Question

Two scientists, working in separate labs, were looking into the properties of limestone. Two months later, both published their results in a highly renowned journal causing a great controversy amoung the cognoscenti of Organic Chemistry. Said one: "Limestone is quite stable, it is able to withstand centuries of atmospheric exposure.' The other said, "Limestone is extremely reactive, dissolving very rapidly in \(\mathrm{HCl}\) to produce \(\mathrm{CO}_{2}\) and \(\mathrm{CaCl}_{2}\). The two statements seem mutually exclusive. Assuming that each scientist is correct, explain.

Short answer

Both scientists are correct in their observations because they are referring to the properties of limestone under different conditions. The first scientist's statement is about the long-term stability of limestone in atmospheric exposure, predominantly due to its calcium carbonate component. On the other hand, the second scientist's statement refers to the reactivity of limestone when it comes into contact with a specific substance (HCl) that can quickly break down the calcium carbonate. The two statements can coexist because they describe different aspects of limestone's behavior under different conditions.

Step-by-step solution

Identify the situations described by each scientist

In this exercise, we have one scientist describing the stability of limestone when exposed to the atmosphere. The second scientist is describing the reactivity of limestone when it comes into contact with hydrochloric acid (HCl).

Explain the stability of limestone

Limestone is composed mostly of calcium carbonate (\(\mathrm{CaCO}_{3}\)), which is a relatively stable compound. When exposed to the atmosphere, limestone will be subjected to weathering, which can be both physical and chemical. Due to its composition, it is able to withstand long periods of exposure to natural elements like air, water, temperature changes, and wind. Although it undergoes these natural processes of erosion, it can still maintain its durability and remain in existence for centuries. This fact explains the first scientist's statement that limestone is quite stable.

Explain the reactivity of limestone

As mentioned earlier, limestone is composed mainly of calcium carbonate. When limestone comes into contact with hydrochloric acid (\(\mathrm{HCl}\)), a chemical reaction occurs: \[ \mathrm{CaCO}_{3}(s) + 2\mathrm{HCl}(aq) \rightarrow \mathrm{CO}_{2}(g) + \mathrm{CaCl}_{2}(aq) + \mathrm{H}_{2}\mathrm{O}(l) \] Calcium carbonate reacts with HCl to form carbon dioxide gas, calcium chloride, and water. The observable result of this reaction is the production of carbon dioxide gas, which is released as bubbles, and the dissolving of limestone. This reaction demonstrates the high reactivity of limestone when exposed to HCl, supporting the second scientist's statement.

Reconcile the two statements

Both scientists are correct in their observations because they are referring to the properties of limestone under different conditions. The first scientist's statement is about the long-term stability of limestone in atmospheric exposure, predominantly due to its calcium carbonate component. On the other hand, the second scientist's statement refers to the reactivity of limestone when it comes into contact with a specific substance (HCl) that can quickly break down the calcium carbonate. The two statements can coexist because they describe different aspects of limestone's behavior under different conditions.

Key concepts

Calcium Carbonate

Calcium carbonate, known chemically as \( \mathrm{CaCO}_3 \), is a common substance found in rocks such as limestone and is a major component of shells of marine organisms, snails, and eggs. It is prevalent in sedimentary rocks and is what predominantly makes up the limestone.

In its pure form, calcium carbonate is a white, insoluble solid (it does not dissolve in water), which allows it to provide structural stability to rocks like limestone over long periods. Despite this insolubility in water, it does react with stronger acids, such as hydrochloric acid. This reaction, as demonstrated in the provided exercise, can be vigorous, releasing carbon dioxide gas and resulting in soluble calcium chloride.

Role in Limestone Stability

The stability of calcium carbonate contributes significantly to limestone's ability to resist atmospheric weathering. Its dissolution or breakdown requires the presence of an acid strong enough to disrupt its ionic structure, which isn't commonly found in nature. This fact is key to understanding the first scientist's claim about limestone's stability in atmospheric conditions.

Chemical Reactivity

Chemical reactivity refers to the tendency of a chemical substance to engage in a reaction with another substance. In the context of limestone, when discussing its chemical reactivity, we are mostly interested in its behavior when exposed to acids.

The calcium carbonate within limestone is not very reactive when left in contact with neutral substances like pure water but reacts readily with acids, including both strong acids like hydrochloric acid (\( \mathrm{HCl} \)) and weaker organic acids.

Reaction with Acids

The reaction between calcium carbonate and \( \mathrm{HCl} \) provided in the solution exemplifies how an otherwise stable compound can rapidly deteriorate in the presence of an acid. The resulting reaction is not just a theoretical consideration but has practical implications in areas such as stone conservation, environmental chemistry, and industrial processes involving limestone.

Atmospheric Weathering

Atmospheric weathering is a natural process that involves the break-down or alteration of rocks and minerals at the Earth's surface through exposure to the atmosphere. It can take place through various means, including temperature fluctuations, moisture, and the presence of certain chemicals in the air.

For limestone, atmospheric weathering is a slow process due to the stability of its main component, calcium carbonate. The process usually involves the interaction of \( \mathrm{CaCO}_3 \) with carbon dioxide (\( \mathrm{CO}_2 \) and water found in air to form a weak acid—carbonic acid—which can gradually dissolve limestone over the course of centuries.

Impact on Limestone

Although atmospheric weathering leads to the eventual erosion and shape changes in limestone formations, it is a much slower process compared to the rapid dissolution caused by strong acids. This gradual mechanism aligns with the first scientist's statement about limestone's ability to endure centuries of exposure without substantial degradation, illustrating its resilience against the elements.