Problem 21 - Chemistry: The Central Science

Question

Using solubility guidelines, predict whether each of the following compounds is soluble or insoluble in water: (a) \(\mathrm{MgBr}_{2}\), (b) \(\mathrm{PbI}_{2}\), (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\), (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\), (e) \(\mathrm{ZnSO}_{4}\).

Step-by-Step Solution

Answer

Based on solubility guidelines, the solubility of the given compounds in water is as follows: a) \(MgBr_2\) is soluble, b) \(PbI_2\) is insoluble, c) \((NH_4)_2CO_3\) is soluble, d) \(Sr(OH)_2\) is soluble (limited solubility), e) \(ZnSO_4\) is soluble.

Step 1: a) MgBr2

Magnesium bromide (MgBr2) is a metal halide, and metal halides are generally soluble in water. Therefore, MgBr2 is soluble in water.

Step 2: b) PbI2

Lead iodide (PbI2) is a metal halide. However, Pb (lead) is an exception in the general rule that halides are soluble. Lead halides tend to be insoluble in water. Therefore, PbI2 is insoluble in water.

Step 3: c) (NH4)2CO3

Ammonium carbonate ((NH4)2CO3) contains the ammonium ion (NH4+). Compounds containing the ammonium ion are typically soluble in water. Therefore, (NH4)2CO3 is soluble in water.

Step 4: d) Sr(OH)2

Strontium hydroxide (Sr(OH)2) is a metal hydroxide. In general, these compounds are either insoluble or only slightly soluble in water. However, Sr(OH)2 has a limited solubility in water, so we can consider it as soluble.

Step 5: e) ZnSO4

Zinc sulfate (ZnSO4) is a sulfate, and most sulfate compounds are soluble in water. Zinc sulfate is no exception, so ZnSO4 is soluble in water.

Key Concepts

Metal Halide Solubility

When it comes to understanding metal halides, you can generally bet on their solubility in water. Metal halides are compounds formed from a halogen (like fluorine, chlorine, bromine, or iodine) and a metal. Considering their structure, the polar nature of water molecules is able to effectively surround the ions in the compound and dissolve them. This is why many compounds such as magnesium bromide \(\mathrm{MgBr}_{2}\) readily dissolve. It's quite interesting to note that this solubility rule has its exceptions, which brings us to the specifically intriguing case of lead halides.

But first, let's remember that halides are a crucial part of chemistry, often applied in daily products like table salt (sodium chloride) or in more complex applications like in photographic films with silver bromide.

Lead Iodide Insolubility

Now, lead iodide \(\mathrm{PbI}_{2}\) stands out from the crowd. Why? Because despite being a metal halide, it’s one of the exceptions to the solubility rules. Lead iodide is poorly soluble in water due to the large size and low charge density of its ions, leading to a weak ability to become hydrated by water molecules. This property is particularly important when conducting chemical reactions where precipitation is required. For instance, forming a yellow precipitate of lead iodide could be a key step in a synthesis process or in diagnostic tests where precipitation indicates the presence of certain ions.

Ammonium Compounds Solubility

Let’s shift our focus to ammonium compounds like ammonium carbonate \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\). These compounds typically break apart in water, releasing the positively charged ammonium ion and the accompanying negative ion. Because of this positive charge and the ammonium ion's small size, which allows water molecules to easily interact, ammonium compounds display a high solubility in water. This is why they are frequently used in fertilizers and cleaning agents, where they need to dissolve to be effective.

Interestingly, their high solubility also makes ammonium compounds great choices for use in chemical reactions where the reagents need to be in an aqueous solution to react fully.

Strontium Hydroxide Solubility

Talking about hydroxides, they usually present a challenge when discussing solubility, but let's dive into strontium hydroxide \(\mathrm{Sr(OH)}_{2}\). Among metal hydroxides, strontium hydroxide is one of the few that are soluble in water, albeit not as freely as many salts. This 'limited solubility' means that while it does dissolve, it does so up to a certain point before reaching saturation. This property is significant for applications in which a partially soluble compound is desired, like in certain industrial processes where strontium hydroxide can be used to refine sugar or in the production of biodegradable plastics.

Sulfate Compounds Solubility

On the sulfate front, these compounds, which include a sulfur atom surrounded by four oxygen atoms, are also primarily water-soluble. Zinc sulfate \(\mathrm{ZnSO}_{4}\) is such a sulfate, and consistent with others in its family, it dissolves in water. This makes it suitable for use in health supplements and agricultural sprays where easy mixing with water is ideal. However, there are notable exceptions like barium sulfate and calcium sulfate in certain conditions, which are sparingly soluble, and their special qualities make them useful for things like medical imaging and setting plasters.

The general solubility of sulfates plays a major role in the transport and cycling of sulfur in the environment, which is a key component in the ecosystem.