Problem 40 - Chemistry and Chemical Reactivity

Question

Predict whether each of the following is insoluble or soluble in water. (a) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{Fe}(\mathrm{OH})_{3}\) (c) \(\mathrm{ZnCl}_{2}\) (d) CuS

Step-by-Step Solution

Answer

(a) Soluble, (b) Insoluble, (c) Soluble, (d) Insoluble.

Step 1: Determine Solubility Rules

Start by understanding basic solubility rules: Nitrates (NO₃⁻) and chlorides (Cl⁻) are usually soluble, while most hydroxides (OH⁻) and sulfides (S²⁻) are insoluble, with exceptions.

Step 2: Check (a) Pb(NO₃)₂ - Nitrates

Lead(II) nitrate, Pb(NO₃)₂, contains the nitrate ion. According to solubility rules, nitrates are soluble in water.

Step 3: Check (b) Fe(OH)₃ - Hydroxides

Iron(III) hydroxide, Fe(OH)₃, is a hydroxide. Solubility rules state most hydroxides are insoluble, except for those of alkali metals and some alkaline earth metals. Fe(OH)₃ is therefore insoluble in water.

Step 4: Check (c) ZnCl₂ - Chlorides

Zinc chloride, ZnCl₂, has the chloride ion, and chlorides are generally soluble. Exceptions include chlorides of lead(II) and silver, which don't apply here, so ZnCl₂ is soluble in water.

Step 5: Check (d) CuS - Sulfides

Copper(II) sulfide, CuS, contains sulfide ions. Most sulfides are insoluble with exceptions for alkali and alkaline earth metals. CuS is therefore insoluble in water.

Key Concepts

Soluble Compounds

Soluble compounds are those that can dissolve in water, forming a homogeneous mixture known as a solution. These compounds have a high affinity for water molecules, which allows them to dissociate and spread evenly throughout the solution. Solubility rules are helpful to predict which compounds are soluble.

• Commonly soluble ions include nitrates (NO₃⁻), acetates (CH₃COO⁻), and most chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻) salts.
• Sulfates (SO₄²⁻) are typically soluble, though there are exceptions.

When presented with a compound such as \( \mathrm{Pb(NO_3)_2} \), knowing that nitrates are universally soluble helps determine its aqueous nature. Similarly, chlorides like \( \mathrm{ZnCl_2} \), which don't contain any of the few exceptions to chloride solubility, are also soluble. This textbook rule often guides whether a compound remains intact or disperses in water.

Insoluble Compounds

Insoluble compounds do not dissolve in water easily. They tend to form precipitates, or solid residues, when mixed with water. The solubility rules aid in identifying such compounds.

• Most carbonates (CO₃²⁻), phosphates (PO₄³⁻), sulfides (S²⁻), and hydroxides (OH⁻) are insoluble.
• There are exceptions, particularly among alkali metal salts and some alkaline earth metals.

For instance, \( \mathrm{CuS} \) is insoluble because sulfides are generally insoluble, except when combined with alkali or a few specific alkaline earth metals. Recognizing these rules helps predict when a compound remains solid and distinct within water.

Solubility of Hydroxides

The solubility of hydroxides varies greatly, and it's essential to know the specific rules. Most hydroxides are only slightly soluble in water. However, there are some notable exceptions.

• Hydroxides of alkali metals, such as sodium hydroxide (\( \mathrm{NaOH} \)), are highly soluble.
• Some alkaline earth metals, like barium hydroxide (\( \mathrm{Ba(OH)_2} \)), also form soluble compounds.

In contrast, \( \mathrm{Fe(OH)_3} \)is an example of a hydroxide that barely dissolves in water, making it insoluble. Understanding these nuances helps in predicting the behavior of compounds when they interact with water.

Solubility of Sulfides

Sulfides' solubility can be quite predictable based on certain rules. Most sulfide compounds tend to be insoluble in water.

• Exceptions are made for sulfides of alkali metals and some alkaline earth metals, like calcium sulfide (\( \mathrm{CaS} \)), which are soluble.
• Heavy metal sulfides generally do not dissolve easily.

For instance, \( \mathrm{CuS} \)is insoluble because copper is a heavy metal and does not fit into the exceptions category. By using these solubility guidelines, one can easily determine whether sulfides will dissolve or remain solid in aqueous environments.