Problem 8 - General Chemistry

Question

Describe in words how you would prepare pure crystalline \(\mathrm{AgCl}\) and \(\mathrm{NaNO}_{3}\) from solid \(\mathrm{AgNO}_{3}\) and solid \(\mathrm{NaCl}\).

Step-by-Step Solution

Answer

Dissolve \( \text{AgNO}_3 \) and \( \text{NaCl} \), mix to form \( \text{AgCl} \) and \( \text{NaNO}_3 \), then isolate, wash, and dry the \( \text{AgCl} \) precipitate; evaporate \( \text{NaNO}_3 \) solution to obtain crystals.

Step 1: Dissolve the Reactants

To begin the preparation, dissolve solid \( \text{AgNO}_3 \) and solid \( \text{NaCl} \) in separate beakers of distilled water. Ensure that the solids fully dissolve to create aqueous solutions of \( \text{Ag}^+ \), \( \text{NO}_3^- \), \( \text{Na}^+ \), and \( \text{Cl}^- \) ions.

Step 2: Mix the Solutions

Pour the \( \text{AgNO}_3 \) solution into the \( \text{NaCl} \) solution. The combination will result in a double displacement reaction, where \( \text{Ag}^+ \) ions react with \( \text{Cl}^- \) ions to form \( \text{AgCl} \) precipitate, and \( \text{Na}^+ \) ions react with \( \text{NO}_3^- \) ions to remain in solution as \( \text{NaNO}_3 \).

Step 3: Collect the Precipitate

The \( \text{AgCl} \) formed will be a white precipitate, which should be separated from the mixture. This can be achieved using filtration. Set up a filter paper in a funnel and pour the reaction mixture through it, capturing the \( \text{AgCl} \) on the paper.

Step 4: Wash the Precipitate

Rinse the \( \text{AgCl} \) on the filter paper with distilled water to remove any residual \( \text{NaNO}_3 \). This helps in purifying the \( \text{AgCl} \).

Step 5: Dry the Precipitate

Transfer the wet \( \text{AgCl} \) from the filter to a drying dish and leave it to dry completely in an oven or air-dry at room temperature until pure crystalline \( \text{AgCl} \) is obtained.

Step 6: Obtain \( \text{NaNO}_3 \)

The filtrate contains dissolved \( \text{NaNO}_3 \). Evaporate distilled water from the solution by heating it slowly until \( \text{NaNO}_3 \) crystals form. Then, allow the crystals to cool and dry.

Key Concepts

Precipitation Reaction

A precipitation reaction is a fascinating type of chemical reaction. It occurs when two soluble salts react in solution to form an insoluble solid—known as a precipitate. When you mix solutions of \( \text{AgNO}_3 \) and \( \text{NaCl} \), a precipitation reaction takes place. Here, \( \text{Ag}^+ \) ions and \( \text{Cl}^- \) ions interact to form solid \( \text{AgCl} \), a white precipitate. This is because \( \text{AgCl} \) is not soluble in water, hence it solidifies and separates from the liquid. This type of reaction is useful in creating pure compounds, like our crystalline silver chloride.

Filtration

Filtration is an essential process in separating a precipitate (like \( \text{AgCl} \)) from the liquid in which it is formed. Once you have your white \( \text{AgCl} \) precipitate, you need to separate it from the surrounding solution.
To do this, you can use a filter paper setup:

• Place filter paper in a funnel.
• Pour the reaction mixture slowly through the funnel.

The filter traps the solid \( \text{AgCl} \) while allowing liquid and any dissolved substances to flow through. After filtration, you'll have your \( \text{AgCl} \) on the paper, ready for further purification.

Solution Preparation

Solution preparation is a critical step in many chemical reactions. It involves dissolving reactants, such as \( \text{AgNO}_3 \) and \( \text{NaCl} \), in a solvent to create solutions of ions.
This step ensures that the ions are free to move and react with each other. Here's how you can prepare the solutions:

• Scoop the solid \( \text{AgNO}_3 \) and \( \text{NaCl} \) separately.
• Add them to distilled water in separate beakers.

Stir the mixtures until all solids are completely dissolved into clear solutions, creating ions in the process. This preparation is crucial for guiding the desired precipitation reaction efficiently.

Crystallization

Crystallization is a key technique for purifying substances, like \( \text{NaNO}_3 \) in this exercise. After separating the \( \text{AgCl} \) precipitate, \( \text{NaNO}_3 \) remains dissolved in the filtrate. To obtain it in pure form, you will need to crystallize it out from the solution.

• First, evaporate the solvent (water) by heating the solution gently.
• As the solution becomes more concentrated, \( \text{NaNO}_3 \) begins to crystallize out.

Cooling the saturated solution helps complete crystallization. Finally, allow the \( \text{NaNO}_3 \) crystals to dry. Crystallization thus helps in obtaining a cleaner, more refined product.