Problem 131 - General Chemistry: Principles and Modern Applications

Question

Briefly describe each concept or process: (a) absolute zero of temperature; (b) collection of a gas over water; (c) effusion of a gas; (d) law of combining volumes.

Step-by-Step Solution

Answer

Absolute Zero is the minimum theoretical temperature, set as 0 Kelvin or -273.15 Celsius. Collection of a gas over water is a laboratory method to measure gas produced in a chemical reaction. Effusion of a gas is when it escapes from a container through a tiny hole into an evacuated space, explained by Graham's law. The law of combining volumes, or Gay-Lussac's law, states that the volume ratios of gases in a reaction are proportional to the stoichiometric ratios.

Step 1: Define Absolute Zero

Absolute zero is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance. It is precisely set as 0 Kelvin, or -273.15 degrees Celsius.

Step 2: Describe the Collection of a Gas Over Water

The Collection of a gas over water is a common laboratory method used to collect the gas produced from a chemical reaction. It is done by displacing water in a container with the gas, which can then be measured. However, the collected gas is often saturated with water vapor, which must be taken into account during calculations.

Step 3: Define Effusion of a Gas

Effusion is a process in which a gas escapes from a container through a tiny hole into an evacuated space. It is described quantitatively by Graham's law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular weight.

Step 4: Explain the Law of Combining Volumes

The law of combining volumes, also known as Gay-Lussac's law, states that the ratio of the volumes of gases involved in a chemical reaction will be directly proportional to the ratio of stoichiometric coefficients in the balanced chemical equation, provided all gases are at the same temperature and pressure.

Key Concepts

Absolute Zero

In the realm of temperature and thermodynamics, absolute zero represents an extraordinary concept. It signifies the lowest theoretical temperature a substance can achieve. At absolute zero, which is 0 Kelvin or -273.15 degrees Celsius, a system is considered to have its minimum thermodynamic energy. This is a point where particles experience minimum motion, and no further heat can be extracted. At this temperature, the entropy of a perfect crystal approaches zero, as postulated by the third law of thermodynamics. Achieving absolute zero is practically impossible, but experiments can get extremely close, providing insight into the quantum mechanical nature of matter. Understanding this concept is crucial in fields like physics and chemistry, as it establishes a baseline for measuring enthalpies and thermal transitions.

Effusion

Effusion defines the process through which gas molecules escape through a tiny hole in their containment into a vacuum or another chamber. This phenomenon helps us understand how gases behave under certain conditions. Effusion is quantitatively expressed by Graham's law. This law states that the rate of effusion for a gas is inversely proportional to the square root of its molar mass. This means:

• Heavier gases effuse more slowly than lighter gases.
• The rate of effusion is critical when separating isotopes or in applications like gas chromatography.
• Effusion assists in determining molecular weights and understanding molecular dynamics.

Understanding effusion is crucial in various scientific processes where the behavior of gases under low pressure becomes significant.

Combining Volumes

The law of combining volumes, discovered by Joseph Louis Gay-Lussac, offers fascinating insights into the world of gaseous chemical reactions. This law posits that, when gases react with each other, they do so in simple whole number ratios by volume, provided they are at the same temperature and pressure. Here's why it's interesting:

• This law works because of the gases' nature, following Avogadro's principle that equal volumes of gases at the same temperature and pressure contain an equal number of molecules.
• It simplifies calculations in chemical reactions, especially when dealing with gases, by allowing stoichiometric calculations using volume ratios instead of mole ratios.
• This concept is pivotal in chemical engineering, helping in processes such as gas production and reaction chamber design.

Recognizing these volume relationships not only aids in understanding chemical equations but also makes the handling of gaseous reactants and products more intuitive.

Collection of Gas Over Water

The collection of gas over water is a classic method used in chemistry labs for gathering gases released from reactions. The process involves displacing water in a container with the gas produced, typically using a beaker or a specialized gas collecting tube. This method allows for the measurement and capture of gases generated, but it comes with a point of caution.
When gases are collected over water, water vapor is also present in the collected sample. The presence of this vapor affects the total pressure and volume of the gas, necessitating corrections in calculations. Here are some key points to note:

• The pressure of the gas collected is a combination of the gas pressure and water vapor pressure.
• Accurate measurements require adjusting for water vapor by using the vapor pressure of water at the collection temperature.
• This technique is particularly useful for gases that do not dissolve significantly in water, such as oxygen or hydrogen.

Understanding the intricacies of collecting gas over water is instrumental for accurate quantitative analysis and ensures the reproducibility of experimental results.