Question

Predict whether a precipitation reaction will occur when aqueous solutions of the following substances are mixed. For those that form a precipitate, write the net ionic reaction. (a) \(\mathrm{NaOH}+\mathrm{HClO}_{4}\) (b) \(\mathrm{FeCl}_{2}+\mathrm{KOH}\) (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}+\mathrm{NiCl}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{Na}+\mathrm{HCl}\)

Short answer

Only reaction (b) forms a precipitate: \( \mathrm{Fe(OH)_2} \).

Step-by-step solution

Understand Solubility Rules

To predict if a precipitation reaction will occur, we need to apply solubility rules. Solubility rules help determine whether a compound is soluble (stays in solution) or insoluble (forms a precipitate) in water. Understanding these rules for common ionic compounds will allow us to determine the products of the reactions and whether they will precipitate.

Analyze Reaction (a)

For the reaction between \( \mathrm{NaOH} \) and \( \mathrm{HClO}_{4} \), both reactants are strong electrolytes and completely dissociate in water:- \( \mathrm{NaOH} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{OH}^-_{(aq)} \)- \( \mathrm{HClO}_4 \rightarrow \mathrm{H}^+_{(aq)} + \mathrm{ClO}_4^-_{(aq)} \)The ions recombine to form water, which is a liquid, and sodium perchlorate, which remains in solution as \( \mathrm{NaClO}_4 \), a soluble compound. Hence, no precipitation occurs.

Analyze Reaction (b)

For the reaction between \( \mathrm{FeCl}_{2} \) and \( \mathrm{KOH} \), both reactants dissociate in water:- \( \mathrm{FeCl}_2 \rightarrow \mathrm{Fe}^{2+}_{(aq)} + 2\mathrm{Cl}^-_{(aq)} \) - \( \mathrm{KOH} \rightarrow \mathrm{K}^+_{(aq)} + \mathrm{OH}^-_{(aq)} \)Ferric hydroxide, \( \mathrm{Fe(OH)_2} \), is an insoluble compound, so it will precipitate out of the solution. The net ionic equation is:\[ \mathrm{Fe}^{2+}_{(aq)} + 2\mathrm{OH}^-_{(aq)} \rightarrow \mathrm{Fe(OH)_2 (s)} \]

Analyze Reaction (c)

For the reaction \( \left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \) and \( \mathrm{NiCl}_{2} \), both compounds dissociate:- \( \left(\mathrm{NH}_4\right)_2\mathrm{SO}_4 \rightarrow 2\mathrm{NH}_4^+_{(aq)} + \mathrm{SO}_4^{2-}_{(aq)} \)- \( \mathrm{NiCl}_2 \rightarrow \mathrm{Ni}^{2+}_{(aq)} + 2\mathrm{Cl}^-_{(aq)} \)Nickel sulfate, \( \mathrm{NiSO_4} \), formed is soluble, so no precipitation occurs in this reaction.

Analyze Reaction (d)

For the reaction between \( \mathrm{CH}_3\mathrm{CO}_2\mathrm{Na} \) and \( \mathrm{HCl} \), these compounds dissociate into:- \( \mathrm{CH}_3\mathrm{CO}_2\mathrm{Na} \rightarrow \mathrm{CH}_3\mathrm{CO}_2^-_{(aq)} + \mathrm{Na}^+_{(aq)} \)- \( \mathrm{HCl} \rightarrow \mathrm{H}^+_{(aq)} + \mathrm{Cl}^-_{(aq)} \)The products, \( \mathrm{NaCl} \) (soluble) and \( \mathrm{CH}_3\mathrm{COOH} \) (acetic acid) remain in solution. Therefore, no precipitate forms.

Key concepts

Solubility Rules

When you're trying to predict if a precipitation reaction will result from mixing certain aqueous solutions, the solubility rules serve as your guiding light. These rules give insight into whether a compound will remain dissolved in water or form a solid, also known as a precipitate.
Here's a quick overview to help you:

• Salts with alkali metal ions (such as Na⁺, K⁺) and ammonium ion (NH₄⁺) are generally soluble.
• Nitrates (NO₃⁻), bicarbonates (HCO₃⁻), and chlorates (ClO₃⁻) are soluble.
• Chlorides, bromides, and iodides are soluble, except for those containing silver (Ag⁺), lead (Pb²⁺), or mercury (Hg₂²⁺).
• Sulfates (SO₄²⁻) are usually soluble, with exceptions like barium sulfate (BaSO₄) and calcium sulfate (CaSO₄).
• Hydroxides are mostly insoluble, except for those formed with alkali metals and barium (Ba²⁺).

By applying these rules, we can predict the outcomes of mixing different ionic compounds in aqueous solutions.

Net Ionic Equations

A net ionic equation gives a more simplified view of chemical reactions, focusing on the ions that actually participate in the reaction. It cuts through the clutter, omitting spectator ions that do not directly contribute to the reaction process.
To construct a net ionic equation, follow these straightforward steps:

• Write the balanced molecular equation for the reaction.
• Dissociate all strong electrolytes into their ions.
• Cancel out the spectator ions appearing on both sides of the equation.
• Write the remaining ions to show the net ionic equation.

For example, in the reaction between \( \mathrm{FeCl}_2 \) and \( \mathrm{KOH} \), the net ionic equation: \[ \mathrm{Fe}^{2+}_{(aq)} + 2\mathrm{OH}^-_{(aq)} \rightarrow \mathrm{Fe(OH)_2 (s)} \] focuses solely on the formation of the insoluble product, ferric hydroxide.

Aqueous Solutions

An aqueous solution is simply a solution where water acts as the solvent. For many reactions in chemistry, particularly those involving ionic compounds, the substances are dissolved in water to separate the ions.
In aqueous solutions, compounds dissociate into individual ions, like this:

• Sodium chloride (NaCl) dissolves to give \( \mathrm{Na}^+_{(aq)} \) and \( \mathrm{Cl}^-_{(aq)} \).
• Potassium hydroxide (KOH) splits into \( \mathrm{K}^+_{(aq)} \) and \( \mathrm{OH}^-_{(aq)} \).

Understanding how substances behave in aqueous solutions is vital. This knowledge lets you predict how different solutions will react when mixed, determining whether new products are formed or a precipitate will occur.

Chemical Reactions

Chemical reactions describe the transformation where reactants convert into products. Their beauty lies in their variety, yet predictability when we understand the underlying principles.
Commonly, chemical reactions involve:

• Reactants that dissociate into ions in solution.
• Reordering of ions to form new compounds.
• Observation of visible changes, such as color change, gas release, or precipitate formation.

In our context, precipitation reactions, characterized by the formation of a solid in a liquid medium, are delineated by the solubility rules. If mixing two solutions results in an insoluble compound, this indicates a precipitation reaction has occurred, showcasing both the predictability and dynamism of chemical transformations.