Problem 25 - Chemistry The Central Science In Si Units

Question

Which ions remain in solution, unreacted, after each of the following pairs of solutions is mixed? (a) potassium carbonate and magnesium sulfate (b) lead nitrate and lithium sulfide (c) ammonium phosphate and calcium chloride

Step-by-Step Solution

Answer

(a) \( \text{K}^+ \) and \( \text{SO}_4^{2-} \); (b) \( \text{Li}^+ \) and \( \text{NO}_3^- \); (c) \( \text{NH}_4^+ \) and \( \text{Cl}^- \).

Step 1: Identify Initial Compounds

First, identify the chemical formulas for each compound in the given pairs of solutions: (a) Potassium carbonate is \( \text{K}_2\text{CO}_3 \) and Magnesium sulfate is \( \text{MgSO}_4 \).(b) Lead nitrate is \( \text{Pb(NO}_3\text{)}_2 \) and Lithium sulfide is \( \text{Li}_2\text{S} \).(c) Ammonium phosphate is \( \text{(NH}_4\text{)}_3\text{PO}_4 \) and Calcium chloride is \( \text{CaCl}_2 \).

Step 2: Understand the Reaction

Determine whether a precipitate is formed by mixing the solutions based on solubility rules:(a) Mixing \( \text{K}_2\text{CO}_3 \) and \( \text{MgSO}_4 \), we form \( \text{K}_2\text{SO}_4 \) and \( \text{MgCO}_3 \) upon mixing. \( \text{MgCO}_3 \) is insoluble.(b) Mixing \( \text{Pb(NO}_3\text{)}_2 \) and \( \text{Li}_2\text{S} \), we form \( \text{PbS} \) and \( \text{LiNO}_3 \). \( \text{PbS} \) is insoluble.(c) Mixing \( \text{(NH}_4\text{)}_3\text{PO}_4 \) and \( \text{CaCl}_2 \), we form \( \text{Ca}_3(\text{PO}_4)_2 \) and \( \text{NH}_4\text{Cl} \). \( \text{Ca}_3(\text{PO}_4)_2 \) is insoluble.

Step 3: Determine Remaining Ions

For each reaction, identify the ions that do not take part in the formation of the insoluble precipitates and thus remain in solution:(a) \( \text{K}^+ \) and \( \text{SO}_4^{2-} \) ions remain in solution.(b) \( \text{Li}^+ \) and \( \text{NO}_3^- \) ions remain in solution.(c) \( \text{NH}_4^+ \) and \( \text{Cl}^- \) ions remain in solution.

Key Concepts

Solubility rules

When dealing with precipitation reactions, solubility rules are essential for predicting which compounds will dissolve in water and which will form a solid, known as a precipitate. These rules help us determine the fate of ions in a solution upon mixing.

• Most nitrates (NO extsubscript{3} extsuperscript{-}) are soluble in water, which means they dissolve easily and do not form precipitates.
• Compounds containing alkali metal ions, such as potassium ( ext{K} extsuperscript{+}) and ammonium ( ext{NH} extsubscript{4} extsuperscript{+}), are also typically soluble.
• Sulfates (SO extsubscript{4} extsuperscript{2-}) are generally soluble, except when paired with lead ( ext{Pb} extsuperscript{2+}), calcium ( ext{Ca} extsuperscript{2+}), or barium ( ext{Ba} extsuperscript{2+}).
• Carbonates (CO extsubscript{3} extsuperscript{2-}) and phosphates (PO extsubscript{4} extsuperscript{3-}), however, are usually insoluble except when they involve alkali metals or ammonium.

By applying these rules, we can anticipate which reactions will produce a solid and which ions will remain free in the solution. This knowledge is crucial in laboratory experiments and real-world applications across chemistry.

Ionic compounds

Ionic compounds are formed from positive and negative ions that bind together as they balance out their charges. When dissolved in water, they break down into their constituent ions, which can move freely in the solution.

• The positive ions, or cations, might include ext{K} extsuperscript{+}, ext{Li} extsuperscript{+}, and ext{NH} extsubscript{4} extsuperscript{+}. These are often responsible for the compounds' name "cation part".
• On the other hand, negative ions, or anions, like SO extsubscript{4} extsuperscript{2-}, NO extsubscript{3} extsuperscript{-}, and Cl extsuperscript{-}, balance the positive charges to form a stable compound.

The behavior of these ions in a solution, whether interacting to form a new compound or remaining unreacted, is what guides precipitation reactions. Understanding the movement and interaction of ions is key to connecting the dots in complex chemical reactions.

Chemical formula identification

Identifying chemical formulas allows chemists to understand and predict the outcomes when substances are mixed. This step involves recognizing the correct chemical symbols and the right proportions of elements in the compound.

• For example, ext{K} extsubscript{2} ext{CO} extsubscript{3} shows two potassium ions combining with one carbonate ion.
• Similarly, compounds like ext{MgSO} extsubscript{4} indicate one magnesium ion paired with one sulfate ion.
• Chemical formulas provide clues about the compound's properties and reactivity in various environments.

Please note that recognizing these formulas is fundamental in predicting chemical behavior, especially in solutions. This knowledge is not only applied during laboratory work but also plays a significant role in industrial processes and even environmental chemistry.