reaction occurs". The student is questioning whether they should be using the standard reduction potential for Ni instead of Cl2. The expert summarizer explains that the half reaction with the greatest reduction potential will occur, and in this case, E^o
  • #1
leroyjenkens
616
49

Homework Statement



Determine what redox reaction, if any, occurs (at 25° C) when lead metal (Pb) is added to 1.0-M solution of NiCl2

Homework Equations



Ecell = Ecathode- Eanode

Here is standard reduction potentials given for Pb and Cl

Pb2+(aq) + 2e- → Pb(s) E°(V) = -0.13

Cl2(g) 2e- → 2Cl-(aq) E°(V) = +1.36

The Attempt at a Solution



According to my book, the half reaction with the greater reduction potential is the one that is the cathode, so that must be +1.36. So using the equation above, the 1.36 - (-0.13) = 1.49, which is a positive number. According to my book, that reaction should occur. But the answer in the back of the book says "no reaction occurs". I don't understand.

Am I supposed to be using the Nickel in the reaction instead of the Chlorine?

The standard reduction potential they give for Nickel is:

Ni2+(aq) + 2e- → Ni(s) E°(V) = -0.25Thanks.

Edit: The next question has me adding Pb to HCl, but in the answers, it shows that the Hydrogen is being used in the equation instead of the Cl. I really don't understand what's going on. How do I know which one to use?
 
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  • #2
The half reaction with the greatest reduction potential will get ectrons and become metal.

but [tex]E^o(Pb) > E^ô(Ni)[/tex], implies no reaction occurs
====
no reaction occurs,
[tex]Pb + Ni^{2+}=/=>Pb^{2+}+Ni[/tex]but the opposite reaction occurs

[tex]Ni + Pb^{2+}==>Ni^{2+}+Pb[/tex]

because
[tex]E^o(Pb) > E^ô(Ni)[/tex],
 
  • #3
Thanks for the response.

I'm given a solution of NiCl2. How do I know to check the reduction potential of Nickel instead of Chlorine?

The worked example they give had them add molecular bromine (Br2) to a solution of NaI, and they ignored the Na and checked the reduction potential of the I. They had another one where they put Br2 in NaCl, and they check the reduction potential of the Cl. But with my problem, I'm putting Pb in NiCl2, and now, unlike the worked example, I ignore the Cl and check the reduction potential of the Ni? How do I know which one to check?

Thanks.
 

FAQ: Standard reduction potential and half reactions

1. What is the significance of standard reduction potential and how is it measured?

Standard reduction potential (E°) is a measure of the tendency of a chemical species to gain electrons and undergo reduction. It is measured by comparing the potential of a half reaction against a standard hydrogen electrode (SHE), which is assigned a potential of 0 volts.

2. How is the standard reduction potential related to the spontaneity of a reaction?

The more positive the standard reduction potential, the greater the tendency for a species to undergo reduction and the more likely the reaction is to be spontaneous. This means that reactions with a higher standard reduction potential are more likely to occur without the need for an external energy source.

3. How does the concentration of a species affect its standard reduction potential?

The standard reduction potential of a species is dependent on its concentration. As the concentration increases, the standard reduction potential becomes more positive, indicating a greater tendency for reduction to occur. This relationship is described by the Nernst equation: E = E° - (RT/nF)ln(Q), where E is the standard reduction potential, E° is the standard reduction potential at 1 M concentration, R is the gas constant, T is the temperature in Kelvin, n is the number of electrons transferred, F is Faraday's constant, and Q is the reaction quotient.

4. Can the standard reduction potential of a species change under different conditions?

Yes, the standard reduction potential of a species can change under different conditions such as temperature, pressure, and concentration. This is because the standard reduction potential is a thermodynamic property and is affected by changes in these conditions.

5. How is the concept of half reactions used in determining standard reduction potentials?

Half reactions are used to split a redox reaction into two separate reactions, one for oxidation and one for reduction. By comparing the standard reduction potentials of these half reactions, we can determine which species is more likely to undergo reduction and therefore which reaction is more spontaneous. The overall standard reduction potential of the redox reaction is then calculated by subtracting the standard oxidation potential from the standard reduction potential.

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