Preparation and Properties of Hydrogen, Oxygen, Nitrogen and Carbon dioxide, Oxidation and Reduction
Preparation and Properties of Hydrogen, Oxygen, Nitrogen and Carbon dioxide, Oxidation and Reduction MCQ Questions
13.
Pure hydrogen of very high purity can be prepared in the laboratory by the electrolysis of a warm, dilute solution of:
B.
Barium hydroxide (using nickel electrodes)
ANSWER :
B. Barium hydroxide (using nickel electrodes)
Approach
The question asks for a method to prepare very high purity hydrogen in the laboratory via electrolysis. We need to evaluate each option based on the purity of hydrogen produced and the feasibility of the electrolytic process.
Step-by-step
- Electrolysis Principle: Electrolysis involves passing an electric current through an electrolyte to cause a non-spontaneous chemical reaction. In the context of hydrogen production, water is typically decomposed into hydrogen and oxygen.
- Purity Requirement: For very high purity hydrogen, the electrolyte and electrodes must not introduce impurities or side reactions that would contaminate the hydrogen gas.
- Evaluating Option B (Barium hydroxide): Electrolysis of warm, dilute barium hydroxide solution using nickel electrodes is a standard laboratory method for producing high-purity hydrogen. Barium hydroxide is a strong electrolyte, and its ions (\(\text{Ba}^{2+}\) and \(\text{OH}^-\)) do not interfere with the production of hydrogen at the cathode. Nickel electrodes are inert and do not corrode or introduce impurities. The reactions are:
- At cathode (reduction): \(\text{2H}_2\text{O}(l) + \text{2e}^- \rightarrow \text{H}_2(g) + \text{2OH}^-(aq)\)
- At anode (oxidation): \[\text{2OH}^-(aq) \rightarrow \text{H}_2\text{O}(l) + \frac{1}{2}\text{O}_2(g) + \text{2e}^-\]
The net reaction is the decomposition of water: \[\text{H}_2\text{O}(l) \rightarrow \text{H}_2(g) + \frac{1}{2}\text{O}_2(g)\]. The hydrogen produced is very pure.
Correct Option:
B) Barium hydroxide (using nickel electrodes) is the correct answer because the electrolysis of a warm, dilute solution of barium hydroxide with nickel electrodes is a well-established method for producing very high purity hydrogen in the laboratory. Barium hydroxide provides sufficient conductivity without introducing interfering ions, and nickel electrodes are inert.
Incorrect Options
- A) Sodium chloride — Electrolysis of sodium chloride solution (brine) produces hydrogen at the cathode, but it also produces chlorine gas at the anode (\(\text{2Cl}^-(aq) \rightarrow \text{Cl}_2(g) + \text{2e}^-\)) and sodium hydroxide in the solution. The presence of chlorine can contaminate the hydrogen if not carefully separated, and the primary goal of brine electrolysis is often chlorine and caustic soda production, not high-purity hydrogen.
- C) Copper sulphate — Electrolysis of copper sulphate solution would lead to the deposition of copper metal at the cathode (\(\text{Cu}^{2+}(aq) + \text{2e}^- \rightarrow \text{Cu}(s)\)) before hydrogen is evolved, especially if the solution is acidic or concentrated. This method is not suitable for producing hydrogen, let alone high-purity hydrogen.
- D) Silver nitrate — Electrolysis of silver nitrate solution would result in the deposition of silver metal at the cathode (\(\text{Ag}^+(aq) + \text{e}^- \rightarrow \text{Ag}(s)\)) and oxygen evolution at the anode (if inert electrodes are used). Similar to copper sulphate, this process is used for silver plating or refining,
A.
Coloured and has a pungent odour
B.
Coloured but odourless
C.
Colourless but has a strong smell
D.
Colourless, odourless and tasteless
ANSWER :
D. Colourless, odourless and tasteless
Approach
The question asks about the physical properties of hydrogen gas, specifically its colour, odour, and taste. To answer this, one needs to recall the characteristic properties of hydrogen, which is the lightest element and the most abundant chemical substance in the universe.
Step-by-step
- Recall the basic properties of hydrogen: Hydrogen (H\(_2\)) is a diatomic gas under standard conditions.
- Consider its colour: Pure hydrogen gas does not absorb or emit light in the visible spectrum, making it colourless.
- Consider its odour: Hydrogen is a simple molecule and does not possess any volatile compounds that would give it a distinct smell, making it odourless.
- Consider its taste: Similar to its odour, hydrogen does not have any chemical properties that would impart a taste sensation, making it tasteless.
- Combine these properties: Therefore, hydrogen gas is colourless, odourless, and tasteless.
