Question

The first stage in corrosion of iron upon exposure to air is oxidation to \(\mathrm{Fe}^{2+}\). (a) Write a balanced chemical equation to show the reaction of iron with oxygen and protons from acid rain. (b) Would you expect the same sort of reaction to occur with a silver surface? Explain.

Short answer

The balanced chemical equation for the oxidation of iron to Fe²⁺ upon exposure to oxygen and protons from acid rain is: \[\mathrm{Fe + O_2 + 4 \ H^+ \longrightarrow Fe^{2+} + 2 \ H_2O}\] As for silver, it is a noble metal with a more positive standard electrode potential (\(\mathrm{Ag^+/Ag = +0.7996 \ V}\)) compared to iron (\(\mathrm{Fe^{2+}/Fe = -0.440 \ V}\)), indicating it has a lower tendency to lose electrons and form positive ions. Therefore, silver is more resistant to oxidation and corrosion compared to iron and we would not expect the same sort of reaction to occur with a silver surface.

Step-by-step solution

Write a balanced chemical equation for the oxidation of iron to Fe²⁺

Initially, we know the iron (\(\mathrm{Fe}\)) is reacting with oxygen (\(\mathrm{O_2}\)) and protons (\(\mathrm{H^+}\)). The oxidation product is Fe²⁺, as given in the exercise. To balance the chemical equation, we must ensure that the number of atoms of the same element is conserved on both sides of the equation. For our reaction, we will first balance the oxygen atoms by including water (\(\mathrm{H_2O}\)) on the right side of the equation: \[\mathrm{Fe + O_2 + 4 \ H^+ \longrightarrow Fe^{2+} + 2 \ H_2O}\] Now, the equation is balanced, and we can see that iron reacts with oxygen and protons from acid rain, forming Fe²⁺ and water.

Would the same sort of reaction occur with a silver surface? Explain.

Silver is a noble metal, which means that it has low reactivity and is resistant to corrosion and oxidation in humid air. The reason for this is that silver has a lower tendency to lose electrons compared to iron and form positive ions. Comparing the standard electrode potentials of silver and iron, we can observe that silver has a more positive value (\(\mathrm{Ag^+/Ag = +0.7996 \ V}\)) compared to iron (\(\mathrm{Fe^{2+}/Fe = -0.440 \ V}\)). The more positive the value, the less likely that the element will undergo oxidation. Therefore, silver has a lower tendency to lose electrons and form positive ions compared to iron. Based on the standard electrode potentials and general knowledge about the reactivity of noble metals, we would not expect the same sort of oxidation reaction to occur with a silver surface as it does for iron. Silver is more resistant to oxidation and corrosion compared to iron.

Key concepts

Oxidation reactions

Oxidation is a key process in the corrosion of metals, especially iron. When we talk about oxidation reactions, we're referring to the loss of electrons by a metal atom. In the context of corrosion, iron atoms lose electrons and are transformed into iron ions (\(\mathrm{Fe^{2+}}\)). This happens when iron comes into contact with oxygen and water, often present in the form of acid rain.
To visualize this, imagine a piece of iron exposed to humid air. The oxygen molecules achieve a more stable state by accepting electrons from the iron. As a result, iron is oxidized and oxygen is reduced.

• Iron transforms from a neutral atom to positively charged \(\mathrm{Fe^{2+}}\) due to electron loss.
• Oxygen, in turn, accepts these electrons and is reduced.

Understanding the environment is important too. Conditions like those created by acid rain, with dissolved oxygen and hydrogen ions (\(\mathrm{H^+}\)), facilitate the oxidation reactions.

Chemical equations

A chemical equation is like a recipe detailing how substances interact. For corrosion, it helps represent how iron reacts with oxygen and protons in the environment to form corrosion products. The key here is balancing a chemical equation to reflect the conservation of mass. In this particular exercise, the oxidation of iron is represented as:\[\mathrm{Fe + O_2 + 4 \ H^+ \longrightarrow Fe^{2+} + 2 \ H_2O}\]Balancing equations involves ensuring that the number of each type of atom is the same on both sides of the equation.

• Oxygen: Needed in both molecular oxygen (\(\mathrm{O_2}\)) and water formed (\(\mathrm{H_2O}\)).
• Iron: One iron atom starts and ends the reaction.
• Protons: Four protons or \(\mathrm{H^+}\) ions react to produce two water molecules.

This chemical equation acts as a snapshot of the initial stage of corrosion, illustrating the transformation and movement of substance during the process.

Electrode potentials

Electrode potentials help us understand a metal's tendency to lose or gain electrons. This is crucial in predicting how different metals react in the environment, such as which will corrode faster. Every metal has a standard electrode potential, which can be found experimentally and tells us how keen a substance is to undergo oxidation or reduction.
For iron, the electrode potential for the reaction \(\mathrm{Fe^{2+}/Fe}\) is -0.440 V. This negative value indicates that iron has a proclivity to lose electrons, thereby being prone to oxidation and corrosion. In contrast, the electrode potential for silver \(\mathrm{Ag^+/Ag}\) is +0.7996 V. This positive value suggests that silver is less likely to oxidize as it doesn’t readily lose electrons.
Silver is what we call a 'noble metal'. It's this intrinsic resistance to corrosion that prevents silver from undergoing the same type of oxidation reaction that happens in iron. It is because of their differing electrode potentials that metals have varied reactivity when exposed to the same conditions.