Problem 23 - General Chemistry

Question

You come across a beaker that contains water, aqueous ammonium acetate, and a precipitate of calcium phosphate. a. Write the balanced molecular equation for a reaction between two solutions containing ions that could produce this solution. b. Write the complete ionic equation for the reaction in part a. c. Write the net ionic equation for the reaction in part a.

Step-by-Step Solution

Answer

a. \(3Ca(NO_3)_2 + 2Na_3PO_4 \rightarrow Ca_3(PO_4)_2 + 6NaNO_3\) b. \(3Ca^{2+} + 6NO_3^- + 6Na^+ + 2PO_4^{3-} \rightarrow Ca_3(PO_4)_2 + 6Na^+ + 6NO_3^-\) c. \(3Ca^{2+} + 2PO_4^{3-} \rightarrow Ca_3(PO_4)_2\).

Step 1: Identify potential reactants

The problem mentions that the beaker contains aqueous ammonium acetate and a precipitate of calcium phosphate. Therefore, potential reactants might include solutions containing phosphate ions and calcium ions. A possible pair of reactants could be calcium nitrate \[(Ca(NO_3)_2)\] and sodium phosphate \[(Na_3PO_4)\], as they contain the necessary ions to create these products.

Step 2: Write the balanced molecular equation

The balanced molecular equation for the reaction between calcium nitrate and sodium phosphate, yielding the precipitate calcium phosphate and soluble sodium nitrate, is:\[ 3Ca(NO_3)_2(aq) + 2Na_3PO_4(aq) \rightarrow Ca_3(PO_4)_2(s) + 6NaNO_3(aq) \]

Step 3: Write the complete ionic equation

For the complete ionic equation, we need to separate all the strong electrolytes into their constituent ions:\[ 3Ca^{2+}(aq) + 6NO_3^-(aq) + 6Na^+(aq) + 2PO_4^{3-}(aq) \rightarrow Ca_3(PO_4)_2(s) + 6Na^+(aq) + 6NO_3^-(aq) \]

Step 4: Write the net ionic equation

To write the net ionic equation, remove the spectator ions (ions that appear on both sides of the equation and do not participate in the reaction). The spectator ions here are \(Na^+\) and \(NO_3^-\). Thus, the net ionic equation is:\[ 3Ca^{2+}(aq) + 2PO_4^{3-}(aq) \rightarrow Ca_3(PO_4)_2(s) \]

Key Concepts

Molecular Equations

In chemical reactions involving ionic compounds, a molecular equation showcases the reactants and products in their molecular form. This means it presents the compounds as if they are molecules, regardless of whether they exist as separate ions in the solution.
For the reaction between calcium nitrate and sodium phosphate, the molecular equation is displayed as follows:

• \[3Ca(NO_3)_2(aq) + 2Na_3PO_4(aq) \rightarrow Ca_3(PO_4)_2(s) + 6NaNO_3(aq)\]
  This equation shows calcium and phosphate ions forming solid calcium phosphate, while the sodium and nitrate ions stay in solution as sodium nitrate.

Molecular equations are essential as they provide the complete picture of the reactants and products, and are especially useful for understanding the overall stoichiometry of the reaction. This kind of equation helps in ensuring all atoms are accounted for on both sides of the equation.

Ionic Equations

Ionic equations breakdown the molecular equation further by showing all aqueous ions separately. This is because in aqueous solutions, many compounds dissociate into ions. For the given reaction, the ionic equation presents all reactants and products as ions, except the solid precipitate.
Here is the ionic equation for the reaction:

• \[3Ca^{2+}(aq) + 6NO_3^-(aq) + 6Na^+(aq) + 2PO_4^{3-}(aq) \rightarrow Ca_3(PO_4)_2(s) + 6Na^+(aq) + 6NO_3^-(aq)\]

This extended version allows us to see the actual positions and movements of the ions during the reaction. Notice how all ions are shown, revealing which ions participate in forming the solid and which remain dissolved in the water. Understanding ionic equations is pivotal as they provide a more granular look at the chemical reaction than molecular equations.

Net Ionic Equations

A net ionic equation simplifies the ionic equation by excluding the spectator ions, which are ions present on both sides of the equation that do not engage directly in the reaction. Removing these ions focuses on the chemical change that occurs.
For this reaction, the net ionic equation is:

• \[3Ca^{2+}(aq) + 2PO_4^{3-}(aq) \rightarrow Ca_3(PO_4)_2(s)\]

Spectator ions, here being sodium ions \((Na^+)\) and nitrate ions \((NO_3^-)\), are left out, highlighting the formation of the insoluble calcium phosphate from calcium and phosphate ions. Net ionic equations are extremely beneficial, as they zero in on the core transformation happening during a chemical reaction, making it easier to grasp what's truly occurring. They are especially useful when analyzing reactions that result in precipitates.

Precipitation Reactions

Precipitation reactions involve the formation of a solid from the reaction of two aqueous solutions. This solid is called a precipitate, and it forms when ions in solution combine to create an insoluble compound.
In the presented scenario, calcium and phosphate ions react to form calcium phosphate:

• \[Ca_3(PO_4)_2(s)\]

For a precipitation reaction to occur, the product must be less soluble in water than the reactants. The outcome of these reactions can often be predicted using solubility rules, which help determine whether an ionic compound will precipitate. Understanding precipitation reactions is crucial for predicting the behavior of substances when they are mixed, as well as in applications such as water treatment or in the field of geology.

Aqueous Solutions

Aqueous solutions play a significant role in chemistry as they are solutions in which the solvent is water. When an ionic compound dissolves in water, it dissociates into its ions, which become surrounded by water molecules.
This process is essential in the reaction between calcium nitrate and sodium phosphate, allowing the ions to mix and react:

• The dissolved ions include:
  \(Ca^{2+}\), \(NO_3^-\), \(Na^+\), and \(PO_4^{3-}\).

Aqueous solutions are important because they facilitate the movement and reaction of ions, enabling many types of reactions, including the formation of precipitates. Additionally, they are integral in many biological systems and chemical industries, making them a foundational concept in both academic and practical chemistry.