Question

As with all quantitative problems in chemistry, make sure not to get "lost in the math." In particular, work on visualizing solutions at a molecular level. For example, consider the following. You have two separate beakers with aqueous solutions, one with 4 "units" of potassium sulfate and one with 3 "units" of barium nitrate. a. Draw molecular-level diagrams of both solutions. b. Draw a molecular-level diagram of the mixture of the two solutions before a reaction has taken place. c. Draw a molecular-level diagram of the product and solution formed after the reaction has taken place.

Short answer

The molecular-level diagrams for the solutions and mixture before the reaction are drawn as follows: 1. Potassium sulfate (K₂SO₄) solution consists of four units with each unit composed of two potassium ions (K⁺) and one sulfate ion (SO₄²⁻). 2. Barium nitrate (Ba(NO₃)₂) solution has three units with each unit composed of one barium ion (Ba²⁺) and two nitrate ions (NO₃⁻). 3. The mixture before the reaction consists of both potassium ions (K⁺), sulfate ions (SO₄²⁻), barium ions (Ba²⁺), and nitrate ions (NO₃⁻), all represented separately in an aqueous solution. Upon reaction, a double displacement Reaction occurs producing potassium nitrate (KNO₃) and barium sulfate (BaSO₄). The final molecular diagram shows potassium ions (K⁺) and nitrate ions (NO₃⁻) in the solution forming potassium nitrate (KNO₃), while barium ions (Ba²⁺) and sulfate ions (SO₄²⁻) form barium sulfate (BaSO₄) precipitates.

Step-by-step solution

Identifying the compounds in the solutions

To draw the molecular-level diagram of each solution, firstly we need to identify the compounds present in them. Here, we have potassium sulfate (K₂SO₄) in one beaker and barium nitrate (Ba(NO₃)₂) in the other.

Drawing the molecular-level diagram for potassium sulfate solution

In the potassium sulfate solution, there are four units of potassium sulfate (K₂SO₄). Each molecule is composed of two potassium ions (K⁺) and one sulfate ion (SO₄²⁻). To draw the molecular-level diagram, we will represent the potassium ions with positively charged small circles (e.g., K⁺), and sulfate ions with negatively charged larger circles (e.g., SO₄²⁻). Place the four potassium sulfate units in the solution and show the interaction between the positively charged potassium ions and the negatively charged sulfate ions.

Drawing the molecular-level diagram for barium nitrate solution

In the barium nitrate solution, there are three units of barium nitrate (Ba(NO₃)₂). Each molecule is composed of one barium ion (Ba²⁺) and two nitrate ions (NO₃⁻). To draw the molecular-level diagram, we'll represent the barium ions with positively charged larger circles (e.g., Ba²⁺), and nitrate ions with negatively charged smaller circles (e.g., NO₃⁻). Place the three barium nitrate units in the solution and show the interaction between the positively charged barium ions and the negatively charged nitrate ions.

Drawing the molecular-level diagram of the mixture before the reaction

Now we'll mix both solutions. In this step, we draw a diagram of the mixture before the reaction takes place. Combine the potassium sulfate and barium nitrate solutions, so that both potassium ions (K⁺) and sulfate ions (SO₄²⁻) from potassium sulfate are mixed with barium ions (Ba²⁺) and nitrate ions (NO₃⁻) from barium nitrate. Separate the ions from each other to show that they are in an aqueous solution, which means they are surrounded by water molecules.

Reaction between potassium sulfate and barium nitrate

When we mix the solutions, we have a double displacement reaction taking place. The reaction equation can be written as: \[K_2SO_4(aq) + Ba(NO_3)_2(aq) \rightarrow 2KNO_3(aq) + BaSO_4(s)\] In this reaction, potassium ions (K⁺) will bond with nitrate ions (NO₃⁻) to form potassium nitrate (KNO₃), which will remain in an aqueous solution, and barium ions (Ba²⁺) will bond with sulfate ions (SO₄²⁻) to form solid barium sulfate (BaSO₄), which will precipitate out of the solution.

