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Ch.20 - Electrochemistry
Chapter 20, Problem 99

Predict whether the following reactions will be spontaneous in acidic solution under standard conditions: (a) oxidation of Sn to Sn2+ by I2 (to form I-), (b) reduction of Ni2+ to Ni by I- (to form I2), (c) reduction of Ce4+ to Ce3+ by H2O2, (d) reduction of Cu2+ to Cu by Sn2+ (to form Sn4+).

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Identify the half-reactions involved in the process. For the reduction of Ce^{4+} to Ce^{3+}, the half-reaction is: Ce^{4+} + e^- \rightarrow Ce^{3+}. For the oxidation of H_2O_2, the half-reaction is: H_2O_2 \rightarrow O_2 + 2H^+ + 2e^-.
Look up the standard reduction potentials (E^\circ) for each half-reaction. The standard reduction potential for Ce^{4+} to Ce^{3+} is E^\circ_{Ce^{4+}/Ce^{3+}}. The standard reduction potential for the reverse of the oxidation of H_2O_2 (i.e., O_2 to H_2O_2) is E^\circ_{O_2/H_2O_2}.
Calculate the standard cell potential (E^\circ_{cell}) by using the formula: E^\circ_{cell} = E^\circ_{reduction} - E^\circ_{oxidation}. Here, E^\circ_{reduction} is the potential for Ce^{4+}/Ce^{3+} and E^\circ_{oxidation} is the potential for H_2O_2/O_2.
Determine the spontaneity of the reaction. If E^\circ_{cell} > 0, the reaction is spontaneous under standard conditions. If E^\circ_{cell} < 0, the reaction is non-spontaneous.
Consider the effect of acidic conditions on the reaction. Since the problem specifies acidic solution, ensure that the half-reactions are balanced with respect to H^+ ions and electrons.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Spontaneity of Reactions

A reaction is considered spontaneous if it occurs without external intervention, typically determined by the change in Gibbs free energy (ΔG). If ΔG is negative, the reaction is spontaneous under the given conditions. Understanding spontaneity involves evaluating thermodynamic parameters and the reaction's enthalpy and entropy changes.
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Standard Electrode Potentials

Standard electrode potentials (E°) are measured under standard conditions and indicate the tendency of a species to be reduced or oxidized. A higher E° value suggests a greater likelihood of reduction. For redox reactions, comparing the E° values of the involved half-reactions helps predict the spontaneity of the overall reaction.
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Acidic Solution Effects

In an acidic solution, the concentration of hydrogen ions (H+) affects the equilibrium of redox reactions. The presence of H+ can shift the reaction's position, influencing the reduction potential of certain species. This is particularly relevant for reactions involving transition metals or complex ions, where pH can significantly alter the reaction dynamics.
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Related Practice
Textbook Question
A mixture of copper and gold metals that is subjected to electrorefining contains tellurium as an impurity. The standard reduction potential between tellurium and its lowest common oxidation state, Te4+, is Te4+1aq2 + 4 e- ¡ Te1s2 E°red = 0.57 V Given this information, describe the probable fate of tellurium impurities during electrorefining. Do the impurities fall to the bottom of the refining bath, unchanged, as copper is oxidized, or do they go into solution as ions? If they go into solution, do they plate out on the cathode?
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Textbook Question

A disproportionation reaction is an oxidation–reduction reaction in which the same substance is oxidized and reduced. Complete and balance the following disproportionation reactions:

(b) MnO42-(aq) → MnO4-(aq) + MnO2(s) (acidic solution)

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Open Question
A common shorthand way to represent a voltaic cell is anode | anode solution || cathode solution | cathode. A double vertical line represents a salt bridge or a porous barrier. A single vertical line represents a change in phase, such as from solid to solution. (a) Write the half-reactions and overall cell reaction represented by Fe | Fe2+ || Ag+ | Ag; calculate the standard cell emf using data in Appendix E. (b) Write the half-reactions and overall cell reaction represented by Zn | Zn2+ || H+ | H2; calculate the standard cell emf using data in Appendix E and use Pt for the hydrogen electrode. (c) Using the notation just described, represent a cell based on the following reaction: ClO3^-_(aq) + 3 Cu_(s) + 6 H+_(aq) -> Cl^-_(aq) + 3 Cu2+_(aq) + 3 H2O_(l); Pt is used as an inert electrode in contact with the ClO3^- and Cl^-. Calculate the standard cell emf given: ClO3^-_(aq) + 6 H+_(aq) + 6 e^- -> Cl^-_(aq) + 3 H2O_(l); E° = 1.45 V.
Textbook Question

Gold exists in two common positive oxidation states, +1 and +3. The standard reduction potentials for these oxidation states are Au+1aq2 + e- ¡ Au1s2 Ered ° = +1.69 V Au3+1aq2 + 3 e- ¡ Au1s2 Ered ° = +1.50 V (c) Miners obtain gold by soaking gold-containing ores in an aqueous solution of sodium cyanide. A very soluble complex ion of gold forms in the aqueous solution because of the redox reaction 4 Au1s2 + 8 NaCN1aq2 + 2 H2O1l2 + O21g2 ¡ 4 Na3Au1CN2241aq2 + 4 NaOH1aq2 What is being oxidized, and what is being reduced in this reaction?

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Open Question
A voltaic cell is constructed from an Ni2+(aq) / Ni(s) half-cell and an Ag+(aq) / Ag(s) half-cell. The initial concentration of Ni2+(aq) in the Ni2+ - Ni half-cell is [Ni2+] = 0.0100 M. The initial cell voltage is +1.12 V. (a) By using data in Appendix E, calculate the standard emf of this voltaic cell.
Open Question
Will the concentration of Ni2+ in the Ni2+ - Ni half-cell increase or decrease as the cell operates?