THINK ABOUT IT

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With all the heat and light it gives off, fire is an obvious source of energy. Less obvious is the possible use of other exothermic chemical reactions as energy sources. Because the oxidation of metals is exothermic, it may be possible to use oxidation reactions to generate power.

How much energy is transferred during oxidation of metals?

To answer this question, you will explore

EXPLORING THE TOPIC

Heat of Formation

The oxidation of most metals is exothermic, just like the combustion of fuels. So, oxidation reactions are a potential source of energy for our daily lives. To explore the usefulness of metal oxidation as an energy source, we must examine and compare the amount of thermal energy transferred during these reactions.

COPPER VERSUS ZINC

Copper metal and zinc metal react with oxygen to form copper (II) oxide, CuO, and zinc oxide, ZnO.

Notice that ΔH is negative for both reactions. As you recall, this indicates that the reactions are exothermic and heat is transferred to the surroundings. Also notice that in both cases the heat of reaction is associated with two moles of metal oxide being produced. So, the formation of one mole of each oxide results in the release of half as much energy.

The energy released when one mole of a compound is formed from elements is called the heat of formation, or ΔH_f. Compare the heats of formation of copper (II) oxide, CuO, and zinc oxide, ZnO:

Heats of formation of copper (II) oxide: ΔH_f (CuO) = –157kJ/mol

Heats of formation of zinc oxide: ΔH_f (ZnO) = –348kJ/mol

These values indicate how much heat transfer to expect when one mole of each compound is formed from its elements. The formation of 1 mol of zinc oxide releases over twice as much energy as the formation of 1 mol of copper (II) oxide. You can find heats of formation values of some metal oxides in the table below. These values are determined under standard conditions, at 1 atm and 25 °C. Note that standard conditions are different from STP, standard temperature and pressure, which is 1 atm and 0 °C.

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Reversing Oxidation

When an oxidation reaction for a metal is reversed, the resulting products are a pure metal and oxygen gas. This reaction is a decomposition reaction. The metal oxide decomposes and one of the products is a metal element. The decomposition of a metal oxide is one way to extract pure metals from metal compounds. But how difficult is it to accomplish energetically? Consider the decomposition of aluminum oxide, Al₂O₃:

2Al₂O₃(s) → 4Al(s) + 3O₂(g)

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Energy diagrams for the oxidation of aluminum and for the decomposition of aluminum oxide are shown here. Take a moment to examine the energy changes.

The oxidation of aluminum to form aluminum oxide is very exothermic. The energy for the reverse reaction is equal and opposite. So the decomposition of aluminum oxide is highly endothermic. This means that a large amount of energy is required to extract pure aluminum from aluminum oxide.

Indeed, aluminum was not discovered and brought into use until about 200 years ago, partly because so much energy is required to extract the pure aluminum metal from aluminum ores. The simple heating of aluminum ores does not provide enough energy to decompose the aluminum compounds and extract aluminum metal.

THE HISTORY OF METAL USE

It is apparent from archeological sites that gold was in common use thousands of years before iron. This makes sense from the energy data for these metals. The metals that have been in use the longest are the ones that are easiest to extract from their compounds.

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This table shows the energy data for the decomposition of metal oxides to metals. To extract the metals, people had to construct furnaces that transferred a sufficient amount of heat.

The difficulty of extraction of various metals from metal compounds is reflected in the chronological “discovery” and use of these metals through history. The more energy it takes to extract a metal, the later the metal was put into practical use.

Metals that are easily oxidized are hard to extract. In other words, the oxidation reaction is harder to reverse. These are also the metals that tarnish and rust easily. Gold is often found in its pure form in nature. This is because gold is not easily oxidized and is fairly unreactive.

LESSON SUMMARY

How much energy is transferred during oxidation of metals?

The formation of metal oxides is usually an exothermic reaction. This means oxidation reactions may be valuable sources of energy. The heat of formation of a compound is the thermal energy change associated with the formation of one mole of that compound from its constituent elements. The decomposition of metal oxides into pure metals and oxygen is the reverse of an oxidation reaction. These reactions are generally endothermic. This means it requires an input of energy to extract a metal from a metal compound. Metals that are easily oxidized (with high heats of formation) are more difficult to extract from their metal compounds than metals that are less easily oxidized (with low heats of formation).