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Unit 9: Equilibrium and Advanced Thermodynamics—Balance in Chemical Reactions

Section 6: Coupling Reactions

As mentioned in Section 1 of this unit, the rusting of iron is a spontaneous chemical reaction. Because iron will spontaneously react with oxygen in the air, pure elemental iron is rarely found in the Earth's crust. Iron mines extract various types of iron ore, which consist largely of compounds made of iron and oxygen. To get pure iron metal for use in manufacturing, there must be some way to reverse the spontaneous reaction of iron with oxygen. In other words, the following reaction must occur:

2Fe2O3(s) → 4Fe(s) + 3O2(g)

Iron Ore Smelting

Figure 9-7. Iron Ore Smelting

© Cristiano Soares, O2HD Media.

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Iron Ore Smelting

Figure 9-7. Iron Ore Smelting

The smelting of iron in a blast furnace requires the coupling of chemical reactions.

The process of smelting iron ore—processing the ore in a blast furnace—reverses the reaction. (Figure 9-7) In the intense heat of the blast furnace, the reaction is coupled to another reaction, the spontaneous reaction of carbon with oxygen:

C(s) + O2(g) → CO2(g)

These reactions are coupled because the O2 produced by the first reaction is consumed in the second reaction. When two chemical reactions are coupled, the overall ΔG of the reaction determines the spontaneity. Combining the two reactions above, we get:

2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g)

The ΔG of this combined reaction is negative, so the reaction is spontaneous.

Coupled reactions are not just important in heavy industry; all living things rely on coupled reactions inside their cells. Thousands of nonspontaneous reactions must occur inside an organism to keep it alive, and spontaneous reactions "drive" the nonspontaneous reactions.

For example, the following reaction shows the synthesis of glutamine, an amino acid, from the glutamate ion and ammonia in the form of ammonium:

glutamate + NH4+ → glutamine + H2O    ΔG = +14.2 kJ/mol

The ΔG for the reaction is positive, so the reaction is nonspontaneous. Many reactions like this are coupled with the hydrolysis of adenosine triphosphate (ATP):

ATP → ADP + phosphate    ΔG = -30.5 kJ/mol

Taken together, the coupled reactions have ΔG = 14.2-30.5 = -16.3 kJ/mol. So, overall, the process is spontaneous.

In this way, ATP drives thousands of biological reactions. Instead of making more ATP from scratch, organisms simply reverse the breakdown of ATP in the nonspontaneous reaction:

ADP + phosphate → ATP

This regeneration is driven, in turn, by the breakdown of nutrients such as glucose:

C6H12O6 + 6O2 → 6H2O + 6CO2    ΔG = -2870 kJ/mol

From Diamond to Graphite

Figure 9-8. From Diamond to Graphite

© Molecules: Science Media Group. Diamond: Wikimedia Commons, CC License 3.0. Author: Mario Sarto, 4 February 2004. Pencil: Wikimedia Commons, Public Domain.

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From Diamond to Graphite

Figure 9-8. From Diamond to Graphite

The transformation of diamond into graphite is spontaneous at room temperature and pressure, but the reaction proceeds extremely slowly.

What about spontaneous reactions: Must they occur? While many reactions are technically spontaneous according to their ΔG values, this tells us nothing about the rate at which the chemical reaction will occur. For example, pure carbon in the form of a diamond will spontaneously turn into carbon's cheap graphite form; the ΔG of this reaction is -2.87 kJ/mol at room temperature. (Figure 9-8) The reaction is so slow, however, that it is not noticeable over human scales of time. However, if we accidentally lost a diamond ring in the batter while baking a cake, the temperatures in the oven would speed up the process and turn the diamond into worthless graphite.


Coupled reactions

When a spontaneous reaction drives a nonspontaneous reaction.


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