Question

The weak electrolyte \(\mathrm{NH}_{3}(g)\) does not obey Henry's law. Why? \(\mathrm{O}_{2}(g)\) obeys Henry's law in water but not in blood (an aqueous solution). Why?

Short answer

Ammonia (NH3) does not obey Henry's law because it forms a weak electrolyte, undergoing a reaction with water molecules to produce ammonium ions (\(\mathrm{NH}_{4}^+\)) and hydroxide ions (\(\mathrm{OH}^-\)). Oxygen (O2) obeys Henry's law in water as it only physically dissolves without undergoing any reaction. However, in blood, the behavior of oxygen is different, as it primarily binds to hemoglobin within red blood cells, rather than physically dissolving in blood plasma. This binding process is not described by Henry's law and is influenced by factors like pH, temperature, and the partial pressure of carbon dioxide.

Step-by-step solution

Understanding Henry's law

Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas on the surface of the liquid, under constant temperature. Mathematically, it can be expressed as: \[C = k_H \times P\] Where: - \(C\) is the concentration of the dissolved gas in the liquid - \(k_H\) is Henry's law constant (depends on the specific gas being dissolved and the temperature) - \(P\) is the partial pressure of the gas above the liquid It is important to note that Henry's law only applies to dilute solutions and to gases that do not undergo any chemical reaction with the solvent.

Explaining the behavior of ammonia (NH3)

Ammonia (NH3) does not obey Henry's law because it forms a weak electrolyte when it dissolves in water. When ammonia dissolves in water, it can react with water molecules to form ammonium ions (\(\mathrm{NH}_{4}^+\)) and hydroxide ions (\(\mathrm{OH}^-\)), as shown in the following reaction: \[\mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l) \rightleftharpoons \mathrm{NH}_{4}^+(aq) + \mathrm{OH}^-(aq)\] This reaction is reversible, and the equilibrium shifts depending on the concentration of ammonia and other ions in the solution. Because ammonia undergoes a chemical reaction with water, it does not follow Henry's law.

Explaining the behavior of oxygen (O2) in water and in blood

In water, oxygen (O2) follows Henry's law because it is only physically dissolved and does not undergo any chemical reaction with water molecules. Thus, the solubility of oxygen in water is directly proportional to the partial pressure of oxygen above the water. However, in blood (which is an aqueous solution), the behavior of oxygen is different. In blood, the majority of oxygen is transported by binding to a protein called hemoglobin within red blood cells. This binding process is not described by Henry's law and is influenced by factors like pH, temperature, and the partial pressure of carbon dioxide. A relatively small amount of oxygen remains physically dissolved in blood plasma, following Henry's law. In conclusion, ammonia (NH3) does not obey Henry's law because it undergoes a chemical reaction with water, while oxygen (O2) obeys Henry's law in water due to physical dissolution but does not follow it in blood due to its binding to hemoglobin.

Key concepts

Solubility of Gases

Solubility refers to how much of a gas can dissolve in a liquid. For gases, this solubility is governed by Henry's law. According to this law, the amount of gas that can be dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. This means the higher the pressure, the more gas can dissolve.

However, Henry's law has its limitations. It only applies to dilute solutions and non-reacting gases. These conditions ensure the gas dissolves physically, without any chemical changes.

• Factors affecting solubility include the nature of the gas, the nature of the liquid, and temperature.
• Increased temperature usually reduces the solubility of gases.

Chemical Equilibrium

When a gas like ammonia dissolves in water, it might undergo a chemical reaction, resulting in a state of equilibrium. Ammonia reacts with water to form ammonium and hydroxide ions.

In this situation, the initial dissolving process is balanced by the formation of these ions, leading to a typical equilibrium described by the equation: \[\mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l) \rightleftharpoons \mathrm{NH}_{4}^+(aq) + \mathrm{OH}^-(aq)\]This reaction shows that ammonia does not just dissolve; it reacts, differentiating it from gases that only dissolve physically under Henry's law.

• An equilibrium occurs when the rate of the forward reaction matches the rate of the reverse reaction.
• This equilibrium can shift based on concentration changes, temperature, and other factors.

Weak Electrolytes

A weak electrolyte refers to a substance that only partly ionizes in a solution. Ammonia is a classic example, as when it dissolves in water, it doesn't fully dissociate into ions.

The partial ionization of weak electrolytes like ammonia results in an equilibrium between the molecular form and its ions. Unlike strong electrolytes, which completely dissociate, weak electrolytes maintain this balance, influencing their behavior in solutions.

• Weak electrolytes have higher resistance than strong electrolytes due to less ion availability.
• They do not follow Henry’s law, because of their chemical interaction with the solvent.

Ammonia Solubility

Ammonia's solubility is unique due to its behavior in water. It’s not merely about how much ammonia can dissolve but also about the subsequent reaction that takes place.

When ammonia dissolves, it forms hydroxide and ammonium ions, which illustrates a typical reaction for a weak electrolyte. This means its solubility includes both physical dissolution and chemical reaction, preventing it from adhering strictly to Henry's law.

• Ammonia's solubility changes with shifts in equilibrium, depending on its concentration.
• The interaction with water changes its behavior compared to inert gases.

Oxygen Transport in Blood

In blood, oxygen's behavior deviates from simple solubility principles. While a small portion dissolves directly in blood plasma as per Henry's law, most oxygen binds to hemoglobin in red blood cells.

This process of binding is crucial for efficient oxygen transport and isn't covered by Henry's law, which deals with direct solubility. Factors such as carbon dioxide levels, pH, and temperature influence this binding, impacting how much oxygen can be carried and released by hemoglobin.

• Oxygen binding to hemoglobin is a reversible process critical for oxygen delivery to tissues.
• This mechanism allows for efficient transport, accommodating physiological changes.
• Disequilibrium in factors like pH can lead to shifts in oxygen affinity, affecting transport efficiency.