Question

Which of the following solutions will not form a precipitate when added to \(0.10 \mathrm{M} \mathrm{NaOH} ?\) (a) \(0.10 \mathrm{M} \mathrm{MgBr}_{2}\) (b) \(0.10 \mathrm{M} \mathrm{NH}_{4} \mathrm{Br}\) (c) \(0.10 \mathrm{M} \mathrm{FeCl}_{2}\)

Short answer

(b) 0.10 M NH₄Br does not form a precipitate with NaOH.

Step-by-step solution

Understanding Precipitation Reactions

A precipitate will form when two solutions are mixed if an insoluble product is formed. We need to determine whether the combination of NaOH with each solution forms an insoluble compound.

Review Solubility Rules

The solubility rules help us determine which compounds are insoluble. Hydroxides (OH⁻) are generally insoluble, except for alkali metal cations and certain others like Ba²⁺, Sr²⁺, and Ca²⁺, while ammonium (NH₄⁺) salts are always soluble.

Examine Each Option: MgBr₂

Magnesium hydroxide, Mg(OH)₂, is insoluble in water. Thus, mixing MgBr₂ with NaOH would result in the formation of an insoluble precipitate, Mg(OH)₂.

Examine Each Option: NH₄Br

Ammonium ions (NH₄⁺) form soluble hydroxides. Therefore, no precipitate forms when NH₄Br reacts with NaOH because NH₄OH is soluble.

Examine Each Option: FeCl₂

Iron(II) hydroxide, Fe(OH)₂, is insoluble in water. Thus, FeCl₂ would form a precipitate, Fe(OH)₂, upon mixing with NaOH.

Determine the Non-Precipitating Solution

By reviewing each combination with NaOH, NH₄Br is the only solution that does not form a precipitate as its hydroxide is soluble.

Key concepts

Solubility Rules

Solubility rules are essential guidelines that help predict whether a compound will dissolve in water or form a precipitate. These rules can be extremely helpful in understanding chemical reactions, especially those involving ions in solution.
First, it's important to know that some salts are nearly always soluble. For instance, compounds containing alkali metal cations, like sodium (Na⁺) and potassium (K⁺), are generally soluble in water. Additionally, ammonium (NH₄⁺) and nitrate (NO₃⁻) salts almost always dissolve, regardless of what they are paired with.
On the other hand, certain anions tend to form insoluble compounds. Hydroxides are an example, as they are typically insoluble except when they combine with alkali metals or specific cations such as barium (Ba²⁺), strontium (Sr²⁺), and calcium (Ca²⁺). This means while NaOH is soluble, compounds like magnesium hydroxide (Mg(OH)₂) are not.

• Most chlorides, bromides, and iodides are soluble, except those of silver, lead, and mercury.
• Carbonates, phosphates, sulfides, and hydroxides are generally insoluble, with some exceptions.

By applying these solubility rules, you can determine whether a precipitation reaction is likely to occur.

Insoluble Compounds

Insoluble compounds are those that do not dissolve readily in water. They tend to form solids, called precipitates, when certain ions meet in an aqueous solution.
For instance, metal hydroxides like magnesium hydroxide (Mg(OH)₂) and iron(II) hydroxide (Fe(OH)₂) do not dissolve well in water. When solutions containing these metal ions (like Mg²⁺ from MgBr₂ and Fe²⁺ from FeCl₂) are mixed with a hydroxide source such as NaOH, the insoluble metal hydroxides can form.
This is because the product of their ionic reaction does not separate into individual ions in water, forming a solid instead. Solubility rules help us predict these interactions, as they provide a quick reference to know if a compound is likely to be soluble or insoluble.

• An insoluble compound results from a chemical reaction that decreases the solubility of the ions involved.
• The solid precipitate formed acts as evidence of the insoluble nature of the compound.

This understanding is crucial in predicting precipitate formation in different chemical reactions.

Precipitate Formation

Precipitate formation is a fascinating chemical process where two soluble substances react to form an insoluble solid, known as a precipitate. This happens due to the chemical properties of ions that result in the formation of an insoluble compound when combined in solution.
For example, when a solution of NaOH is mixed with MgBr₂, magnesium ions (Mg²⁺) react with hydroxide ions (OH⁻) to form magnesium hydroxide (Mg(OH)₂), which is insoluble. This process results in the formation of a solid dispersed in the solution, which can usually be seen as a cloudiness or sediment.
The formation of precipitates can be predicted using solubility rules, helping chemists to understand and anticipate the outcomes of their reactions.

• A precipitate indicates a chemical change where the ions have rearranged to form a solid product.
• Precipitates are key in many industrial and laboratory processes, including water treatment and chemical analysis.

Knowing how precipitates form aids in mastering many practical applications and theoretical aspects of chemistry.