What happens when a weak base such as NH3 is dissolved in water?

Ch.10 Acids and Bases

McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition

McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187당신이 사용하는 게 아니라요?교과서 변경

모든 교과서

McMurry 8th Edition

Ch.10 Acids and Bases

문제 41

10장, 문제 41

What happens when a weak base such as NH₃ is dissolved in water?

검증된 단계별 안내

When a weak base like NH₃ (ammonia) is dissolved in water, it reacts with water molecules in a reversible reaction. This means the reaction does not go to completion and establishes an equilibrium.

The weak base NH₃ accepts a proton (H⁺) from a water molecule (H₂O), forming NH₄⁺ (ammonium ion) and OH⁻ (hydroxide ion). The chemical equation for this reaction is:

NH 3 + H_{2} O \leftrightarrow {NH}_{4} + {OH}_{-}

Since NH₃ is a weak base, it does not ionize completely in water. Only a small fraction of NH₃ molecules react with water to form NH₄⁺ and OH⁻ ions, leaving most of the NH₃ molecules unreacted.

The extent of ionization of NH₃ in water is determined by its base dissociation constant, Kb. The Kb value for NH₃ is relatively small, indicating that it is a weak base and produces a low concentration of OH⁻ ions in solution.

The solution becomes slightly basic due to the presence of OH⁻ ions, which increase the pH of the solution. However, the pH will not be as high as that of a strong base because NH₃ does not fully ionize.

비슷한 문제에 대한 검증된 영상 답변:

이 영상 해법은 위 문제에 도움이 된다고 튜터들이 추천한 것입니다.

영상 길이:

도움이 되었나요?

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Weak Bases

Weak bases are substances that partially ionize in solution, meaning they do not completely dissociate into ions. Ammonia (NH₃) is a common example, which reacts with water to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). This partial ionization results in a lower pH compared to strong bases, which fully dissociate.

Equilibrium in Chemical Reactions

In the context of weak bases, equilibrium refers to the state where the rate of the forward reaction (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻) equals the rate of the reverse reaction. This dynamic balance means that not all NH₃ molecules will convert to ions, leading to a mixture of both reactants and products in solution.

pH and Basicity

The pH scale measures the acidity or basicity of a solution, with values below 7 indicating acidity and above 7 indicating basicity. When NH₃ dissolves in water, it increases the concentration of hydroxide ions (OH⁻), resulting in a pH greater than 7, thus making the solution basic. The extent of this increase depends on the concentration of the weak base.