19.2.4 Using Standard Reduction Potentials

Question 1

Using Standard Reduction Potentials

Answer 1

• Standard reduction potentials (E ̊) describe the relative ability of a substance to be reduced.
• Standard reduction potentials can be compared to predict the spontaneity of a reaction.
• Trends in standard reduction potentials can be explained based on properties such as the relative stability of ions, atoms, and molecules.

Question 2

note

Answer 2

• Standard reduction potentials (E ̊) describe the relative ability of a substance to be reduced.
• Substances with high standard reduction potentials are easily reduced (are good oxidants).
• If a substance has a low standard reduction potential, its conjugate is easily oxidized (is a good reductant). For example, iron cation (Fe 2+ ) has a low standard reduction potential, and is therefore a weak oxidant. Therefore, iron metal (Fe) is a strong reductant.
• Standard reduction potentials (E ̊) can be used to predict the spontaneity of a reaction.
• First, a given reaction must be broken down into
  half-reactions. One of the half-reactions will be an
  oxidation, rather than a reduction. This reaction will have to be reversed in the balanced equation.
• For example, Zn 0 -> Zn 2+ + 2e – is the reverse of
  Zn 2+ + 2e – -> Zn 0 . Since this is now an oxidation, the standard reduction potential (E ̊) must be multiplied by –1 to give the standard oxidation potential.
• Finally, adding this potential to the standard reduction potential for the Ni 2+ half-reaction gives the cell potential of 0.532 V.
• Since this number is positive, the forward reaction is
  spontaneous. If this number had been negative, the reverse reaction would have been spontaneous.
• Trends in standard reduction potentials can be explained based on properties such as the relative stability of atoms, ions, and molecules.
• Electronegative elements are stronger oxidants.
• Metals with full d orbitals are weaker oxidants than metals with partially filled d orbitals.
• Metals with large nuclei are stronger oxidants than metals with small nuclei.
• Many ionic compounds are weaker oxidants than the
  equivalent metal cation.

Question 3

Examine the standard reduction potential for the following half-reactions:

Li+ + e- -> Li E = -3.05V
Na+ + e- -> Na E = -2.71 V

Why is the lithium cation a weaker oxidant than the sodium cation?

Answer 3

Li+ is more stable in aqueous solution than Na+.

Question 4

Study the series of the standard aqueous electrode potentials at 25°C. Which is the strongest reducing agent in the group?

Answer 4

Lithium

Question 5

Using the table below, place the following in order of decreasing strength as oxidizing agents:

Answer 5

F2 > ClO2 > AgCl > Fe2+

Question 6

Examine the standard reduction potential for the following half-reactions:

AgCl + e- -> Ag + Cl- E = +0.22 V
Ag+ + e- -> Ag E = +0.80 V

Why is the silver chloride a weaker oxidant than the Ag+ cation?

Answer 6

It takes energy to break the AgCl bond

Question 7

Examine the standard reduction potential for the following half-reactions:

Ni2+ + 2e- -> Ni E = -0.23 V
Zn2+ + 2e- -> Zn E = -0.76 V

Why is the zinc cation a weaker oxidant than the nickel cation?

Answer 7

The nickel cation has room in the d shell for extra electrons.

Question 8

The standard reduction potential (E) for the Pb2+ cation in the reduction half-reaction

Pb2+ + 2e- -> Pb is -0.126 V,

and the standard reduction potential (e) for the Fe2+ cation in the reduction half-reaction

Fe2+ + 2e- -> Fe is -0.44 V

From this information, we can conclude that:

Answer 8

Lead metal is a weaker reducing agent than iron metal

Question 9

All of the following can be significant factors in explaining variations in standard reduction potentials among elements except:

Answer 9

Coefficients used to balance the electron transfer

Question 10

Which of the following statements best explains why fluorine is such a strong oxidant and lithium is such a weak oxidant?

Answer 10

F is highly electronegative and Li is not very electronegative.

Question 11

The standard reduction potential (E) for the Ag+ cation in the reduction half-reaction

Ag+ + e- -> Ag is +0.7994 V,

and the standard reduction potential (e) for the Cu2+ cation in the reduction half-reaction

Cu2+ + 2e- -> Cu is +0.337 V

From this information, we can conclude that:

Answer 11

Silver is a better oxidizing agent than copper.

Question 12

Which of the following equations describes a complete reaction?

Answer 12

Cu2+ + Zn -> Cu + Zn2+