Correct Option:
D) Colourless, odourless and tasteless
This option accurately describes the physical characteristics of pure hydrogen gas. It is a transparent gas that cannot be detected by sight, smell, or taste.
Incorrect Options
- A) Coloured and has a pungent odour — This is incorrect. Hydrogen is colourless and odourless. Gases like chlorine are coloured (greenish-yellow) and have a pungent odour.
- B) Coloured but odourless — This is incorrect. Hydrogen is colourless. Some gases are coloured (e.g., nitrogen dioxide is reddish-brown), but hydrogen is not.
- C) Colourless but has a strong smell — This is incorrect. While hydrogen is colourless, it is also odourless. Gases like hydrogen sulfide are colourless but have a strong, characteristic smell (rotten eggs).
15.
Hydrogen is the lightest of all gases known, which is why it was once used to fill:
ANSWER :
D. Balloons and airships
Approach
The question asks about the historical use of hydrogen, specifically leveraging its property of being the lightest gas. We need to identify an application where lightness is a primary requirement.
Step-by-step
- Identify the key property: The question states that hydrogen is the "lightest of all gases known." This property is crucial for applications where buoyancy is desired.
- Understand buoyancy: Lighter gases rise in heavier gases (like air) due to buoyancy. This principle is used in devices that need to float or lift.
- Evaluate the options based on lightness:
- Oxygen cylinders: Used for breathing or medical purposes, not for buoyancy.
- Refrigerators: Use refrigerants (like CFCs, HFCs, ammonia) for cooling, not hydrogen for lightness.
- Fire extinguishers: Use agents like carbon dioxide or dry chemicals to suppress fires, not hydrogen.
- Balloons and airships: These vehicles are designed to float in the air. Filling them with a gas lighter than air (like hydrogen or helium) provides the necessary lift.
- Recall historical context: Historically, hydrogen was indeed used to fill balloons and airships because of its extreme lightness, despite its flammability. Later, helium, being non-flammable, replaced hydrogen in many applications.
Therefore, the application that directly benefits from hydrogen's lightness is filling balloons and airships.
Correct Option:
D) Balloons and airships — Hydrogen's extreme lightness (low density) makes it highly buoyant in air, which is essential for lifting balloons and airships. It was widely used for this purpose before the flammability risk led to its replacement by helium in many applications.
Incorrect Options
- A) Oxygen cylinders — Oxygen cylinders are used to store oxygen for breathing, medical use, or industrial processes. Hydrogen's lightness is irrelevant for this purpose, and it would be dangerous to mix hydrogen with oxygen due to its flammability.
- B) Refrigerators — Refrigerators use refrigerants (e.g., freon, ammonia, isobutane) that undergo phase changes to transfer heat. Hydrogen is not used as a refrigerant.
- C) Fire extinguishers — Fire extinguishers use substances like carbon dioxide, water, foam, or dry chemicals to suppress fires. Hydrogen is highly flammable and would exacerbate a fire, making it entirely unsuitable for fire extinguishers.
16.
Compared to air, hydrogen gas diffuses:
A.
Much faster, being the lightest gas with the smallest molecular mass
B.
It cannot diffuse at all
C.
At exactly the same rate
D.
Much more slowly, being a heavier gas
ANSWER :
A. Much faster, being the lightest gas with the smallest molecular mass
Approach
This question tests the understanding of Graham's Law of Diffusion, which relates the rate of diffusion of a gas to its molecular mass. We need to compare the molecular masses of hydrogen and air to determine their relative diffusion rates.
Step-by-step
- Understand Graham's Law of Diffusion: Graham's Law states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its molar mass. Mathematically, it is expressed as:
\[ \frac{\text{Rate}_1}{\text{Rate}_2} = \sqrt{\frac{M_2}{M_1}} \]
where \(\text{Rate}_1\) and \(\text{Rate}_2\) are the diffusion rates of gas 1 and gas 2, respectively, and \(M_1\) and \(M_2\) are their respective molar masses.
- Determine the molar masses:
- The molar mass of hydrogen gas (\(\text{H}_2\)) is approximately \(2 \times 1 = 2 \text{ g/mol}\).
- Air is a mixture of gases, primarily nitrogen (\(\text{N}_2\), molar mass \(\approx 28 \text{ g/mol}\)) and oxygen (\(\text{O}_2\), molar mass \(\approx 32 \text{ g/mol}\)). The average molar mass of air is approximately \(29 \text{ g/mol}\).
- Compare molar masses: Hydrogen (\(2 \text{ g/mol}\)) has a significantly smaller molar mass than air (\(29 \text{ g/mol}\)).
