Course Outline


RedOx Reactions



 

You should be able to recognize when an element has been oxidized or reduced in a chemical reaction.  Oxidation can be defined as the loss of electrons or combination with oxygen.  Most metals react with acids to give hydrogen gas; the activity series ranks metals according to their ease of oxidation; metals can be oxidized by metal ions lower in the activity series; this property of metals can be used to generate electricity using electrochemical cells.

Reaction of Metals with Acids

Some metals, zinc for example, dissolve in acidic solutions to give salts and hydrogen gas:


Zn (s)  +  H2SO4 (aq)

ZnSO4 (aq) +  H2 (g)

We can describe this as a single replacement reaction, in which one element (zinc) has displaced another element (hydrogen) from its compound (in this case, the sulfate).

In contrast to the neutralization and precipitation reactions we have looked at up to this point, in which the metallic element(s) existed as ions on both sides of the equation, in this reaction the metal exists as atoms on one side and as ions on the other side of the equation.  Metal atoms become cations by giving up electrons, and as we have described previously this process is called oxidation.  If we write total and net ionic equations for this reaction, we can see where the electrons go:


Zn (s)  +  2 H+ (aq)  +  SO42- (aq)

Zn2+ (aq)  +  SO42- (aq) +  H2 (g)

Zn (s)  +  2 H+ (aq)

Zn2+ (aq)  +  H2 (g)

We see that sulfate ion is the same on both sides, a spectator ion, so it cancels out from the total ionic equation.  From the net ionic equation we see that two hydrogen ions combine with two electrons to form a hydrogen molecule.  Recall that gain of electrons is called reduction; hydrogen ions have been reduced to elemental hydrogen.  Reactions of this type, in which electrons are transferred, are classified as redox reactions, and always include at least one species that is reduced and another that is oxidized.

Some metals are more easily oxidized than others.  Sodium and calcium, for example, give up electrons so easily that they react violently with pure, cold water:


2 Na (s)  +  2 H2O

2 NaOH (aq) +  H2 (g)

Other metals, gold for example, are so unreactive that they do not dissolve even in concentrated sulfuric or hydrochloric acid.  Most metals have reactivity somewhere between these extremes.  Iron, for example, dissolves readily in dilute hydrochloric acid, producing a gentle stream of hydrogen gas which can be used for other reactions in the laboratory:


Fe (s)  +  2 HCl (aq)

FeCl2 (aq) +  H2 (g)

The Activity Series

A table of metallic elements listed in order of decreasing ease of oxidation is called an activity series, and is useful for predicting when certain types of reaction might occur.  In the table below, elements to the left are most easily oxidized while elements to the right are most difficult to oxidize.  Hydrogen is included in the table to separate those metals that will not react with acids (the noble metals) from those that will.

K > Ca > Na > Mg > Al > Zn > Cr > Fe > Ni > Sn > Pb > (H) > Cu > Ag > Hg > Au

The activity series can be used to predict when a metal will displace another metal from one of its compounds in aqueous solution.  For example, copper metal reacts with silver nitrate in aqueous solution:


Cu (s)  +  2 AgNO3 (aq)

Cu(NO3)2 (aq) +  2 Ag (s)

Cu (s)  +  2 Ag+ (aq)

Cu2+ (aq) +  2 Ag (s)

The net ionic equation shows us that copper atoms, the more easily oxidized metal, lose electrons and silver ions gain electrons.  Metallic silver can be seen to precipitate as the originally colorless solution turns blue due to the presence of dissolved copper(II) ion.

In this reaction we can think of copper being oxidized by silver ion, so silver ion is the oxidizing agent.  Alternatively we can think of silver ion being reduced by copper metal, so copper is the reducing agent.

If we attempt to do a similar reaction by adding copper metal to zinc nitrate solution, we find that no reaction takes place.  Zinc metal is more easily oxidized than copper, so zinc ions cannot oxidize copper metal.  On the other hand if we were to add zinc metal to copper nitrate solution, a reaction would take place:

Zn (s)  +  Cu(NO3)2 (aq)

Zn(NO3)2 (aq) +  Cu (s)

Electrochemistry

The different tendencies of metallic elements to give up electrons is used in batteries, where the electrons can be made to flow around a circuit and do electrical work. The first battery, a voltaic pile or stack of galvanic cells, was made in 1800 by Alessandro Volta, building on initial observations made twenty years earlier by Luigi Galvani.

 

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