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.
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.
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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