Problem 54 - Introductory Chemistry Atoms First

Question

When an aqueous solution of ammonium sulfate is added to an aqueous solution of calcium nitrate, a precipitate forms. (a) Write a net ionic equation for the precipitation. (b) Write the precipitation reaction in a way that emphasizes that a double- replacement reaction has taken place.

Step-by-Step Solution

Answer

(a) The net ionic equation for the precipitation is: \(Ca^{2+} (aq) + SO_4^{2-} (aq) \rightarrow CaSO_4 (s)\) (b) The precipitation reaction emphasizing a double-replacement reaction is: \(2NH_4^+ (aq) + SO_4^{2-} (aq) + Ca^{2+} (aq) + 2NO_3^- (aq) \rightarrow Ca^{2+} (aq) + SO_4^{2-} (aq) + 2NH_4^+ (aq) + 2NO_3^- (aq)\)

Step 1: Write the balanced molecular equation

Write the balanced molecular equation for the reaction between ammonium sulfate and calcium nitrate. Ammonium sulfate is (NH4)2SO4 and calcium nitrate is Ca(NO3)2. This reaction generates calcium sulfate (CaSO4) and ammonium nitrate (NH4NO3). The balanced molecular equation for the reaction is: (NH4)2SO4 (aq) + Ca(NO3)2 (aq) -> CaSO4 (s) + 2 NH4NO3 (aq)

Step 2: Break the molecular equation into ions

Now, we need to break up the molecular equation into its ionic components: 2 NH4+ (aq) + SO4^2- (aq) + Ca^2+ (aq) + 2 NO3- (aq) -> CaSO4 (s) + 2 NH4+ (aq) + 2 NO3- (aq)

Step 3: Write the net ionic equation

First, remove the spectator ions which are the ions that appear on both sides of the equation. In this case, 2 NH4+ (aq) and 2 NO3- (aq) are spectator ions. By removing them, we get the net ionic equation: Ca^2+ (aq) + SO4^2- (aq) -> CaSO4 (s)

Step 4: Write the precipitation reaction for a double-replacement reaction

To emphasize the double-replacement aspect of the reaction, we will rewrite the molecular equation using the ions. This will demonstrate that the ions from the reactants are swapping places to form the products: 2NH4+ (aq) + SO4^2- (aq) + Ca^2+ (aq) + 2NO3- (aq) -> Ca^2+ (aq) + SO4^2- (aq) + 2NH4+ (aq) + 2NO3- (aq) This is our double-replacement reaction.

Key Concepts

Precipitation Reaction

A precipitation reaction is a type of chemical reaction that results in the formation of an insoluble product, called a precipitate, from the reaction of two soluble reactants. Precipitation reactions often occur when two ionic compounds in aqueous solutions are mixed together, leading to the exchange of ions and the formation of a solid.
In the given exercise, when ammonium sulfate \[(\text{NH}_4)_2\text{SO}_4\] and calcium nitrate \[\text{Ca(NO}_3)_2\] are combined in water, they form calcium sulfate \[\text{CaSO}_4\], a solid precipitate. This happens because calcium sulfate is not soluble in water, thus it separates from the solution as a solid.

• Key characteristic: formation of a solid precipitate
• Result: insoluble product from soluble reactants

Understanding precipitation reactions helps in predicting the outcomes of chemical reactions in solutions and is crucial in fields such as chemistry, environmental science, and materials science.

Double-Replacement Reaction

A double-replacement reaction, also known as a metathesis reaction, involves the exchange of ions between two compounds, leading to the formation of two new compounds. This type of reaction is characterized by the swapping of components between reactants.
In the exercise, this can be seen when ammonium sulfate \[(\text{NH}_4)_2\text{SO}_4\] reacts with calcium nitrate \[\text{Ca(NO}_3)_2\]. In the balanced equation, the ammonium \((\text{NH}_4^+)\) and sulfate \((\text{SO}_4^{2-})\) ions exchange places with calcium \((\text{Ca}^{2+})\) and nitrate \((\text{NO}_3^-)\) ions, forming calcium sulfate \((\text{CaSO}_4)\) and ammonium nitrate \((\text{NH}_4\text{NO}_3)\) in the process.

• Key characteristic: exchange or swapping of ions between reactants
• Outcome: formation of two different products

This type of reaction is common in various chemical processes, including those used in industrial and environmental applications.

Spectator Ions

Spectator ions are ions that do not participate in the actual chemical reaction. They remain unchanged and in solution throughout the reaction. These ions are found on both the reactant and product sides of a chemical equation, and do not contribute to the formation of the precipitate.
In our example, when ammonium sulfate and calcium nitrate react, the ammonium \((\text{NH}_4^+)\) and nitrate \((\text{NO}_3^-)\) ions appear on both sides of the ionic equation, hence they are marked as spectator ions.
This allows us to simplify the equation by removing them, leading to the net ionic equation focused solely on the ions directly involved:
\[\text{Ca}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{CaSO}_4(\text{s})\]

• Not involved in reaction's main change
• Key for simplifying with net ionic equations

Understanding spectator ions is important for simplifying reactions and focusing on the components that are actively changing in a reaction.