Problem 16 - Chemistry and Chemical Reactivity

Question

Decide whether each of the following is water-soluble. If soluble, tell what ions are produced when the compound dissolves in water. (a) \(\mathrm{NiCl}_{2}\) (c) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\operatorname{Cr}\left(\mathrm{NO}_{3}\right)_{3}\) (d) \(\mathrm{BaSO}_{4}\)

Step-by-Step Solution

Answer

NiCl₂, Pb(NO₃)₂, and Cr(NO₃)₃ are soluble. BaSO₄ is not soluble.

Step 1: Determine Solubility of NiCl2

Refer to the solubility rules: Chlorides (Cl⁻) are generally soluble in water, with exceptions like AgCl, PbCl₂, and Hg₂Cl₂. Since NiCl₂ is not an exception, it is soluble in water.

Step 2: Identify Ions from Dissolution of NiCl2

When NiCl₂ dissolves in water, it dissociates into Ni²⁺ cations and Cl⁻ anions. The equation for dissolution is: \[ \text{NiCl}_2 \rightarrow \text{Ni}^{2+} + 2\text{Cl}^- \]

Step 3: Determine Solubility of Pb(NO3)2

Nitrates (NO₃⁻) are soluble in water without exceptions. Therefore, Pb(NO₃)₂ is water-soluble.

Step 4: Identify Ions from Dissolution of Pb(NO3)2

When Pb(NO₃)₂ dissolves in water, it dissociates into Pb²⁺ cations and NO₃⁻ anions. The equation for dissolution is: \[ \text{Pb(NO}_3)_2 \rightarrow \text{Pb}^{2+} + 2\text{NO}_3^- \]

Step 5: Determine Solubility of Cr(NO3)3

Like Pb(NO₃)₂, Cr(NO₃)₃ also contains the nitrate ion (NO₃⁻), which makes it soluble according to the solubility rules.

Step 6: Identify Ions from Dissolution of Cr(NO3)3

When Cr(NO₃)₃ dissolves in water, it dissociates into Cr³⁺ cations and NO₃⁻ anions. The equation for dissolution is: \[ \text{Cr(NO}_3)_3 \rightarrow \text{Cr}^{3+} + 3\text{NO}_3^- \]

Step 7: Determine Solubility of BaSO4

Refer to the solubility rules for sulfates (SO₄²⁻). Most sulfates are soluble, but BaSO₄ is an exception and is insoluble in water.

Key Concepts

Dissolution Equations

When a compound dissolves in water, it undergoes a process known as dissolution. This process involves the separation of the compound into its constituent ions. The equation that represents this process is a dissolution equation. For example, when nickel(II) chloride (\( \text{NiCl}_2 \)) dissolves in water, it separates into its constituent ions, nickel (II) ions \( (\text{Ni}^{2+}) \) and chloride ions \( (\text{Cl}^-) \). The dissolution equation for this process is written as: \[ \text{NiCl}_2 \rightarrow \text{Ni}^{2+} + 2\text{Cl}^- \] Dissolution equations are crucial for understanding how chemical compounds behave in water. They inform us about the type and number of ions produced in solution. This information is essential for predicting the outcome of chemical reactions that occur in aqueous solutions, such as those in chemistry labs and in the environment. In general, when writing dissolution equations:

• Identify the compound and determine if it is soluble or insoluble in water.
• If the compound is soluble, write the dissolution equation to show the compound dissociating into its respective ions.

Understanding how to write and interpret dissolution equations is foundational for chemistry students and provides insight into the interactions between solute and solvent in solutions.

Ions in Solution

When ionic compounds dissolve in water, they disintegrate into their constituent ions. This is because the water molecules surround and stabilize the ions, allowing them to move independently in the solution. These free-moving ions make up an electrolytic solution, capable of conducting electricity. Knowing which ions are present in a solution is integral to understanding its chemical properties and potential reactions. For instance, in our examples:

• \( \text{NiCl}_2 \) dissociates into \( \text{Ni}^{2+} \) and \( \text{Cl}^- \) ions.
• \( \text{Pb(NO}_3)_2 \) produces \( \text{Pb}^{2+} \) and \( \text{NO}_3^- \) ions.
• \( \text{Cr(NO}_3)_3 \) forms \( \text{Cr}^{3+} \) and \( \text{NO}_3^- \) ions.

Every ion in solution contributes to the overall properties of that solution.

• \( \text{Cations} \) often determine the acidity or basicity.
• \( \text{Anions} \) can influence the reactivity with other compounds dissolved in the solution.

Comprehending these ions is vital for anyone studying chemical solutions, as it enables predictions about how solutions will behave during various chemical processes.

Water-Soluble Compounds

Water-soluble compounds are those that easily dissolve in water to form ions. Whether a compound will dissolve in water is determined chiefly by the solubility rules. These rules provide guidelines on the solubility of different ionic compounds in water. Some basic solubility rules are:

• Compounds containing alkali metal ions \((\text{Li}^+, \text{Na}^+, \text{K}^+, \text{Rb}^+, \text{Cs}^+)\) and the ammonium ion \((\text{NH}_4^+)\) are always soluble.
• Nitrates \((\text{NO}_3^- )\) and acetates \((\text{C}_2\text{H}_3\text{O}_2^- )\) are generally soluble.
• Most chlorides \((\text{Cl}^- )\), bromides \((\text{Br}^- )\), and iodides \((\text{I}^- )\) are soluble, except those of silver, lead, and mercury.
• Sulfates \((\text{SO}_4^{2-})\) are usually soluble, except for those with barium \((\text{Ba}^{2+})\), strontium \((\text{Sr}^{2+})\), and lead \((\text{Pb}^{2+})\).

Exceptions to these guidelines must always be considered. For example, barium sulfate \((\text{BaSO}_4)\) is an exception to the sulfate rule and is insoluble in water.Understanding these rules helps predict a compound's behavior in water, which is vital for various applications in chemistry, from laboratory experiments to industrial processes and environmental studies.