Question

What would be the nature of aqueous solutions of the following salts? (i) \(\mathrm{KNO}_{3}\) (ii) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (iii) \(\mathrm{CH}_{3} \mathrm{COONa}\)

Short answer

Question: Determine the nature of the aqueous solutions of the following salts: (i) Potassium nitrate (KNO3), (ii) Sodium carbonate (Na2CO3), and (iii) Sodium acetate (CH3COONa). Answer: (i) The aqueous solution of Potassium nitrate (KNO3) is neutral. (ii) The aqueous solution of Sodium carbonate (Na2CO3) is basic. (iii) The aqueous solution of Sodium acetate (CH3COONa) is basic.

Step-by-step solution

(i) Potassium nitrate (KNO3)

Potassium nitrate dissociates into potassium (K+) and nitrate (NO3-) ions in water. According to the Arrhenius definition, acids are the substances that donate H+ ions, and bases are the substances that donate OH- ions. Potassium ion (K+) is formed from the neutralization of a strong base, KOH (potassium hydroxide), and the nitrate ion (NO3-) is formed from a strong acid, HNO3 (nitric acid). Since both ions are formed from strong acid and strong base, their aqueous solution will be neutral.

(ii) Sodium carbonate (Na2CO3)

Sodium carbonate dissociates into sodium (Na+) and carbonate (CO3^2-) ions in water. The sodium ion (Na+) is formed from the neutralization of a strong base, NaOH (sodium hydroxide), and the carbonate ion (CO3^2-) is formed from a weak acid, H2CO3 (carbonic acid). Since the ions are formed from a strong base and weak acid, their aqueous solution will be basic.

(iii) Sodium acetate (CH3COONa)

Sodium acetate dissociates into sodium (Na+) and acetate (CH3COO-) ions in water. The sodium ion (Na+) is formed from the neutralization of a strong base, NaOH (sodium hydroxide), and the acetate ion (CH3COO-) is formed from a weak acid, CH3COOH (acetic acid). Since the ions are formed from a strong base and weak acid, their aqueous solution will be basic.

Key concepts

Acid-Base Neutralization

Neutralization is a key concept in acid-base chemistry. It occurs when an acid and a base react to form water and a salt. During this process, the hydrogen ions \(\text{(H}^+\text{)}\) from the acid react with the hydroxide ions \(\text{(OH}^-\text{)}\) from the base. This reaction results in the formation of water \((H_2O)\), which is neutral.

• An acid-base neutralization is a balancing act.
• The goal is for the acidic protons to perfectly react with the basic hydroxides.
• When this condition is met, the resulting solution is neither acidic nor basic.

The product of this reaction, the salt, will have properties based on the strengths of the original acid and base. Whether this salt makes the solution neutral, basic, or acidic will depend on its constituents.

Strong Acid and Strong Base

When a strong acid and a strong base react, the resulting salt typically forms neutral aqueous solutions. This is because both the acid and base completely dissociate in water. For instance, the reaction between nitric acid \((HNO_3)\) and potassium hydroxide \((KOH)\) produces potassium nitrate \((KNO_3)\) and water.

• Strong acids like \(HNO_3\) donate \(H^+\) ions readily.
• Strong bases like \(KOH\) provide \(OH^-\) ions without hesitation.

When the components are strong, they perfectly counteract each other, ensuring no excess \(H^+\) or \(OH^-\) remains. Therefore, the perceived neutrality of potassium nitrate arises from the complete and efficient neutralization of its parent acid and base.

Weak Acid and Strong Base

The interaction between a weak acid and a strong base results in a basic aqueous solution. This is because the weak acid doesn't completely dissociate in water. Take sodium carbonate \((Na_2CO_3)\) for example. It originates from sodium hydroxide \((NaOH)\) and carbonic acid \((H_2CO_3)\). While \(NaOH\) is a strong base that dissociates fully, \(H_2CO_3\) dissociates only partially.

• Sodium hydroxide's \(Na^+\) ion is a product of a strong base.
• Carbonate \((CO_3^{2-})\), resulting from a weak acid, maintains a potential to bind \(H^+\) and thus acts as a base in solution.

Consequently, the leftover hydroxide ions make the solution basic. Sodium acetate \((CH_3COONa)\) behaves similarly due to its weak acidic component, acetate \((CH_3COOH)\), resulting in a basic solution when dissolved.

Salt Hydrolysis

Salt hydrolysis refers to the reaction of salt ions with water, which can change the pH of the solution. Derived from the terms formed during neutralization, the behavior of the salt determines the nature of the solution.

• Salts from strong acids and bases typically maintain neutrality.
• Salts from strong bases and weak acids, like sodium acetate or sodium carbonate, hydrolyze to make the solution basic.

Acetate ions \((CH_3COO^-)\) from sodium acetate react with water, creating OH- ions, which increase the pH, making it basic. Thus, understanding the origins of these ions is crucial in predicting the resultant pH and behavior of the solution upon dissolution.