Problem 12 - Chemistry and Chemical Reactivity

Question

Predict the products of each precipitation reaction. Balance the completed equation, and then write the net ionic equation. (a) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{KBr}(\mathrm{aq}) \longrightarrow ?\) (b) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{KF}(\mathrm{aq}) \longrightarrow ?\) (c) \(\operatorname{Ca}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{C}_{2} \mathrm{O}_{4}(\mathrm{aq}) \longrightarrow ?\)

Step-by-Step Solution

Answer

(a) \( \mathrm{PbBr_2(s)} \), (b) \( \mathrm{CaF_2(s)} \), (c) \( \mathrm{CaC_2O_4(s)} \) are precipitates.

Step 1: Identify the Products of the Reactions

For each given reaction, use the concept of double displacement to identify the potential products which may be formed when the solutions mix. (a) Mixing \( \mathrm{Pb(NO_3)_2}(aq) \) and \( \mathrm{KBr}(aq) \) leads to potential products: \( \mathrm{PbBr_2} \) and \( \mathrm{KNO_3} \).(b) Mixing \( \mathrm{Ca(NO_3)_2}(aq) \) and \( \mathrm{KF}(aq) \) the possible products are \( \mathrm{CaF_2} \) and \( \mathrm{KNO_3} \).(c) Mixing \( \mathrm{Ca(NO_3)_2}(aq) \) and \( \mathrm{Na_2C_2O_4}(aq) \), the potential products are \( \mathrm{CaC_2O_4} \) and \( \mathrm{NaNO_3} \).

Step 2: Determine Precipitation

Check the solubility of the products from Step 1 to determine if any of them form a precipitate.(a) \( \mathrm{PbBr_2} \) forms a precipitate as it is insoluble in water.(b) \( \mathrm{CaF_2} \) is insoluble in water and forms a precipitate.(c) \( \mathrm{CaC_2O_4} \) forms a precipitate as it is typically insoluble.

Step 3: Write and Balance the Complete Molecular Equations

Arrange and balance the complete chemical equations by combining the reactants and the identified precipitate products.(a) \[ \mathrm{Pb(NO_3)_2(aq) + 2KBr(aq) \rightarrow PbBr_2(s) + 2KNO_3(aq)} \](b) \[ \mathrm{Ca(NO_3)_2(aq) + 2KF(aq) \rightarrow CaF_2(s) + 2KNO_3(aq)} \](c) \[ \mathrm{Ca(NO_3)_2(aq) + Na_2C_2O_4(aq) \rightarrow CaC_2O_4(s) + 2NaNO_3(aq)} \]

Step 4: Write the Net Ionic Equations

Write the net ionic equations by removing the spectator ions from the balanced molecular equations.(a) \[ \mathrm{Pb^{2+}(aq) + 2Br^-(aq) \rightarrow PbBr_2(s)} \](b) \[ \mathrm{Ca^{2+}(aq) + 2F^-(aq) \rightarrow CaF_2(s)} \](c) \[ \mathrm{Ca^{2+}(aq) + C_2O_4^{2-}(aq) \rightarrow CaC_2O_4(s)} \]

Key Concepts

Double Displacement Reactions

In chemistry, double displacement reactions play a significant role in understanding different chemical processes. These reactions involve the exchange of ions between two reacting compounds to form two new compounds. Imagine a dance where two pairs switch partners. Here, the cations (positively charged ions) and anions (negatively charged ions) swap places.
This can be thought of as occurring in two steps:

• First, the cations and anions in the initial compounds are identified.
• Next, the ions switch places to form new compounds.

In the provided exercise:

• When \( \mathrm{Pb(NO_3)_2} \) and \( \mathrm{KBr} \) are mixed, \( \mathrm{PbBr_2} \) and \( \mathrm{KNO_3} \) are formed.
• For \( \mathrm{Ca(NO_3)_2} \) and \( \mathrm{KF} \), the products are \( \mathrm{CaF_2} \) and \( \mathrm{KNO_3} \).
• Mixing \( \mathrm{Ca(NO_3)_2} \) and \( \mathrm{Na_2C_2O_4} \) results in \( \mathrm{CaC_2O_4} \) and \( \mathrm{NaNO_3} \).

Double displacement reactions often yield a product that is a gas, precipitate, or a weak electrolyte, like water.

Solubility Rules

Solubility rules are essential guidelines that chemists use to predict whether a substance will dissolve in water. Understanding these rules helps us determine if a double displacement reaction will produce a solid precipitate or remain soluble.
Solubility rules generally tell us:

• Nitrates (\( \mathrm{NO_3^-} \)) are always soluble.
• Compounds containing alkali metal ions (like \( \mathrm{K^+} \)) and ammonium (\( \mathrm{NH_4^+} \)) are typically soluble.
• Most chlorides, bromides, and iodides are soluble, except those of lead (\( \mathrm{Pb^{2+}} \)), silver (\( \mathrm{Ag^+} \)), and mercury (\( \mathrm{Hg_2^{2+}} \)).
• Carbonates and phosphates are usually insoluble except when paired with alkali metals or ammonium.

In the reactions we've looked at:

• \( \mathrm{PbBr_2} \) is insoluble, forming a precipitate.
• \( \mathrm{CaF_2} \) is insoluble, forming a precipitate.
• \( \mathrm{CaC_2O_4} \) is insoluble and also precipitates out of solution.

Knowing which compounds will form precipitates is crucial in predicting the outcome of reactions.

Net Ionic Equations

Net ionic equations provide a simplified way to represent chemical reactions. They highlight only the ions and molecules directly involved in the chemical change, removing the spectator ions that do not participate.
To write a net ionic equation:

• Start with a balanced chemical equation for the reaction.
• Identify the ions present in the solution on both sides of the reaction.
• Eliminate the spectator ions that appear unchanged on both sides of the equation.

This process leaves you with the simplest representation of the actual chemical change. For example:

• For the reaction between \( \mathrm{Pb(NO_3)_2} \) and \( \mathrm{KBr} \): The net ionic equation is \( \mathrm{Pb^{2+}(aq) + 2Br^-(aq) \rightarrow PbBr_2(s)} \).
• In the case of \( \mathrm{Ca(NO_3)_2} \) and \( \mathrm{KF} \): \( \mathrm{Ca^{2+}(aq) + 2F^-(aq) \rightarrow CaF_2(s)} \).
• For \( \mathrm{Ca(NO_3)_2} \) and \( \mathrm{Na_2C_2O_4} \): \( \mathrm{Ca^{2+}(aq) + C_2O_4^{2-}(aq) \rightarrow CaC_2O_4(s)} \).

Net ionic equations help students and chemists by focusing on the essence of the reaction, making it easier to understand and analyze the transformation occurring.