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Direct Redox Reactions

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Direct Redox Reactions
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Gold and silver are often used for jewelry because they are very unreactive

 How much for that necklace?

Gold and silver are widely used metals for making jewelry. One of the reasons these metals are employed for this purpose is that they are very unreactive. They do not react in contact with most other metals, so they are more likely to stay intact under challenging conditions. Who wants their favorite piece of jewelry to fall apart on them?

Direct Redox Reactions

When a strip of zinc metal is placed into a blue solution of copper(II) sulfate ( Figure below ), a reaction immediately begins as the zinc strip begins to darken. If left in the solution for a longer period of time, the zinc will gradually decay due to oxidation to zinc ions. At the same time, the copper(II) ions from the solution are reduced to copper metal (see Figure below ), which causes the blue copper(II) sulfate solution to become colorless.

Copper sulfate is initially a blue solution

Copper sulfate solution.

The reaction of zinc with copper sulfate is a redox reaction

Reaction of zinc metal in copper sulfate solution.

The process that occurs in this redox reaction is shown below as two separate half-reactions, which can then be combined into the full redox reaction.

& \text{Oxidation}:  \quad \quad \ \text{Zn}(s) \rightarrow \text{Zn}^{2+} (aq) + 2e^- \\&  \underline{\text{Reduction}: \quad \ \quad \text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s) \qquad \qquad \qquad \ }\\&\text{Full Reaction}: \quad \text{Zn}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{Cu}(s)

Why does this reaction occur spontaneously? The activity series is a listing of elements in descending order of reactivity. An element that is higher in the activity series is capable of displacing an element that is lower on the series in a single-replacement reaction. This series also lists elements in order of ease of oxidation. The elements at the top are the easiest to oxidize, while those at the bottom are the most difficult to oxidize. The Table below shows the activity series together with each element’s oxidation half-reaction.

Activity Series of Metals
Element Oxidation Half Reaction
Most active or most easily oxidized Lithium Li( s ) → Li + ( aq ) + e -
Potassium K( s ) → K + ( aq ) + e -
Barium Ba( s ) → Ba 2+ ( aq ) + 2e -
Calcium Ca( s ) → Ca 2+ ( aq ) + 2e -
Sodium Na( s ) → Na + ( aq ) + e -
Magnesium Mg( s ) → Mg 2+ ( aq ) + 2e -
Aluminum Al( s ) → Al 3+ ( aq ) + 3e -
Zinc Zn( s ) → Zn 2+ ( aq ) + 2e -
Iron Fe( s ) → Fe 2+ ( aq ) + 2e -
Nickel Ni( s ) → Ni 2+ ( aq ) + 2e -
Tin Sn( s ) → Sn 2+ ( aq ) + 2e -
Lead Pb( s ) → Pb 2+ ( aq ) + 2e -
Hydrogen H 2 ( g ) → 2H + ( aq ) + 2e -
Copper Cu( s ) → Cu 2+ ( aq ) + 2e -
Mercury Hg( l ) → Hg 2+ ( aq ) + 2e -
Silver Ag( s ) → Ag + ( aq ) + e -
Platinum Pt( s ) → Pt 2+ ( aq ) + 2e -
Least active or most difficult to oxidiz Gold Au( s ) → Au 3+ ( aq ) + 3e -

Notice that zinc is listed above copper on the activity series, which means that zinc is more easily oxidized than copper. That is why copper(II) ions can act as an oxidizing agent when put into contact with zinc metal. Ions of any metal that is below zinc, such as lead or silver, would oxidize the zinc in a similar reaction. These types of reactions are called direct redox reactions because the electrons flow directly from the atoms of one metal to the cations of the other metal. However, no reaction will occur if a strip of copper metal is placed into a solution of zinc ions, because the zinc ions are not able to oxidize the copper. In other words, such a reaction is nonspontaneous.

Summary

  • The activity series of metal reactivities is given.
  • Parameters for spontaneous reactions between metals are described.

Practice

Questions

Watch the video at the link below and answer the following questions:

http://www.youtube.com/watch?v=2MawIDT5DFU

  1. What happened when Mg and Zn were placed in the Pb 2+ solution?
  2. Did the Zn strip react in the Mg 2+ solution?
  3. How was Ag shown to be least reactive?

Review

Questions

  1. What metals are high in the activity series?
  2. What metals are low in the activity series?
  3. Is tin easier to oxidize than magnesium?

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