121 Experiment 10 A QUALITATIVE STUDY OF A REDUCTION POTENTI

121 Experiment 10. A QUALITATIVE STUDY OF A REDUCTION POTENTIAL TABLE An oxidation-reduction reaction, or redos reaction, is one in which one or more elements change oxidation number in the course of the reaction. Oxidation is the increase in the oxidation number of an atom as that atom loses electrons. Conversely, reduction is the decrease in the oxidation number of an atom as the atom gains electrons. (See Section 4.9, Zumdahl for a review of oxidation numbers.) The chemical species containing the atom that is oxidized is called the reducing agent. The chemical species that contains the atom that is reduced is called the oxidizing agent A reducing agent always reacts with an oxidizing agent in a redox reaction, with the reducing agent transferring electrons to the oxidizing agent. Consider, for example, the spontaneous reaction of Ni with Cu Ni(s) Cu2.(aq) Ni>(aq) Cu(s) + .- , + Ni is the reducing agent in this reaction because the oxidation number of Ni increases from 0 to +2 as two electrons are transferred to Cu\". Cu is the oxidizing agent because the oxidation number of Cu decreases from +2 to 0 as Cu accepts two electrons from Ni. The transfer of two electrons in this process is more apparent when the reaction is broken up into two half reactions an oxidation half-reaction: and a reduction half-reaction: Ni(s) Ni(aq) 2e If the reaction is considered in reverse, Cu(s) + Ni(aq)Cu (ag) Ni(s) Cu is now the reducing agent and Ni2 is the oxidizing agent. Write the corresponding half-reactions for the reverse reaction here: oxidation: reduction: Only one direction will be favored for the overall reaction. A redox reaction is spontaneous when the reducing agent for the forward reaction is stronger than the reducing agent for the reverse reaction. Thus, for redox reactions, one can envision two reducing agents competing to lose electrons. The stronger reducing agent prevails and the reaction proceeds spontaneously in the direction of the weaker reducing agent. Similarly,

Solution

1.

Cu, I-, Br-

1a.

Cu --> Cu²⁺ + 2e-

2I^- --> I₂ + 2e-

2Br- --> Br₂ + 2e-

1b.

Reduction potential of

Zn < H₂ < Cu < I- < Fe2+ < Br- < Cl-

1c.

Cu + I₂ --> 2 I^- + Cu²⁺

Cu + Br₂ --> 2 Br- + Cu²⁺

1d.

Ag⁺ + Br- --> AgBr (s)

Ag⁺ + I- --> AgI (s)

Cu²⁺ + 4 NH₃ --> Cu(NH₃)₄²⁺