Question

Consider the mixing of aqueous solutions of lead(II) nitrate and sodium iodide to form a solid. a. Name the possible products, and determine the formulas of these possible products. b. What is the precipitate? How do you know? c. Must the subscript for an ion in a reactant stay the same as the subscript of that ion in a product? Explain your answer.

Short answer

The possible products of the reaction between lead(II) nitrate (Pb(NO₃)₂) and sodium iodide (NaI) are lead(II) iodide (PbI₂) and sodium nitrate (NaNO₃). The precipitate is lead(II) iodide (PbI₂) as it is insoluble in water, according to solubility rules. The subscript for an ion in a reactant does not necessarily need to remain the same in the product, as it depends on the charge balance between the ions in the new compounds formed during the reaction.

Step-by-step solution

Write down the reactants

We are given the reactants as lead(II) nitrate and sodium iodide. Write their chemical formulas: Pb(NO₃)₂ and NaI.

Perform a double replacement reaction

In a double replacement reaction, the positive ions in each compound switch places. So, the products formed will be: 1. Lead(II) iodide (PbI₂) 2. Sodium nitrate (NaNO₃) So, the possible products are lead(II) iodide and sodium nitrate with the formulas PbI₂ and NaNO₃.

Identify the precipitate

We need to use solubility rules to determine which of the products is insoluble (precipitate). According to the solubility rules: 1. Most nitrate salts are soluble. 2. Most iodide salts are soluble, except for those of lead(II), silver, and mercury(I). From these rules, we can deduce that sodium nitrate (NaNO₃) is soluble in water, whereas lead(II) iodide (PbI₂) is insoluble. Therefore, the precipitate is lead(II) iodide (PbI₂).

Subscript in ions

The subscript for an ion in a reactant does not necessarily need to remain the same in the product. Subscripts in chemical formulas represent the ratio between the ions in the compound. When compounds interact with each other during a chemical reaction, the charge balance between the ions in the new compound must be established. This may lead to different subscripts in the products compared to the reactants as long as the overall charge in the new compound remains balanced. For instance, in our exercise, the reactants have subscripts of 2 for nitrate (NO₃¹⁻) and 1 for iodide (I⁻). However, the products formed have different subscripts for these ions (i.e., 1 for nitrate in sodium nitrate and 2 for iodide in lead(II) iodide) to maintain charge balance in the products.

Key concepts

Double Replacement Reaction

A double replacement reaction is a type of chemical reaction where two compounds exchange ions to form two new compounds. This kind of reaction typically occurs in aqueous solutions. The general formula for a double replacement reaction can be written as \( AB + CD \to AD + CB \), where the cations (positive ions) and anions (negative ions) switch places.

In the original exercise, the reactants—lead(II) nitrate \( \text{Pb(NO}_3\text{)}_2 \) and sodium iodide \( \text{NaI} \)—undergo a double replacement reaction. The lead(II) ions \( \text{Pb}^{2+} \) pair with iodide ions \( \text{I}^- \), and the sodium ions \( \text{Na}^+ \) pair with nitrate ions \( \text{NO}_3^- \), resulting in the formation of lead(II) iodide \( \text{PbI}_2 \) and sodium nitrate \( \text{NaNO}_3 \).

These reactions often result in the formation of a precipitate, a solid that emerges from the liquid solution.

Solubility Rules

Solubility rules help predict whether a compound that forms in aqueous solution will dissolve in water or remain as a solid (precipitate). Knowing these rules is crucial when determining the outcomes of double replacement reactions.

Here are some general solubility rules to keep in mind:

• Most nitrate (\( \text{NO}_3^- \)) salts are soluble in water.
• Alkali metal (\( \text{Na}^+, \text{K}^+, \text{etc.} \)) salts and ammonium (\( \text{NH}_4^+ \)) salts are soluble.
• Most chloride, bromide, and iodide salts are soluble, with notable exceptions for salts containing silver (Ag), lead(II) (\( \text{Pb}^{2+} \)), and mercury(I) (\( \text{Hg}_2^{2+} \)).

In the context of the exercise, these rules explain why sodium nitrate \( \text{NaNO}_3 \) is soluble, while lead(II) iodide \( \text{PbI}_2 \) is not soluble in water, as it contains lead iodide which is generally insoluble.

Chemical Formulas

Chemical formulas are symbolic representations of compounds, indicating the elements present and their relative ratios. These formulas are vital for understanding the composition and function of compounds in reactions.

For example, lead(II) nitrate \( \text{Pb(NO}_3\text{)}_2 \) consists of one lead (\( \text{Pb} \)) ion and two nitrate (\( \text{NO}_3^- \)) ions. Similarly, sodium iodide \( \text{NaI} \) is made up of one sodium (\( \text{Na}^+ \)) ion and one iodide (\( \text{I}^- \)) ion.

In chemical reactions, especially those involving a double replacement, the formulas of reactants and products can differ as ions recombine. Keeping an eye on the charges and how they balance out helps in correctly writing formulas for new compounds.

During reactions, it is essential to correctly write and balance chemical formulas to ensure that the charge balance and stoichiometry (ratio of reactants to products) of the reaction are maintained.

Precipitate Identification

Precipitate identification is a critical part of analyzing chemical reactions, especially double replacement reactions. A precipitate is an insoluble solid that emerges from a liquid solution during a reaction.

The formation of a precipitate can be predicted using solubility rules. In the given exercise, when lead(II) nitrate \( \text{Pb(NO}_3\text{)}_2 \) reacts with sodium iodide \( \text{NaI} \), lead(II) iodide \( \text{PbI}_2 \) forms as an insoluble solid, visible as a yellow precipitate.

To identify a precipitate:

• Write down the chemical reaction and possible products.
• Apply the solubility rules to determine which products are insoluble in water.
• Look for any visible changes, such as color or formation of solid particles in the solution.

Understanding which compounds form precipitates in reactions is helpful for predicting the outcomes of chemical experiments and real-world applications, such as waste treatment and mineral recovery.