Problem 103 - General Chemistry: Principles and Modern Applications

Question

Saturated solutions of sodium phosphate, copper(II) chloride, and ammonium acetate are mixed together. The precipitate is (a) copper(II) acetate; (b) copper(II) phosphate; (c) sodium chloride; (d) ammonium phosphate; (e) nothing precipitates.

Step-by-Step Solution

Answer

The precipitate that forms when saturated solutions of sodium phosphate, copper(II) chloride, and ammonium acetate are mixed together is copper(II) phosphate.

Step 1: Identify Potential Products

When solutions are mixed, the ions can potentially rearrange to form different compounds. In this case, by exchanging the cations and anions of the original compounds, one can predict the potential products: copper(II) phosphate, copper(II) acetate, sodium chloride, and ammonium phosphate.

Step 2: Check Solubility Rules

Consult a solubility rules chart. These rules provide information on which ions form soluble compounds. Most notably: (1) salts containing Na+, NH4+, and NO3- are typically soluble; (2) salts containing Cl-, Br-, and I- are typically soluble, apart from when these anions are combined with Ag+, Pb2+, Hg2+; (3) salts containing PO4 3-, OH-, S2-, CO3 2-, CrO4 2- are generally not soluble, apart from when these anions are combined with the cations mentioned in rule 1.

Step 3: Apply Solubility Rules to Potential Products

Apply the rules to the potential products: (1) Copper(II) phosphate is not soluble because phosphates are generally not soluble apart from when combined with Na+ or NH4+; (2) Copper(II) acetate is soluble because acetates are generally soluble; (3) Sodium chloride is soluble as per rule 2; (4) Ammonium phosphate is soluble as per rule 1.

Step 4: Identify the Precipitate

The precipitate will be the product that is not soluble in water. In this case, the precipitate is therefore copper(II) phosphate.

Key Concepts

Solubility Rules

Understanding solubility rules is vital for predicting the outcome when various ionic compounds are mixed in solutions. These rules provide a guideline on the likelihood that a compound will dissolve in water, which is directly related to its solubility.

Generally speaking, solubility rules state that most nitrate (NO3-) salts, acetate (CH3COO-) salts, and salts containing the alkali metal ions (such as sodium - Na+ and potassium - K+) as well as ammonium (NH4+) are soluble in water. This means that when they are in solution, they will dissociate into their respective ions freely.

Conversely, compounds containing certain ions like phosphate (PO4 3-), carbonate (CO3 2-), sulfide (S2-), and hydroxide (OH-) are usually insoluble, with exceptions existing for combinations with the previously mentioned soluble cations. When faced with a chemical problem or a laboratory experiment, reference to these rules quickly guides us in predicting which compounds will remain dissolved and which will form a precipitate.

Ionic Compounds

Ionic compounds are substances composed of positively charged ions (cations) and negatively charged ions (anions) held together by strong electrostatic forces known as ionic bonds. They often form crystalline solids and have high melting and boiling points.

Formation of Ionic Compounds

These compounds typically form when a metal reacts with a non-metal, transferring electrons from the metal to the non-metal, resulting in the formation of oppositely charged ions. The robust attraction between these charges leads to the characteristic lattice structure of ionic compounds.

Dissociation in Water

In water, ionic compounds can dissociate into their constituent ions, a process greatly influenced by the solubility of the compound. If the ionic compound is soluble, it will dissociate completely, allowing the ions to move freely in solution and potentially engage in further chemical reactions. If it is insoluble, the ionic compound will remain as a solid precipitate.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions of ionic compounds are combined and an insoluble ionic compound is formed. The resultant solid, referred to as the 'precipitate,' is evidence of a chemical reaction having taken place.

Determining the Precipitate

Using the solubility rules, one can predict the outcome of mixing various solutions. In a typical precipitation reaction, the anions and cations swap partners, potentially forming a new insoluble compound. As seen in the exercise, the process involves identifying all possible products and then applying solubility rules to find the solid precipitate.

Importance of Precipitation Reactions

These reactions are crucial in various applications, ranging from waste water treatment to qualitative chemical analysis. By precipitating out certain components from a mixture, it allows for separation and purification of different substances. Understanding and applying the concept of precipitation reactions is not only useful for theoretical chemistry problems but also for real-world applications in the laboratory and industry.