Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following mixture of gases does not obey Dalton's law of partial pressure? (a) \(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\) (b) \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\) (c) \(\mathrm{Cl}_{2}\) and \(\mathrm{SO}_{2}\) (d) \(\mathrm{NH}_{3}\) and \(\mathrm{HCl}\)

Step-by-Step Solution

Answer

(d) frac{mathrm{NH}_{3})} and frac{mathrm{HCl})} do not obey Dalton's law.

Step 1: Understanding Dalton's Law

Dalton's law of partial pressure states that in a mixture of non-reacting gases, the total pressure is equal to the sum of the partial pressures of individual gases. It assumes gases do not react chemically with each other.

Step 2: Analyze Each Gas Mixture

(a) Oxygen and carbon dioxide are non-reactive and thus follow Dalton's law. (b) Nitrogen and oxygen are also non-reactive and follow Dalton's law. (c) Chlorine and sulfur dioxide are non-reactive and follow Dalton’s law. (d) Ammonia and hydrochloric acid react chemically to form ammonium chloride, violating Dalton's law due to chemical reaction.

Step 3: Identify the Exception

In option (d), ammonia (frac{mathrm{NH}_{3})} and hydrochloric acid (frac{mathrm{HCl})} undergo a chemical reaction to form solid ammonium chloride (frac{mathrm{NH}_{4}Cl}). This mixture does not obey Dalton's law because the reaction changes the gas concentration.

Key Concepts

Gas Mixtures

Gas mixtures consist of two or more gases combined together in an environment, and each gas retains its individual properties. The behavior of these mixtures can generally be predicted based on the nature of the gases involved.
In chemical terms, we treat these mixtures as solutions where different gases act as solutes that dissolve in one another to form a homogeneous mixture. Dalton's Law of Partial Pressure aids us in determining the pressure exerted by each gas in the mixture and relies on the principle that each gas exerts pressure independently.

• Inert behavior is assumed, meaning gases do not interact chemically, altering concentrations or identities.
• Oxygen and nitrogen, or oxygen and carbon dioxide, are such examples where the gases in the mixture are non-reactive.

Understanding the nature of gas mixtures helps to predict how they will behave under different conditions, critical in fields ranging from chemical engineering to environmental science.

Non-reacting Gases

Non-reacting gases are gases that do not engage in chemical reactions when combined. For example, gases like nitrogen and oxygen in the Earth's atmosphere exist together without reacting under normal conditions. This non-reactivity is critical for the application of Dalton's law, which assumes that the gases do not change chemically when mixed.
The principles guiding non-reacting gases include:

• Each gas contributes to the total mixture's pressure independently based on its own partial pressure.
• Non-reactivity allows for predictions using Dalton's Law, where the total pressure is the sum of individual partial pressures.

Understanding the behavior of non-reacting gases is essential for various applications, from industrial gases to ensuring safety in breathing mixtures used in deep-sea diving.

Chemical Reactions in Gases

When gases react chemically, their fundamental nature changes, and this alters the overall behavior of the mixture. A prominent example from the original exercise is when ammonia ( fNH3) reacts with hydrochloric acid ( fHCl) to form ammonium chloride ( fNH4Cl). This chemical reaction results in a significant deviation from the predictions of Dalton's Law.
Some key points about gas reactions include:

• Reactions result in new compounds, often with different states, such as the solid ammonium chloride formed in the reaction between ammonia and hydrochloric acid.
• These reactions typically change the pressure dynamics within the mixture, as the reactants' pressure is altered by their chemical transformation into products.

Hence, knowing whether gases will react or not is vital for correctly applying laws like Dalton’s and anticipating the changes in gas composition and pressure.