- Apply Graham's Law: Since the rate of diffusion is inversely proportional to the square root of the molar mass, a gas with a smaller molar mass will diffuse faster. Because hydrogen has a much smaller molar mass than air, it will diffuse much faster than air.
Correct Option: A) Much faster, being the lightest gas with the smallest molecular mass. This statement correctly identifies that hydrogen is the lightest gas and, therefore, diffuses much faster than air, in accordance with Graham's Law.
Incorrect Options
- B) It cannot diffuse at all. This is incorrect. All gases diffuse, as their molecules are in constant random motion.
- C) At exactly the same rate. This is incorrect. Gases diffuse at different rates depending on their molecular masses. Only gases with identical molecular masses would diffuse at the same rate under the same conditions.
- D) Much more slowly, being a heavier gas. This is incorrect. Hydrogen is much lighter than air, not heavier, and therefore diffuses much faster, not slower.
17.
Hydrogen gas is collected in the laboratory by the method of downward displacement of water because it is:
B.
Highly soluble in water
C.
Reactive with water at room temperature
D.
Only slightly soluble in water
ANSWER :
D. Only slightly soluble in water
Approach
The question asks why hydrogen gas is collected by the downward displacement of water in a laboratory setting. This method is chosen based on the physical and chemical properties of the gas in relation to water.
Step-by-step
- Understanding Downward Displacement of Water: This collection method involves inverting a gas jar filled with water into a trough of water. The gas produced is then bubbled into the inverted jar, displacing the water.
- Conditions for using Downward Displacement of Water: This method is suitable for gases that are:
- Insoluble or only slightly soluble in water.
- Do not react with water.
- Properties of Hydrogen Gas: Hydrogen gas (\(H_2\)) is known to be very sparingly soluble in water and does not react with water at room temperature. Its density is also much lower than water.
- Evaluating the Options:
- If hydrogen were denser than water, it would not displace water upwards in an inverted jar.
- If hydrogen were highly soluble in water, it would dissolve instead of collecting as a gas.
- If hydrogen reacted with water, the collected gas would not be pure hydrogen.
- Since hydrogen is only slightly soluble in water, it can be efficiently collected by displacing the water in the inverted jar without significant loss due to dissolution.
Correct Option:
D) Only slightly soluble in water. This is the primary reason why hydrogen gas is collected by the downward displacement of water. Its low solubility ensures that a significant amount of gas does not dissolve in the water during collection, allowing for efficient and pure collection.
Incorrect Options
- A) Denser than water — Hydrogen gas is actually much less dense than water. If it were denser, it would not rise to displace the water in an inverted container; instead, it would sink, making this collection method unsuitable.
- B) Highly soluble in water — If hydrogen were highly soluble, it would dissolve in the water rather than collecting as a gas, making the downward displacement of water an ineffective collection method.
- C) Reactive with water at room temperature — Hydrogen gas is generally unreactive with water at room temperature. If it reacted, the collected gas would not be pure hydrogen, and the method would be inappropriate.
18.
When a burning splint is brought near a test tube of hydrogen gas, it burns with a characteristic sound and a pale blue flame. This sound is described as a:
Approach
The question describes a common laboratory test used to identify hydrogen gas. When hydrogen gas burns, it reacts with oxygen in the air. This reaction is exothermic and produces water. The specific sound produced during this combustion is a key characteristic for identifying hydrogen.
Step-by-step
- Understanding the reaction: Hydrogen gas (\(H_2\)) is highly flammable. When ignited in the presence of oxygen (\(O_2\)), it undergoes a combustion reaction to form water (\(H_2O\)). The chemical equation is: \[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \]
- Nature of the combustion: This reaction is rapid and releases a significant amount of energy, causing a sudden expansion of gases.
- Sound produced: The rapid combustion and expansion of gases create a small, sharp explosion. This explosion is heard as a distinctive "pop" sound. This test is often referred to as the "pop test" for hydrogen.
- Flame characteristics: Hydrogen burns with a pale blue, almost invisible flame, as mentioned in the question.
Correct Option
A) Pop sound — When a burning splint is brought near hydrogen gas, the hydrogen ignites rapidly, producing a small explosion that results in a characteristic "pop" sound. This is a standard test for hydrogen gas in chemistry.
Incorrect Options
- B) Crackling sound — A crackling sound is typically associated with burning wood or other materials that produce sparks or uneven combustion, not pure hydrogen gas.
- C) No sound at all — If there were no hydrogen gas, or if the concentration was too low to ignite, there might be no sound. However, the question states it burns, implying a reaction.
- D) Hissing sound — A hissing sound is usually associated with gas escaping under pressure or a slower, continuous burning process, not the rapid combustion of hydrogen.
