Question

INORGANIC CHEMISTRY: Using fundamental thermodynamics, and ignoring reaction kinetics, explain which of the following materials is most susceptible to corrosion in fully aerated (O₂ saturated) seawater (pH = 6.5) -- aluminum metal, iron metal, glass (silicon dioxide), or gold.

Answer 1

Corrosion is a natural process of oxidation of metals to their stable oxides. It’s well known that iron forms the corresponding iron oxide (Fe2O3) during rust formation because of corrosion.

To explain the corrosion in simple thermodynamics terms one has to consider the heat of formation of the metal oxides in the given question. Thus the heats of formation of oxides of aluminum metal, iron metal, glass (silicon dioxide), and gold need to be considered in determining their ease of formation in terms of thermodynamics. Following is the data obtained from literature about the heat of formation and standard free energy of formation of the oxides in question.

Iron oxide, Fe2O3…………………delta H_f ……= -824.2 kJ/mole……….delta G = -742.2 kJ/mole

Aluminum oxide, Al2O3………………………….= -1675.7 kJ/mole……. delta G = -1582.3 kJ/mole

Silicon dioxide, SiO2……………………………..= -910.86 kJ/mole….... .delta G = -856.4 kJ/mole

Gold (III) oxide, Au2O3……………………………= 81 kJ/mole

A negative value of delta H_f and delta G indicates ease of formation of these oxides except for gold oxide which has a positive delta H_f value. A positive delta H_f value for gold oxide indicates that it is less readily formed under standard conditions. Among the iron, aluminium and silicon oxides aluminium oxide is readily formed because of its higher negative delta H_f value whereas iron and silicon oxides are formed less readily in comparison to aluminum oxide. Standard free energy of formation (delta G) is also following similar pattern as that of delta H_f values for these oxides.