Problem 38 - Chemistry and Chemical Reactivity

Question

Name two insoluble salts of each of the following ions. (a) \(\mathrm{Cl}^{-}\) (b) \(\mathrm{Zn}^{2+}\) (c) \(\mathrm{Fe}^{2+}\)

Step-by-Step Solution

Answer

(a) \(AgCl\), \(PbCl_2\); (b) \(Zn_3(PO_4)_2\), \(ZnCO_3\); (c) \(FeS\), \(FeCO_3\).

Step 1: Understanding the Solubility Rule

To determine insoluble salts, we first need to understand basic solubility rules. Generally, chlorides (\(Cl^-\)) are soluble, except when combined with silver (\(Ag^+\)), lead (\(Pb^{2+}\)), or mercury (\(Hg_2^{2+}\)). Moreover, compounds of zinc (\(Zn^{2+}\)) and iron (\(Fe^{2+}\)) might be insoluble based on the anion they are paired with.

Step 2: Identify Insoluble Chlorides

For the chloride ion (\(Cl^-\)), two salts that are insoluble include silver chloride (\(AgCl\)) and lead(II) chloride (\(PbCl_2\)). These do not dissolve in water as per the solubility rules.

Step 3: Determine Insoluble Zinc Compounds

Zinc typically forms soluble salts. However, zinc phosphate (\(Zn_3(PO_4)_2\)) and zinc carbonate (\(ZnCO_3\)) are known to be insoluble in water due to their particular anion's insolubility in water.

Step 4: Identify Insoluble Iron(II) Compounds

Iron(II) compounds may also be insoluble depending on the anion. Iron(II) sulfide (\(FeS\)) and Iron(II) carbonate (\(FeCO_3\)) are examples of insoluble iron salts.

Key Concepts

Solubility Rules

The solubility rules help us predict whether a salt will dissolve in water. These rules stem from the interaction of ions with water. For common ions, solubility rules are as follows:

• Most salts containing alkali metal ions (like sodium and potassium) and the ammonium ion are soluble.
• Nitrates ( NO_3^- ) and acetates ( CH_3COO^- ) are generally soluble.
• Chlorides ( Cl^- ), bromides ( Br^- ), and iodides ( I^- ) are soluble, except those of silver ( Ag^+ ), lead ( Pb^{2+} ), and mercury ( Hg_2^{2+} ).
• Sulfates ( SO_4^{2-} ) are generally soluble, but here exceptions include barium ( BaSO_4 ), lead ( PbSO_4 ), and calcium ( CaSO_4 ).
• Compounds with CO_3^{2-} , PO_4^{3-} , and sulfide ( S^{2-} ) ions are typically insoluble, except when paired with alkali metals or ammonium.

Understanding these rules allows us to anticipate which combinations of ions will form insoluble compounds, which are particularly relevant in predicting reactions and product formation.

Chloride Ion

The chloride ion, Cl^- , is a common anion found in many salts, known for its ability to form soluble compounds. However, there are notable exceptions based on the cation it pairs with. Chloride is typically soluble in water, except when combined with certain metal ions.

Common insoluble chloride compounds include:

• Silver chloride ( AgCl ): Forms a white precipitate in water and is used in photographic materials.
• Lead(II) chloride ( PbCl_2 ): Sparingly soluble at room temperature, used in certain types of metal plating and pigments.

Recognizing these exceptions is crucial for explaining why certain reactions result in the formation of precipitates.

Zinc Compounds

Zinc cations, Zn^{2+} , are often capable of forming soluble salts, yet some zinc compounds are notable for their insolubility. This is largely due to the nature of the anion paired with the zinc ion.

Examples of insoluble zinc compounds include:

• Zinc phosphate ( Zn_3(PO_4)_2 ): Used in dental cement and corrosion-resistant coatings.
• Zinc carbonate ( ZnCO_3 ): Occurs naturally as smithsonite, historically used in calamine lotion for skin conditions.

The behavior of zinc compounds in reactions depends on their solubility, which in turn affects the applications and techniques used in chemical processes.

Iron Compounds

Iron, a versatile element, often forms compounds like iron(II) and iron(III) that display varied solubility properties. Iron(II), represented as Fe^{2+} , can form insoluble compounds, which have implications in fields like water treatment and mineral extraction.

Two common examples of insoluble iron(II) compounds are:

• Iron(II) sulfide ( FeS ): Black precipitate often encountered in the refining of metal ores.
• Iron(II) carbonate ( FeCO_3 ): Known as siderite, often found in iron ores and used for its potential magnetic properties.

Understanding the conditions under which these compounds form is important for industries involved in chemistry-heavy manufacturing and environmental management.