Problem 25 - Chemistry The Central Science

Question

Which ions remain in solution, unreacted, after each of the following pairs of solutions is mixed? \begin{equation} \begin{array}{l}{\text { (a) potassium carbonate and magnesium sulfate }} \\\ {\text { (b) lead nitrate and lithium sulfide }} \\ {\text { (c) ammonium phosphate and calcium chloride }}\end{array} \end{equation}

Step-by-Step Solution

Answer

For the given pairs of solutions, the ions that remain unreacted in the solution are: (a) K⁺ and SO₄²⁻ (b) Li⁺ and NO₃⁻ (c) NH₄⁺ and Cl⁻

Step 1: Identifying possible reactions for part (a)

For the pair potassium carbonate (K2CO3) and magnesium sulfate (MgSO4), first write down the dissociated ions as K⁺, CO₃²⁻, Mg²⁺, and SO₄²⁻. Now, consider all possible combinations of cations and anions: K⁺ with SO₄²⁻, yielding potassium sulfate (K2SO4), and Mg²⁺ with CO₃²⁻, yielding magnesium carbonate (MgCO3).

Step 2: Determine solubility for part (a)

Using solubility rules, we can determine that potassium sulfate (K2SO4) is soluble, while magnesium carbonate (MgCO3) is not. Therefore, magnesium carbonate will precipitate out of the solution.

Step 3: Identify unreacted ions for part (a)

Since potassium sulfate remains soluble, the K⁺ and SO₄²⁻ ions remain unreacted in the solution.

Step 1: Identifying possible reactions for part (b)

For the pair lead nitrate (Pb(NO₃)₂) and lithium sulfide (Li₂S), first write down the dissociated ions as Pb²⁺, NO₃⁻, Li⁺, and S²⁻. Now, consider all possible combinations of cations and anions: Pb²⁺ with S²⁻, yielding lead sulfide (PbS), and Li⁺ with NO₃⁻, yielding lithium nitrate (LiNO₃).

Step 2: Determine solubility for part (b)

Using solubility rules, we can determine that lead sulfide (PbS) is not soluble, while lithium nitrate (LiNO₃) is. Therefore, lead sulfide will precipitate out of the solution.

Step 3: Identify unreacted ions for part (b)

Since lithium nitrate remains soluble, the Li⁺ and NO₃⁻ ions remain unreacted in the solution.

Step 1: Identifying possible reactions for part (c)

For the pair ammonium phosphate ((NH₄)₃PO₄) and calcium chloride (CaCl₂), first write down the dissociated ions as NH₄⁺, PO₄³⁻, Ca²⁺, and Cl⁻. Now, consider all possible combinations of cations and anions: NH₄⁺ with Cl⁻, yielding ammonium chloride (NH₄Cl), and Ca²⁺ with PO₄³⁻, yielding calcium phosphate (Ca₃(PO₄)₂).

Step 2: Determine solubility for part (c)

Using solubility rules, we can determine that ammonium chloride (NH₄Cl) is soluble, while calcium phosphate (Ca₃(PO₄)₂) is not. Therefore, calcium phosphate will precipitate out of the solution.

Step 3: Identify unreacted ions for part (c)

Since ammonium chloride remains soluble, the NH₄⁺ and Cl⁻ ions remain unreacted in the solution.

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two solutions containing soluble salts are mixed, leading to the formation of an insoluble compound. These reactions are essential to many processes in chemistry and are often depicted in chemical equations.

For instance, when mixing solutions of potassium carbonate and magnesium sulfate, we observe the outcome of a precipitation reaction. Two new compounds can form: potassium sulfate (K2SO4) and magnesium carbonate (MgCO3). However, the solubility rules tell us that magnesium carbonate is insoluble in water and therefore precipitates out of the solution as a solid.

Applying these principles helps identify which ions will remain in solution and which will form the precipitate. Soluble ions, like the potassium (K⁺) and sulfate (SO₄²⁻) ions in our example, will not participate in the precipitate formation and will remain in the solution as spectator ions.

Ionic Equations

Ionic equations provide a deeper understanding of the reactions happening in a solution by showing which ions are involved and their states. The process begins by writing the complete ionic equation, which lists all ions in their dissociated form. For example, lead nitrate and lithium sulfide dissociate into their constituent ions Pb²⁺, NO₃⁻, Li⁺, and S²⁻ when dissolved in water.

In an ionic equation, the ions that do not participate in the formation of the precipitate are termed 'spectator ions'. These ions do not change state and are not part of the net ionic equation. For instance, when lead nitrate reacts with lithium sulfide, the net ionic equation would only include the ions that form the lead sulfide precipitate, not the spectator ions Li⁺ and NO₃⁻ which are soluble and remain in the solution.

Soluble and Insoluble Compounds

Understanding the concept of solubility is essential for predicting the outcome of reactions in aqueous solutions. Solubility rules are guidelines that allow us to predict whether a compound will dissolve in water (soluble) or form a precipitate (insoluble).

Compounds containing alkali metal ions and the ammonium ion are typically soluble. Hence, potassium sulfate and ammonium chloride remain in solution in the given examples. On the other hand, many carbonates, phosphates, and sulfides, such as magnesium carbonate, calcium phosphate, and lead sulfide, are generally insoluble, except when paired with specific ions that can make them soluble.

These solubility rules come in handy when predicting which ions will remain unreacted in a solution, such as Ca²⁺ and PO₄³⁻ ions forming the insoluble compound calcium phosphate, whereas NH₄⁺ and Cl⁻ remain in solution as they form the soluble compound ammonium chloride.