Question

Predict whether aqueous solutions of the following compounds are acidic, basic, or neutral: \((\mathbf{a})\mathrm{NH}_{4} \mathrm{Br},(\mathbf{b}) \mathrm{FeCl}_{3}\) \((\mathbf{c}) \mathrm{Na}_{2} \mathrm{CO}_{3},(\mathbf{d}) \mathrm{KClO}_{4},(\mathbf{e}) \mathrm{NaHC}_{2} \mathrm{O}_{4}\)

Short answer

The predicted behaviors of the given aqueous solutions are: a) NH₄Br: acidic b) FeCl₃: acidic c) Na₂CO₃: basic d) KClO₄: neutral e) NaHC₂O₄: acidic

Step-by-step solution

(Step 1: Identify Cations and Anions)

In this step, we will identify the cations and anions in each of the given compounds. This will help us understand how they will interact with water. a) NH₄Br: Cation: NH₄⁺ (ammonium) Anion: Br⁻ (bromide) b) FeCl₃: Cation: Fe³⁺ (ferric) Anion: Cl⁻ (chloride) c) Na₂CO₃: Cation: Na⁺ (sodium) Anion: CO₃²⁻ (carbonate) d) KClO₄: Cation: K⁺ (potassium) Anion: ClO₄⁻ (perchlorate) e) NaHC₂O₄: Cation: Na⁺ (sodium) Anion: HC₂O₄⁻ (hydrogen oxalate)

(Step 2: Determine Cation and Anion Behavior in Water)

In this step, we will determine whether the cations and anions will act as acids or bases when they react with water. NH₄⁺: acidic (ammonium ion behaves as a weak acid) Br⁻: neutral (conjugate base of a strong acid, HBr) Fe³⁺: acidic (transition metal ions with positive charges tend to be acidic because they can hydrolyze water) Cl⁻: neutral (conjugate base of a strong acid, HCl) Na⁺: neutral (alkali metal ions do not react with water) CO₃²⁻: basic (conjugate base of a weak acid, H₂CO₃) K⁺: neutral (alkali metal ions do not react with water) ClO₄⁻: neutral (conjugate base of a strong acid, HClO₄) Na⁺: neutral (alkali metal ions do not react with water) HC₂O₄⁻: acidic (hydrogen oxalate is an acidic ion since it can donate an H⁺ ion)

(Step 3: Predict Solution Behavior)

Based on the behavior of the cations and anions in water, we can predict whether the overall aqueous solution will be acidic, basic, or neutral. a) NH₄Br: NH₄⁺ (acidic) + Br⁻ (neutral) → acidic solution b) FeCl₃: Fe³⁺ (acidic) + Cl⁻ (neutral) → acidic solution c) Na₂CO₃: Na⁺ (neutral) + CO₃²⁻ (basic) → basic solution d) KClO₄: K⁺ (neutral) + ClO₄⁻ (neutral) → neutral solution e) NaHC₂O₄: Na⁺ (neutral) + HC₂O₄⁻ (acidic) → acidic solution So the final predictions are as follows: a) NH₄Br: acidic b) FeCl₃: acidic c) Na₂CO₃: basic d) KClO₄: neutral e) NaHC₂O₄: acidic

Key concepts

Cation and Anion Identification

Cations and anions are positively and negatively charged ions found in compounds. Identifying them is the first step in predicting how a compound will behave in a solution. In each compound, the cation is usually listed first and tends to lose electrons to form a positive charge. Anions, on the other hand, gain electrons to form a negative charge. Understanding the nature of these ions helps in predicting how they will react with water.

For example, in the compound ammonium bromide ( ext{NH}_4 ext{Br}), the cation is ext{NH}_4^+ (ammonium), a positively charged ion. The anion is ext{Br}^- (bromide), a negatively charged ion. Identifying these ions allows us to anticipate their potential interactions when dissolved in water. Cations like ext{Fe}^{3+} (in ext{FeCl}_3) show strong tendencies to interact due to their higher positive charge, indicating a possible acidic behavior. Similarly, distinguishing whether an anion such as ext{CO}_3^{2-} (in ext{Na}_2 ext{CO}_3) is basic or neutral further guides us in predicting the pH of the solution.

Aqueous Solutions

An aqueous solution refers to a substance dissolved in water. Water acts as a solvent, facilitating the dissociation of ionic compounds into their constituent cations and anions. This dissociation is crucial because it determines how the solution behaves, whether acidic, basic, or neutral. When a compound dissolves, its cations and anions disperse through the solution, each potentially affecting the solution's properties.

This dissociation is determined by the nature of the ions themselves. Remember that ext{Na}^+ and ext{K}^+ ions, often found in salts, typically remain neutral in water and do not affect its acidity or basicity. On the other hand, transition metal ions like ext{Fe}^{3+} can hydrolyze water, increasing the solution's acidity. Understanding these dynamics helps us predict the behavior of the entire solution effectively, allowing us to anticipate whether it will be acidic, basic, or neutral. This is especially important when dealing with chemical reactions in real-world applications.

Solution pH Prediction

Predicting the pH of a solution involves understanding the behavior of both cations and anions upon dissolution. Solutions with acidic behaviors arise when cations act as weak acids and can donate ext{H}^+ ions. An excellent example is the ammonium ion ( ext{NH}_4^+), which releases ext{H}^+ ions, tilting the solution towards acidity when coupled with neutral anions like bromide ( ext{Br}^-).

Conversely, if anions behave as bases, such as ext{CO}_3^{2-} from ext{Na}_2 ext{CO}_3, they will attract ext{H}^+ ions from water, raising the pH and making the solution basic. Neutral solutions, like those formed from ext{KClO}_4, occur when neither ion affects the pH. This balance or imbalance of acidic and basic tendencies in solution is what ultimately defines the solution's pH.

• Acidic Solution Example: ext{NH}_4 ext{Br} contains ext{NH}_4^+, an ion that makes the solution acidic.
• Basic Solution Example: ext{Na}_2 ext{CO}_3 releases ext{CO}_3^{2-}, making the solution basic.
• Neutral Solution Example: ext{KClO}_4 neither increases nor decreases the pH, maintaining neutrality.

With these insights, one can confidently predict how a compound will impact a solution's overall pH, facilitating better planning and execution of chemical processes.