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

(a) The balanced chemical equation for the reaction of iron with oxygen and protons from acid rain is \(4\,\mathrm{Fe} + 3\,\mathrm{O}_{2} + 12\,\mathrm{H}^{+} \rightarrow 4\,\mathrm{Fe}^{2+} + 6\,\mathrm{H}_{2}\mathrm{O} \). (b) No, the same sort of reaction does not occur with a silver surface as it does with an iron surface, because silver is less reactive and has a higher resistance to oxidation due to its more positive standard reduction potential in the electrochemical series.

Step-by-step solution

(a) Balanced Chemical Equation for Iron Oxidation

: To write the balanced chemical equation for the reaction of iron with oxygen and protons from acid rain, we should first identify the reactants and the products involved in the reaction. The reactants are iron (Fe), oxygen (O2), and protons from acid rain (H+). The main product is the oxidized iron, which is \(\mathrm{Fe}^{2+}\), and water (H2O). Now, we can write down the balanced chemical equation as follows: \(4\,\mathrm{Fe} + 3\,\mathrm{O}_{2} + 12\,\mathrm{H}^{+} \rightarrow 4\,\mathrm{Fe}^{2+} + 6\,\mathrm{H}_{2}\mathrm{O} \) It is balanced because there are equal numbers of each element on both sides of the equation.

(b) Reaction of Silver with Oxygen and Protons

: Now, we need to discuss whether the same sort of reaction would occur with a silver surface and provide an explanation. Silver (Ag) is a much less reactive metal than iron. This is mainly due to the difference in their positions on the electrochemical series, where silver is close to the bottom (more positive standard reduction potential) and iron is closer to the top (more negative standard reduction potential). This difference in reactivity indicates that silver is more resistant to oxidation. The reaction of silver with oxygen and protons from acid rain can be represented as: \(4\,\mathrm{Ag} + 2\,\mathrm{H}_{2}\mathrm{O} + 2\,\mathrm{H}^{+} \rightarrow 4\,\mathrm{Ag}^{+} + 4\,\mathrm{H}_{2}\mathrm{O} \) However, this reaction is much slower and less favorable than the reaction involving iron. Silver ions would rather be reduced back to metallic silver, which is why silver surfaces tarnish (a combination of silver sulfide) rather than corrode in the same manner as iron. This is also the reason why silver is often used for jewellery and other decorative items due to its resistance to oxidation and corrosion. In conclusion, the same sort of reaction does not occur with a silver surface as it does with an iron surface due to silver's lower reactivity and higher resistance to oxidation.

Key concepts

Iron Oxidation

Iron oxidation is a fundamental process that plays a key role in the corrosion of iron. When iron is exposed to oxygen and water, particularly from sources like acid rain, it begins to oxidize. This process involves iron atoms losing electrons to form iron ions. In simple terms, iron turns into a rust-like material through a chemical reaction.

The balanced chemical equation detailing this reaction is:

• \(4\,\mathrm{Fe} + 3\,\mathrm{O}_{2} + 12\,\mathrm{H}^{+} \rightarrow 4\,\mathrm{Fe}^{2+} + 6\,\mathrm{H}_{2}\mathrm{O}\)

Here, elementary iron (\(\mathrm{Fe}\)) comes into contact with oxygen (\(\mathrm{O}_{2}\)) and protons from acid rain (\(\mathrm{H}^{+}\)), leading to the formation of ferrous ions (\(\mathrm{Fe}^{2+}\)). The reaction is balanced, meaning the number of each type of atom on both sides of the equation is the same.

Iron oxidation is the initial step in the formation of rust, which further progresses to iron(III) oxide if allowed to continue. Understanding this process is crucial for learning how to protect iron structures and objects from corrosion.

Chemical Equation Balancing

Balancing chemical equations is an essential skill in chemistry, allowing us to represent the conservation of mass and charge in a chemical reaction. Every chemical equation consists of reactants on the left side and products on the right side.

For the reaction of iron with oxygen and protons in acid rain, we wrote:

• \(4\,\mathrm{Fe} + 3\,\mathrm{O}_{2} + 12\,\mathrm{H}^{+} \rightarrow 4\,\mathrm{Fe}^{2+} + 6\,\mathrm{H}_{2}\mathrm{O}\)

This equation is balanced. There are equal numbers of iron, oxygen, and hydrogen atoms on both sides. Balancing involves adjusting coefficients, which are the numbers in front of molecules, but not subscripts which define the identity of the molecules themselves.

When balancing equations:

• Start with the most complex molecule.
• Ensure total atoms for each element and total charge are the same on both sides.
• Make use of coefficients, not subscripts, to balance the equation.

This practice ensures equations obey the law of conservation of mass and charge.

Metal Reactivity

Metal reactivity refers to how easily a metal can undergo a chemical reaction, often resulting in oxidation. Different metals have varying propensities to react, which is determined by their position in the electrochemical series. This series lists metals in order of their tendency to oxidize.


Iron is relatively reactive. It oxidizes easily, particularly in environments with moisture and acids, leading to rust. This is due to its higher negative standard reduction potential compared to other metals.

Silver, on the other hand, is less reactive. It sits lower in the electrochemical series with a higher positive standard reduction potential. This makes silver more resistant to losing electrons and undergoing oxidation.

Understanding metal reactivity helps in predicting how metals will behave in different environments, informs alloy selection, and helps in preventing corrosion through strategies like galvanization.

Silver Tarnishing

Silver tarnishing is a process different from the type of corrosion that affects metals like iron. Instead of forming oxide layers, tarnishing results in silver forming a thin layer of silver sulfide on its surface. This occurs when silver reacts with sulfur compounds present in the atmosphere.

Due to its low reactivity, silver does not readily undergo further oxidation like iron does. Instead, when silver tarnishes, it forms:

• \(2\,\mathrm{Ag} + \mathrm{S} \rightarrow \mathrm{Ag}_2\mathrm{S}\)

This reaction is much slower and less destructive, primarily affecting the surface of silver items, resulting in a darkening rather than structural damage.

Because the tarnishing only affects the surface, it is often removable through polishing. Additionally, silver's resistance to deeper corrosion is why it is used for jewellery and decorative items. Understanding tarnishing can extend the life and preserve the appearance of silver products.