Problem 105 - Chemistry and Chemical Reactivity

Question

Suggest a method for separating a precipitate consisting of a mixture of solid CuS and solid \(\mathrm{Cu}(\mathrm{OH})_{2}.\)

Step-by-Step Solution

Answer

React the mixture with dilute HCl to dissolve \( \mathrm{Cu(OH)_2} \), then filter to separate remaining \( \mathrm{CuS} \).

Step 1: Understand the Problem

The problem involves separating two solid compounds: Copper(II) sulfide (CuS) and Copper(II) hydroxide (\( \mathrm{Cu(OH)_2} \)). Both are components of the given precipitate. To separate them, we need to find a property or reaction that affects one compound but not the other.

Step 2: Identify Chemical Properties

\( \mathrm{CuS} \) is insoluble in water and many acids, while \( \mathrm{Cu(OH)_2} \) reacts with some acids to form soluble products. This difference in chemical reactivity can be used as a basis for separation.

Step 3: Choose a Suitable Reagent

Select a dilute acid, such as hydrochloric acid (HCl), which can react with \( \mathrm{Cu(OH)_2} \) to form soluble \( \mathrm{CuCl_2} \) and water, but not react significantly with \( \mathrm{CuS} \).

Step 4: Perform the Reaction

Add the dilute HCl to the precipitate mixture. The \( \mathrm{Cu(OH)_2} \) will react to form water and \( \mathrm{CuCl_2} \), which dissolves in the acid, leaving \( \mathrm{CuS} \) as an undissolved solid.

Step 5: Separate the Mixture

Once the reaction is complete, filter the mixture. The undissolved \( \mathrm{CuS} \) will remain on the filter paper as a solid. The filtrate will contain dissolved \( \mathrm{CuCl_2} \).

Key Concepts

Separation Techniques

In chemistry, when faced with a mixture of substances, separation techniques are essential to isolate the individual components. For our problem, we need to separate solid Copper(II) sulfide (CuS) from solid Copper(II) hydroxide (Cu(OH)_2). One effective technique is based on the different solubilities and reactivities of these compounds. We leverage a process called "selective dissolution," where only one component reacts with an added reagent.

In this exercise, the approach involves using a dilute acid to selectively dissolve one part of the mixture. In this case, Copper(II) hydroxide reacts with the acid to form a soluble compound, while Copper(II) sulfide remains unreacted and insoluble, making it easy to separate through filtration.

An important step in separation is filtering the mixture after reaction to isolate the unreacted solid component. This keeps the method simple, effective, and perfect for creating a distinct boundary between the active and unreacted substances.

Chemical Reactivity

Understanding chemical reactivity is critical in selecting the correct method for separating substances. This concept refers to how different substances undergo chemical reactions under various conditions. Copper(II) hydroxide and Copper(II) sulfide exhibit different reactivities.

Copper(II) hydroxide, when exposed to dilute hydrochloric acid (HCl), reacts to form copper(II) chloride, a soluble compound, and water. This characteristic makes it easier to separate using selective dissolution. On the other hand, Copper(II) sulfide does not readily react with the acid, maintaining its solid state.

By targeting varying reactivity, chemists can design experiments, like in our example, to achieve efficient separation. This process shows how manipulating chemical properties enable us to effectively control and direct reactions as needed to reach our desired separation outcome.

Chemical Properties

The chemical properties of substances give insights into how they react or behave under certain conditions. These include solubility, reactivity with acids or bases, and stability.

In separating Copper(II) sulfide and Copper(II) hydroxide, their differing chemical properties play a fundamental role. The main property used here is solubility. Cu(OH)_2 has the chemical property that makes it reactive with acids to produce soluble products. This trait is crucial for its selection in the separation method.

Conversely, CuS is insoluble in water and a typical strong acid. Its resistance to dissolve or react ensures it remains as a solid, thus facilitating its separation from the soluble forms. Understanding these distinct chemical properties helps predict how they interact with other substances, which is vital in chemical processes, including separations.