Question

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

Short answer

(a) NH4Br forms an acidic solution because it increases the concentration of H+ ions (protons) in the solution. (b) FeCl3 is acidic, as it generates H+ ions in an aqueous solution. (c) Na2CO3 forms a basic aqueous solution as it increases the concentration of OH- ions (hydroxide ions) in the solution. (d) KClO4 forms a neutral solution as both K+ and ClO4- ions do not affect its acidity or basicity. (e) NaHC2O4 forms a basic solution since it increases the concentration of OH- ions (hydroxide ions) in the solution.

Step-by-step solution

- Compound (a) - NH4Br

First, let's analyze NH4Br. When it dissolves in water, it breaks down as follows: NH4Br (s) ⟶ NH4+ (aq) + Br- (aq) NH4+ is the ammonium ion, which can donate a proton (H+) to the water: NH4+ (aq) + H2O(l) ⟶ NH3(aq) + H3O+ (aq) Therefore, NH4Br gives acidic solution because it increases the concentration of H+ ions (protons) in the solution.

- Compound (b) - FeCl3

Now let's analyze FeCl3. When it dissolves in water, it dissociates as follows: FeCl3 (s) ⟶ Fe3+ (aq) + 3Cl- (aq) Fe3+ is the ferric ion, which can hydrolyze water molecules and generate H+ ions: Fe3+ (aq) + 3H2O(l) ⟶ Fe(OH)3(s) + 3H3O+ (aq) FeCl3 is therefore considered acidic, as it generates H+ ions in an aqueous solution.

- Compound (c) - Na2CO3

Now let's look at Na2CO3. When it dissolves in water, it dissociates as follows: Na2CO3 (s) ⟶ 2Na+ (aq) + CO3^2- (aq) CO3^2-, the carbonate ion, can accept H+ ions from water: CO3^2- (aq) + H2O(l) ⟶ HCO3- (aq) + OH- (aq) Because it increases the concentration of OH- ions (hydroxide ions) in the solution, Na2CO3 forms a basic aqueous solution.

- Compound (d) - KClO4

Now let's analyze KClO4. When it dissolves in water, it dissociates as follows: KClO4 (s) ⟶ K+ (aq) + ClO4- (aq) Both K+ and ClO4- are considered spectator ions, which means they don't participate in any reactions and don't affect the acidity or basicity of the solution. Therefore, KClO4 forms a neutral solution.

- Compound (e) - NaHC2O4

Finally, let's analyze NaHC2O4. When it dissolves in water, it dissociates as follows: NaHC2O4 (s) ⟶ Na+ (aq) + HC2O4- (aq) HC2O4- can both accept and donate a proton, but its tendency to accept a proton is more significant. It means it acts as a base: HC2O4- (aq) + H2O(l) ⟶ C2O4^2- (aq) + H3O+ (aq) NaHC2O4 forms a basic solution since it increases the concentration of OH- ions (hydroxide ions) in the solution.

Key concepts

Aqueous Solutions

When a substance is dissolved in water, it forms an aqueous solution. This is indicated by the abbreviation (aq). Water acts as a solvent, allowing various reactions to occur between the dissolved substances, also known as solutes.
In chemistry, understanding how compounds interact in aqueous solutions is vital. This is because many reactions, especially acid-base reactions, occur in these solutions. For example, when NH4Br dissolves, it separates into its ionic components, NH4+ and Br-.
Aqueous solutions allow ions to move freely, enabling these ions to react with water molecules. This understanding helps predict whether the resulting solution will be acidic, basic, or neutral.

Ionic Compounds

Ionic compounds are formed from positively and negatively charged ions. They dissolve in water by breaking apart into their respective ions.
This dissociation process is crucial in determining the nature of the solution. For instance,

• The dissociation of Na2CO3 into Na+ and CO3^2- results in a basic solution because the carbonate ion (CO3^2-) can accept protons (H+).
• On the other hand, FeCl3 dissociates into Fe3+ and Cl-, resulting in an acidic solution as the Fe3+ ion hydrolyzes water, producing H+ ions.

Understanding the behavior of these ions helps in predicting how they affect the solution, like increasing hydroxide or hydronium ions.

Spectator Ions

Spectator ions are ions present in a solution that do not participate in the chemical reaction. They remain unchanged before and after the reaction.
For instance, when KClO4 dissolves in water, it splits into K+ and ClO4- ions. However, neither of these ions reacts with water or alters its pH. They are spectators in the chemical process, resulting in a neutral solution.
Recognizing spectator ions is essential for simplifying reactions, as it allows us to focus on the ions that actively participate in the reaction and influence the solution's properties, like acidity or basicity.