Problem 24 - Chemistry: The Central Science in SI Units, Global Edition

Question

Identify the precipitate (if any) that forms when the following solutions are mixed, and write a balanced equation for each reaction. (a) \(\mathrm{NH}_{4} \mathrm{I}\) and \(\mathrm{CuCl}_{2},\) (b) LiOH and \(\mathrm{MnCl}_{2}\), (c) \(\mathrm{K}_{3} \mathrm{PO}_{4}\) and \(\mathrm{CoSO}_{4}\)

Step-by-Step Solution

Answer

(a) Precipitate: CuI; Balanced equation: \[ 2 NH_4I + CuCl_2 \rightarrow 2 NH_4Cl + CuI \] (b) Precipitate: Mn(OH)2; Balanced equation: \[ 2 LiOH + MnCl_2 \rightarrow 2 LiCl + Mn(OH)_2 \] (c) Precipitate: Co3(PO4)2; Balanced equation: \[ 2 K_3PO_4 + 3 CoSO_4 \rightarrow 6 K_2SO_4 + Co_3(PO_4)_2 \]

Step 1: (a) Identify the ions in the first pair of solutions

The ions in the first solutions are NH4+ (ammonium) and I- (iodide) from NH4I, and Cu2+ (copper) and Cl- (chloride) from CuCl2.

Step 2: (a) Predict the products of possible precipitation reactions

The possible products of the reaction are: 1. NH4Cl (formed from NH4+ and Cl- ions) 2. CuI (formed from Cu2+ and I- ions)

Step 3: (a) Determine the solubility of the predicted products

Using solubility rules: 1. NH4Cl is soluble because it contains ammonium (NH4+). 2. CuI is moderately soluble, meaning it could still form a precipitate in some conditions. After considering solubility, CuI is the potential precipitate.

Step 4: (a) Write a balanced chemical equation for the reaction

The balanced chemical equation for the reaction is: \[2 NH_4I + CuCl_2 \rightarrow 2 NH_4Cl + CuI\]

Step 5: (b) Identify the ions in the second pair of solutions

The ions in the second solutions are Li+ (lithium) and OH- (hydroxide) from LiOH, and Mn2+ (manganese) and Cl- (chloride) from MnCl2.

Step 6: (b) Predict the products of possible precipitation reactions

The possible products of the reaction are: 1. LiCl (formed from Li+ and Cl- ions) 2. Mn(OH)2 (formed from Mn2+ and OH- ions)

Step 7: (b) Determine the solubility of the predicted products

Using solubility rules: 1. LiCl is soluble because it contains a Group 1 element (Li). 2. Mn(OH)2 is insoluble because it is a hydroxide salt. After considering solubility, Mn(OH)2 is the precipitate.

Step 8: (b) Write a balanced chemical equation for the reaction

The balanced chemical equation for the reaction is: \[2 LiOH + MnCl_2 \rightarrow 2 LiCl + Mn(OH)_2\]

Step 9: (c) Identify the ions in the third pair of solutions

The ions in the third solutions are K+ (potassium) and PO4^3- (phosphate) from K3PO4, and Co2+ (cobalt) and SO4^2- (sulfate) from CoSO4.

Step 10: (c) Predict the products of possible precipitation reactions

The possible products of the reaction are: 1. K2SO4 (formed from K+ and SO4^2- ions) 2. Co3(PO4)2 (formed from Co2+ and PO4^3- ions)

Step 11: (c) Determine the solubility of the predicted products

Using solubility rules: 1. K2SO4 is soluble because it contains a Group 1 element (K). 2. Co3(PO4)2 is insoluble because it is a phosphate salt. After considering solubility, Co3(PO4)2 is the precipitate.

Step 12: (c) Write a balanced chemical equation for the reaction

The balanced chemical equation for the reaction is: \[2 K_3PO_4 + 3 CoSO_4 \rightarrow 6 K_2SO_4 + Co_3(PO_4)_2\]

Key Concepts

Solubility Rules

Solubility rules are essential guidelines that help predict whether a salt will dissolve in water. They simplify the process of determining the outcome of mixing different ionic compounds.
These rules state that:

• Compounds containing alkali metals (Group 1 elements) like lithium, sodium, or potassium, are generally soluble.
• Ammonium compounds, such as those containing NH₄⁺, are soluble.
• Nitrates (NO₃^-) and acetates (CH₃COO^-) also tend to dissolve in water.
• Most chlorides, bromides, and iodides are soluble, except when paired with silver, lead, or mercury.
• Hydroxides are generally insoluble, except for those involving Group 1 elements and some Group 2 elements like barium or strontium.
• Phosphates, carbonates, and sulfides usually form precipitates, except when they are paired with Group 1 elements or ammonium.

Applying these rules helps identify potential precipitates, such as the CuI precipitate from NH₄I and CuCl₂, Mn(OH)₂ from LiOH and MnCl₂, and Co₃(PO₄)₂ from K₃PO₄ and CoSO₄.

Balanced Chemical Equations

Balanced chemical equations are indispensable for representing chemical reactions accurately. They ensure that the number of atoms of each element is consistent on both the reactant and product sides. This is due to the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
Creating a balanced equation involves:

• Identifying the reactants and products involved in the reaction.
• Counting the number of each type of atom in the reactants and products.
• Using coefficients to balance the number of atoms on both sides of the equation.

For example, when mixing NH₄I and CuCl₂, the balanced equation is \(2 \text{NH}_4\text{I} + \text{CuCl}_2 \rightarrow 2 \text{NH}_4\text{Cl} + \text{CuI}\). Similarly, mixing LiOH and MnCl₂ results in \(2 \text{LiOH} + \text{MnCl}_2 \rightarrow 2 \text{LiCl} + \text{Mn(OH)}_2\). This process maintains balance and clarity in representing chemical transformations.

Ionic Compounds

Ionic compounds are a class of chemicals formed by the electrostatic attraction between oppositely charged ions. Typically, they are composed of metal cations and non-metal anions. The nature of this bond provides ionic compounds with unique properties.
The key characteristics of ionic compounds include:

• Formation of crystal lattice structures due to the regular repeating pattern of ions.
• High melting and boiling points due to strong ionic bonds.
• Solubility in polar solvents like water, where they often dissociate into their respective ions.
• Electrical conductivity in the melted state or when dissolved in a solvent, as free ions are present to carry electric current.

When NH₄I mixes with CuCl₂, each breaks into ionic components: NH₄⁺, I^-, Cu²⁺, and Cl^-. These ions can recombine to form different ionic compounds, which may precipitate if insoluble. Thus, ionic compounds are fundamental to understanding how precipitation reactions work and the resulting substances they can form.