Problem 55 - Chemistry and Chemical Reactivity

Question

When solutions of \(\mathrm{BaCl}_{2}\) and \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) are mixed, the mixture becomes cloudy. After a few days, a white solid is observed on the bottom of the beaker with a clear liquid above it. (a) Write a balanced equation for the reaction that occurs. (b) Why is the solution cloudy at first? (c) What happens during the few days of waiting?

Step-by-Step Solution

Answer

The balanced equation is \( \mathrm{BaCl}_2 + \mathrm{Na}_2 \mathrm{SO}_4 \rightarrow \mathrm{BaSO}_4 + 2 \mathrm{NaCl} \). The initial cloudiness is due to \( \mathrm{BaSO}_4 \) precipitate forming, and over days, it settles at the bottom.

Step 1: Identify the Reaction

When solutions of barium chloride \( \mathrm{BaCl}_2 \) and sodium sulfate \( \mathrm{Na}_2 \mathrm{SO}_4 \) are mixed, a double displacement reaction occurs forming barium sulfate \( \mathrm{BaSO}_4 \) and sodium chloride \( \mathrm{NaCl} \). The chemical reaction can be represented as: \[ \mathrm{BaCl}_2 (aq) + \mathrm{Na}_2 \mathrm{SO}_4 (aq) \rightarrow \mathrm{BaSO}_4 (s) + 2 \mathrm{NaCl} (aq) \] In this reaction, \( \mathrm{BaSO}_4 \) is an insoluble precipitate, while \( \mathrm{NaCl} \) remains in solution.

Step 2: Write the Balanced Equation

The balanced equation for the reaction between \( \mathrm{BaCl}_2 \) and \( \mathrm{Na}_2 \mathrm{SO}_4 \) is: \[ \mathrm{BaCl}_2 (aq) + \mathrm{Na}_2 \mathrm{SO}_4 (aq) \rightarrow \mathrm{BaSO}_4 (s) + 2 \mathrm{NaCl} (aq) \] This equation shows one molecules of barium chloride reacting with one molecule of sodium sulfate to produce barium sulfate and two molecules of sodium chloride.

Step 3: Explain Initial Cloudiness

The solution becomes cloudy because \( \mathrm{BaSO}_4 \) begins to form as a precipitate as soon as the solutions are mixed. This precipitate scatters light, giving the solution a cloudy appearance.

Step 4: Describe Changes over Time

Over a few days, the particles of \( \mathrm{BaSO}_4 \) slowly settle at the bottom of the beaker due to gravity. This leads to the formation of a clear solution above the accumulated white solid precipitate, which is \( \mathrm{BaSO}_4 \). The clarity of the liquid indicates that most of the solid particles have settled.

Key Concepts

Double Displacement Reaction

In chemistry, a double displacement reaction is a particular type of chemical reaction where two compounds exchange components. It usually involves the exchange of ions between two reacting compounds. When solutions of barium chloride (\(\mathrm{BaCl}_2\)) and sodium sulfate (\(\mathrm{Na}_2 \mathrm{SO}_4\)) are mixed, a double displacement reaction occurs. In this process:

• The barium ion (\(\mathrm{Ba}^{2+}\)) from barium chloride swaps places with the sodium ion (\(\mathrm{Na}^{+}\)) from sodium sulfate.
• This exchange results in the formation of barium sulfate (\(\mathrm{BaSO}_4\)) and sodium chloride (\(\mathrm{NaCl}\)).

Double displacement reactions are fundamental in forming precipitates, such as barium sulfate in this example.

Barium Sulfate

Barium sulfate (\(\mathrm{BaSO}_4\)) plays a critical role in this reaction. It is an insoluble compound, which means it does not dissolve in water. This insolubility leads to it forming a solid precipitate. When it precipitates out of the solution, the liquid becomes cloudy due to the suspended particles.After some time, gravity causes these particles to settle, forming a white solid at the bottom. You might often see this in laboratory experiments where precipitate formation is essential for substance identification and separation. Barium sulfate, due to its insolubility, is commonly used in various industries, including medical imagining as a contrast agent in X-rays.

Chemical Equation

The chemical equation for the reaction between barium chloride and sodium sulfate is a balanced representation of this double displacement reaction. It is written as:\[\mathrm{BaCl}_2 (aq) + \mathrm{Na}_2 \mathrm{SO}_4 (aq) \rightarrow \mathrm{BaSO}_4 (s) + 2 \mathrm{NaCl} (aq)\]This equation implies:

• One molecule of barium chloride reacts with one molecule of sodium sulfate.
• It produces one molecule of the insoluble precipitate, barium sulfate, and two molecules of sodium chloride, which remain dissolved.

Balanced equations are a vital tool in chemistry for ensuring matter is conserved and for providing insights into reaction dynamics. Understanding the formation and ratios of products in a reaction is essential in both academic and industrial chemical applications.

Solubility Rules

Solubility rules are guidelines that predict whether a particular ionic compound will dissolve in water. These rules help chemists understand and predict solubility behavior in reactions, like in the case of forming barium sulfate:

• Compounds containing sodium ions (\(\mathrm{Na}^{+}\)) are generally soluble in water, explaining why sodium chloride remains dissolved.
• Sulfates (\(\mathrm{SO}_4^{2-}\)) are generally soluble but with exceptions, including barium sulfate which is insoluble.

These predictions aid in anticipating the formation of precipitates and designing chemical processes efficiently. By understanding which compounds are soluble, chemists can control reactions to achieve desired products, as seen when barium sulfate precipitates from the mixture.