Drawing the molecular-level diagram of the product and solution formed after the reaction

In this last step, we draw the molecular-level diagram after the reaction has taken place. In the solution, we have potassium nitrate (KNO₃), which is composed of potassium ions (K⁺) and nitrate ions (NO₃⁻). Represent them with their respective charges in the aqueous solution. The solid formed after the reaction is barium sulfate (BaSO₄), composed of barium ions (Ba²⁺) and sulfate ions (SO₄²⁻). Represent them together with interaction between the positive and negative ions and show it as a precipitate which is not dissolved in the solution.

Key concepts

Molecular-Level Diagrams

Molecular-level diagrams are visual representations of molecules and ions in a solution that help us understand the interactions between different chemicals at a microscopic scale. These diagrams show how ions are arranged and how they interact with each other in a solution. For example, in a potassium sulfate (\( K_2SO_4 \)) solution, two potassium ions (\( K^+ \)) can be depicted as small positively charged circles while the sulfate ion (\( SO₄^{2-} \)) could be a larger negatively charged circle. Similarly, in a barium nitrate (\( Ba(NO_3)_2 \)) solution, one barium ion (\( Ba^{2+} \)) is a larger positive circle and the nitrate ions (\( NO_3^- \)) are smaller negative circles. By visualizing these interactions, students can better grasp the concept of ionic solutions and the specificity of ionic interactions.

Aqueous Solutions

An aqueous solution is a type of solution in which the solvent is water. This means that the solute (the substances being dissolved) exists in a dissociated form as separate ions. For instance, when potassium sulfate (\( K_2SO_4 \)) is dissolved in water, it dissociates into potassium ions (\( K^+ \)) and sulfate ions (\( SO₄^{2-} \)). Similarly, barium nitrate (\( Ba(NO_3)_2 \)) in water dissociates into barium ions (\( Ba^{2+} \)) and nitrate ions (\( NO_3^- \)).

In such solutions, the water molecules interact with the ions and help keep them apart, allowing them to float freely in the solution. This interaction is crucial as it helps facilitate the movement of ions and potential reactions with other dissolved species without the interference of the strong ionic bonds they are initially a part of.

Double Displacement Reaction

A double displacement reaction, or metathesis reaction, involves the exchange of ions between two compounds to form new compounds. In the exercise presented, barium nitrate (\( Ba(NO_3)_2 \)) and potassium sulfate (\( K_2SO_4 \)) react in this manner. The general formula is: \[ AB + CD \rightarrow AD + CB \] In our example, \[ K_2SO_4(aq) + Ba(NO_3)_2(aq) \rightarrow 2KNO_3(aq) + BaSO_4(s) \] The barium (Ba²⁺) swaps partners with the sulfate (SO₄²⁻), forming a new compound, barium sulfate, and potassium (K⁺) swaps with nitrate (NO₃⁻) to form potassium nitrate. This reaction's main driving force is the formation of \( BaSO₄ \), an insoluble precipitate, which further solidifies the product's formation by removing it from the aqueous mixture.

Precipitation in Chemistry

Precipitation in chemistry refers to the process of forming an insoluble solid when two aqueous solutions are mixed. During a chemical reaction, the formation of a precipitate indicates that an insoluble product has been generated, typically falling out of the solution and forming a solid residue at the bottom. In the discussed double displacement reaction, barium sulfate (\( BaSO_4 \)) is the precipitate.

This occurs because barium sulfate is not soluble in water, unlike potassium nitrate (\( KNO_3 \)), which remains dissolved. The formation of a precipitate is fundamental to understanding the outcomes of chemical reactions, especially in laboratory and industrial settings. Observing this process helps confirm that a chemical reaction has occurred, and it's also an excellent tool for purifying compounds by removing unwanted ions from solutions.