Oxidation-Reduction Chemistry

The chemistry of rusting and of batteries

Oxidation and Reduction
Batteries

General Principles: Remember that chemical change occurs when electrons are exchanged between two substances or when atoms are exchanged and new covalent bonds are formed. In the process being modeled here, metallic zinc (the gray spheres, Zn) react with copper cations (the blue spheres, Cu²⁺). The copper cations are in a solution (normally water). There are also anions present, but they are not illustrated, since they do not take part in this particular reaction. Copper cations have fewer electrons than they do protons in their nuclei. In this reaction, two electrons are lost by a zinc atom and gained by a copper cation. Note that there are two distinct processes:

Process Process Name What Happens

Zn --> Zn²⁺ + 2e^- Oxidation Zinc loses 2 electrons

Cu²⁺ + 2e^- --> Cu Reduction Copper ion gains 2 electrons

Oxidation is defined as the loss of electrons. Zinc is oxidized and becomes zinc ion.
Reduction is defined as gaining electrons. Copper ions gain electrons and are reduced to metallic copper.
Even though there are two distinct processes, they cannot occur without each other because in fact zinc is losing electrons to copper ion.

You will examine this process in

lab by dropping a piece of zinc in a solution containing copper ions. The solution will be blue, reflecting the fact that copper ions interacting with water molecules are blue. Over time, you'll see two changes. The solution will become less blue (because copper ions are being changed to metallic copper). Secondl, the appearance of the metal surface will change and eventually become copper-colored. The reaction happens at the surface between the metal and the solution. In the model here, note that two electrons leave a zinc atom and attach themselves to a copper ion. After that happens, the newly-formed zinc ion floats away and the newly-formed copper atom attaches itself to the surface of the metal.

What causes this process? We won't answer that question except to note that copper ion in direct contact with zinc metal is not stable and a spontaneous reaction occurs. It is interesting that if zinc ion is brought into contact with copper metal, no reaction takes place. Copper ion has a stronger tendency to be reduced than does zinc ion. Or phrased in terms of oxidation, zinc metal has a stronger tendency to be oxidized than copper metal.

The important points;

There is a direct transfer of electrons from atoms of zinc metal to copper ions.
Copper ions gain electrons and are reduced.
Zinc atoms lose electrons and are oxidized

There are many other oxidation and reduction reactions. In the formation of rust, for instance, atoms of iron metal (Fe) lose electrons. Fe³⁺ cations are the product of this oxidation. The electrons are gained by oxygen molecules from the air. O₂ molecules are reduced to the oxide anion, O^2-. This reaction requires the presence of water and is greatly accelerated by the presence of other non-reacting electrolytes, like salt. The product is a hydrated iron oxide.

4 Fe + 3 O₂ + 6 H₂O --> 2 Fe₂O₃^.6H₂O

Since the iron oxide is an ionic compound, the formula Fe₂O₃^.6H₂O means that there are two ions of Fe³⁺ for every three O^2- ions. The "^.6H₂O" part of the formula indicates that there are six water molecules hydrating the structure for each two iron and three oxide ions. In fact, the number of water molecules is not always six so it would be more correct to write Fe₂O₃^.xH₂O.

Batteries

What produces the electrical energy in a battery? An oxidation-reduction reaction! The only thing that makes an oxidation-reduction reaction in a battery different from simply placing a piece of zinc, for instance, into a copper ion solution, is that the oxidation reaction is separated in space from the reduction reaction. The reactants are not allowed to come into direct contact.

ANODE CATHODE

Zn-->Zn²⁺+ 2e^- Cu²⁺+2e^--->Cu

Zinc oxidized Cu²⁺ reduced

Zn²⁺ions -----> Cu²⁺ ions---->

<----SO₄^2- ions <----SO₄^2- ions

In the battery illustrated here, there are two compartments separated by a porous clay or class partition.

In the left compartment, a piece of zinc metal (colored gray) is placed in a colorless solution of zinc sulfate . Zinc sulfate is an ionic compound, so the solution really contains Zn²⁺ and SO₄2^-ions. The zinc metal is an electrode, specifically, it is called the anode because oxidation occurs at its surface.
In the right compartment, a piece of copper metal is placed in a solution of copper sulfate. That solution, which is colored blue, contains Cu²⁺ and SO₄2^-ions. The copper electrode is called a cathode because reduction takes place at its surface.

The porous partition prevents liquids from flowing back and forth. It keeps the reactants from coming into direct contact. It is important to realize, though, that ions can move through the pores.

Finally, a wire connects the two pieces of metals. Remember that for electricity to flow, a complete "circuit" must exist. Since the wire can conduct electrons (electronic conduction) and ions can move through the two solution and the partition (electrolytic or ionic conduction), such a complete circuit does, in fact, exist.

In the left compartment, the metallic zinc strip is gradually oxidized. The products of this process are:

Zn²⁺ ions that diffuse away from the anode into the solution.
Two electrons that move away through the wire toward the right compartment, creating an electrical current.

In the right compartment,

Cu²⁺ ions diffuse up to the surface of the cathode and
each ion receives two electrons that have arrived through the wire, producing
atoms of Cu that deposit onto the surface of the copper electrode.

The anode is negatively charged, since electrons are created there as zinc atoms are oxidized. The cathode, where copper ions "remove" electrons, has a slight negative charge. [The negative sign on one "terminal" of lead-acid storage batteries used in cars and photovoltaic systems for the home indicates the anode, where oxidation is occurring.] Notice that cations move from the left (where cations are produced), through the partition, to the right, where cations are consumed. Negatively charged SO₄2^-anions move in the opposite direction so that electrical neutrality is preserved everywhere.

Why does all this occur? For exactly the same (unexplained) reason stated in the oxidation-reduction section.

Direct Contact	Battery
Copper ions in direct contact with zinc metal are not stable and a spontaneous reaction occurs. Electrons are directly transferred from the zinc metal to copper ions.	Copper ions in indirect contact with the zinc electrode are not stable and a spontaneous reaction occurs. Electrons flow from the zinc metal electrode to the anode where they are passed to copper ions. The circuit is completed by electrolytic conduction.

Copper ions have a greater tendancy to be reduced than zinc ions. (Zinc atoms have a greater tendancy to be oxidized than copper ions). The voltage of the battery (in this case about 1.10 volts) is directly related to this differing reduction tendancy. In other batteries, larger or smaller voltages relate to larger or smaller differences in tendancy.

Is your battery run down? The tendancies to be reduced depend on concentrations of the ions. The smaller the concentration of an ion in a cell, the smaller the tendancy to be reduced, and the lower the observed voltage. So as the reaction proceeds, copper ions get used up, their concentration falls, and the voltage decreases. If the voltage decreases all the way to zero, no further reaction occurs and no electrons flow. Dead battery!

Here's a few redox practice questions to try.

RETURN

Process	Process Name	What Happens
Zn --> Zn²⁺ + 2e^-	Oxidation	Zinc loses 2 electrons
Cu²⁺ + 2e^- --> Cu	Reduction	Copper ion gains 2 electrons