Question

You are presented with a white solid and told that due to careless labeling it is not clear if the substance is barium chloride, lead chloride, or zinc chloride. When you transfer the solid to a beaker and add water, the solid dissolves to give a clear solution. Next an \(\mathrm{Na}_{2} \mathrm{SO}_{4}(a q)\) solution is added and a white precipitate forms. What is the identity of the unknown white solid? [Section 4.2\(]\)

Short answer

The unknown white solid is either barium chloride (BaCl₂) or lead chloride (PbCl₂), as both of these substances form insoluble sulfate salts when reacted with sodium sulfate, resulting in a white precipitate. Zinc chloride can be ruled out, as it forms a soluble sulfate salt with no precipitate. Further tests would be needed to confirm the exact identity of the solid.

Step-by-step solution

Write the general form of the reactions

First, we need to write the general form of the reactions of the unknown solid with water and with sodium sulfate. Since all three possible solids are in the form of metal chlorides (MCl₂), the general form of their reactions can be written as: Reaction with water: MCl₂(s) + H₂O(l) → M²⁺(aq) + 2 Cl⁻(aq) Reaction with sodium sulfate: M²⁺(aq) + 2 Cl⁻(aq) + 2 Na⁺(aq) + SO₄²⁻(aq) → M(??) + 2 NaCl(aq)

Identify the products formed in the reaction with sodium sulfate

Using the solubility rules, we can predict the products formed when each of the possible metal cations reacts with sulfate ions: - Barium sulfate (BaSO₄): Insoluble - Lead sulfate (PbSO₄): Insoluble - Zinc sulfate (ZnSO₄): Soluble Based on this information, we can rewrite the second equation for each metal cation: Reaction with sodium sulfate (barium case): Ba²⁺(aq) + 2 Cl⁻(aq) + 2 Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2 NaCl(aq) Reaction with sodium sulfate (lead case): Pb²⁺(aq) + 2 Cl⁻(aq) + 2 Na⁺(aq) + SO₄²⁻(aq) → PbSO₄(s) + 2 NaCl(aq) Reaction with sodium sulfate (zinc case): Zn²⁺(aq) + 2 Cl⁻(aq) + 2 Na⁺(aq) + SO₄²⁻(aq) → ZnSO₄(aq) + 2 NaCl(aq)

Compare the results with the given observations

The initial observation states that when the unknown solid is added to water, it dissolves to give a clear solution. This confirms that the metal chloride is soluble in water, which is true for all three candidates (barium chloride, lead chloride, and zinc chloride). The second observation states that when an aqueous solution of sodium sulfate is added, a white precipitate forms. We can now use this information to eliminate possibilities based on the products formed in the second equation: - Barium case: A white precipitate (BaSO₄) is formed. - Lead case: A white precipitate (PbSO₄) is formed. - Zinc case: No precipitate is formed (ZnSO₄ is soluble).

Identify the unknown solid

Since a white precipitate forms when the sodium sulfate solution is added, the unknown solid must be one of the reactants forming an insoluble sulfate salt. Based on our analysis in step 3, we can conclude that the unknown solid is either: - Barium chloride (BaCl₂) or - Lead chloride (PbCl₂) However, we can not definitively identify the unknown solid as either barium chloride or lead chloride solely based on the given observations. Further tests would be needed to confirm the exact identity of the solid.

Key concepts

Solubility Rules

Solubility rules are guidelines used to predict whether a compound will dissolve in water and form an aqueous solution or form a precipitate. These rules are essential in understanding chemical reactions, especially those involving ionic compounds. Generally, there are certain patterns:

• Most salts of alkali metals (like sodium and potassium) and ammonium are soluble.
• Most nitrates, acetates, and chlorates are soluble.
• Chlorides, bromides, and iodides are usually soluble, except for those of silver, lead, and mercury.

However, there are exceptions, particularly with compounds like sulfates. Many sulfates are soluble, but there are notable exceptions, such as barium sulfate and lead sulfate, both of which are insoluble. These rules help us predict outcomes, such as in the exercise, where knowing lead sulfate does not dissolve in water explains the formation of a precipitate when certain conditions are met.

Metal Chlorides

Metal chlorides, like those mentioned in the problem - barium chloride ( BaCl₂ ), lead chloride ( PbCl₂ ), and zinc chloride ( ZnCl₂ ) - are common compounds in chemistry that consist of a metal ion and chloride ions. These are typically ionic compounds that can dissolve in water to form clear solutions.
When metal chlorides dissolve in water, the metal ions ( M²⁺ ) and chloride ions ( Cl⁻ ) separate and interact freely in the solution, a process known as dissociation. For the three metal chlorides in this exercise, all dissolve to give clear solutions initially.
Understanding the properties of metal chlorides is key, especially when trying to identify unknown substances in chemistry experiments. This is because their reactivity can vary, leading to different outcomes when they encounter other chemical reagents, such as sodium sulfate, as in the original problem.

Precipitate Formation

Precipitate formation is a crucial concept in identifying unknown substances. It occurs when ions in solution combine to form an insoluble compound that "falls out" of the solution. This process is dictated by the solubility rules.
In the provided exercise, the addition of sodium sulfate ( Na₂SO₄ ) to solutions of different metal chlorides resulted in either the formation or non-formation of a precipitate:

• Barium sulfate ( BaSO₄ ) and lead sulfate ( PbSO₄ ) are both insoluble and form white precipitates.
• Zinc sulfate ( ZnSO₄ ) is soluble and does not precipitate.

Through observing the formation of a white precipitate, one can narrow down the identity of the metal chloride present in the solution. In this case, since a precipitate forms, we know the original solid contains either barium or lead, whose sulfates do not dissolve in water.

Aqueous Solutions

An aqueous solution is a solution where water acts as the solvent to dissolve a solute, which in this case, is the metal chloride. When a chemical compound dissolves in water, its component ions break apart and become surrounded by water molecules.

• This results in a homogeneous mixture where the solute is uniformly distributed within the water.
• The ions are free-moving and can engage in subsequent chemical reactions with other ions or molecules in the solution.

In the context of the exercise, when each of the chloride compounds dissolves, it means that they are soluble in water, which is known as forming an aqueous solution. The presence of an aqueous solution is the first step that sets the stage for other chemical reactions, such as the precipitation reaction that follows when sodium sulfate is added. Understanding this concept is vital, as it helps predict how substances will behave in water, a common medium for chemical reactions in both the lab and nature.