What happens when a weak base such as NH 3 is dissolved in water?

Hello. In this problem, we are told an example of a weak base is pyridine. We're asked to explain what happens when pyridine is dissolved in water. We are given four possible answers. Answer A says when dissolved in water, a small amount of pyridine acts as a base that accepts hydros from water yielding a solution of unreactive pyridine and a small amount of the protid form of pyridine and hydroxides. Answer B says when dissolved in water, a small amount of pyridine acts as a base that accepts hydrogen ions from water yielding a solution of the proton form of pyrogen and hydroxid. Answer C says when dissolved in water, pyro acts as a base that accepts hydros from water, yielding a solution of the pron form of pyro and hydroxid. And answer D says when dissolved in water, pyridine acts as a base that accepts hydrogen from water, building a solution of pyridine and hydroxid ions recall that the weak base then is the one that does not go completely to form ions in aqueous solution. Pyridine well, react with water. Water will function as an acid. Pyridine is a base. So the water will donate a proton to pyridine, we will form some of the pron form of pine and some hydroxid ions. But we will have predominantly the unproven form or the base present when we reach a state of chemical equilibrium. So when we consider the four answers that we were provided, the solution should contain our base and the perate form of our base and the hydroxide ions, the major component should be that of the un perate form of our base. So answer B says that we just have the ions present. That's also what C says D says that we have the un perate form of our base and hydroxide ions. So we do have some of the perna form as well. So that's also incorrect. So BC and D do not include the proper components within our solution. When we look at answer A it says when dissolved in water, a small amount of pyridine acts as a base that accepts hydron water using a solution of unreactive pyro and a small amount of the perate form of pyridine and hydroxid ions. So this is our correct answer. Answer. A thanks for watching. Hope this helps.

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10. Acids and Bases

Bronsted Lowry Acid and Base

GOB Chemistry

10. Acids and Bases

Bronsted Lowry Acid and Base

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Problem 41

Textbook Question

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

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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.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.

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