Problem 91 - Introductory Chemistry Atoms First

Question

Write the formula of the precipitate formed when these solutions are mixed: (a) Aluminum nitrate and sodium hydroxide (b) Potassium phosphate and calcium chloride (c) Magnesium sulfate and sodium carbonate

Step-by-Step Solution

Answer

(a) Al(OH)3 (b) Ca3(PO4)2 (c) MgCO3

Step 1: (a) Identify the reactants and possible products in the reaction

In this case, the given reactants are aluminum nitrate (Al(NO3)3) and sodium hydroxide (NaOH). We can carry out a double displacement reaction between these two reactants by exchanging the positive ions (cations: Al3+ and Na+) and negative ions (anions: NO3- and OH-). This gives us the possible products, sodium nitrate (NaNO3) and aluminum hydroxide (Al(OH)3).

Step 2: (a) Determine the precipitate formed in the reaction

We can determine which of these products is likely to form a precipitate by referring to the solubility rules. Nitrates are generally soluble, so sodium nitrate (NaNO3) will remain in solution. Hydroxides are generally insoluble (except for Group 1 and 2 elements), so aluminum hydroxide (Al(OH)3) will form a precipitate. Thus, the formula of the precipitate formed in this reaction is Al(OH)3.

Step 3: (b) Identify the reactants and possible products in the reaction

In this case, the given reactants are potassium phosphate (K3PO4) and calcium chloride (CaCl2). We can carry out a double displacement reaction between these two reactants by exchanging the cations (K+ and Ca2+) and anions (PO4^3- and Cl-). This gives us the possible products, potassium chloride (KCl) and calcium phosphate (Ca3(PO4)2).

Step 4: (b) Determine the precipitate formed in the reaction

We can determine which of these products is likely to form a precipitate by referring to the solubility rules. Chlorides are generally soluble (except for silver and lead), so potassium chloride (KCl) will remain in solution. Phosphates are generally insoluble (except for Group 1 elements), so calcium phosphate (Ca3(PO4)2) will form a precipitate. Thus, the formula of the precipitate formed in this reaction is Ca3(PO4)2.

Step 5: (c) Identify the reactants and possible products in the reaction

In this case, the given reactants are magnesium sulfate (MgSO4) and sodium carbonate (Na2CO3). We can carry out a double displacement reaction between these two reactants by exchanging the cations (Mg 2+ and Na+) and anions (SO4^2- and CO3^2-). This gives us the possible products, sodium sulfate (Na2SO4) and magnesium carbonate (MgCO3).

Step 6: (c) Determine the precipitate formed in the reaction

We can determine which of these products is likely to form a precipitate by referring to the solubility rules. Sulfates are generally soluble (except for barium, calcium, and lead), so sodium sulfate (Na2SO4) will remain in solution. Carbonates are generally insoluble (except for Group 1 elements), so magnesium carbonate (MgCO3) will form a precipitate. Thus, the formula of the precipitate formed in this reaction is MgCO3.

Key Concepts

Precipitate Formation

Precipitate formation occurs when two soluble ionic compounds react in solution, producing an insoluble solid. This solid is the precipitate that forms as the result of a chemical reaction. When two solutions are mixed, the ions in the solutions can potentially rearrange to form new combinations.
For instance, when aluminum nitrate reacts with sodium hydroxide, the ions Al\(^{3+}\) and OH\(^-\) combine to form aluminum hydroxide, a solid precipitate. This process is especially noteworthy in reactions called double displacement reactions.

• A double displacement reaction involves the exchange of ions between two compounds.
• The resulting products may include one or more solids, gases, or water.

In each instance, precipitate formation is a clear indication that a chemical change has occurred, visible to the naked eye as the appearance of a solid.

Solubility Rules

Solubility rules are guidelines that help predict whether an ionic compound will dissolve in water. Understanding these rules is crucial for determining which products of a chemical reaction will precipitate.
These rules are based on empirical observations:

• Nitrates (NO\(_3^-\)) and Group 1 salts such as sodium (Na\(^+\)) and potassium (K\(^+\)) are generally soluble in water.
• Most hydroxides, carbonates, and phosphates are insoluble, except for alkali metals and some specific exceptions.
• Chlorides are soluble, with notable exceptions such as silver chloride and lead chloride.

By applying these solubility principles:- Aluminum hydroxide is insoluble, making it a common precipitate.- Calcium phosphate is also insoluble, leading to its formation as a solid.Solubility rules arm chemists with the ability to predict and understand the behavior of ions during chemical reactions.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products via the rearrangement of atoms. In double displacement reactions, two ionic compounds exchange ions to form new compounds.
The basic process can be outlined as follows:

• Two soluble ionic compounds are dissolved in water, and their ions dissociate in the solution.
• The ions exchange partners, based on attraction and solubility, to form new compounds.
• Some of these new compounds may be insoluble, leading to precipitate formation.

For instance, when potassium phosphate and calcium chloride meet, they undergo a double displacement reaction that results in calcium phosphate, a solid precipitate.
Such reactions are vital for understanding various processes, from natural phenomena to industrial applications, showcasing the beautiful complexity and order found in chemistry.