Problem 84 - Chemistry The Science in Context

Question

Liquid hydrogen boils at \(-253^{\circ} \mathrm{C} .\) What is the boiling point of \(\mathrm{H}_{2}\) on the Kelvin scale?

Step-by-Step Solution

Answer

Answer: The boiling point of liquid hydrogen in Kelvin is 20.15 K.

Step 1: Convert Celsius to Kelvin

To convert the given temperature in Celsius to Kelvin, use the conversion formula \(K = C + 273.15\). With \(C = -253^{\circ}\mathrm{C}\), we have: \(K = -253 + 273.15\)

Step 2: Calculate the Kelvin temperature

Now calculate the result: \(K = 20.15\)

Step 3: Final answer

The boiling point of \(\mathrm{H}_{2}\) on the Kelvin scale is \(20.15\,\mathrm{K}\).

Key Concepts

Boiling Point

Understanding the boiling point of a substance is fundamental in various fields including chemistry and environmental science. The boiling point is the temperature at which a liquid turns into a vapor. This happens when the vapor pressure of the liquid equals the atmospheric pressure surrounding the liquid.

When discussing boiling points, it's important to note that they can change under different conditions. For instance, at higher altitudes, the atmospheric pressure is lower, which means liquids boil at lower temperatures. Conversely, when the pressure increases, such as in a pressure cooker, the boiling point rises.

• Water boils at 100°C at sea level but boils at lower temperatures on a mountain top.
• Liquid nitrogen, often used in cryogenics, boils at -196°C.

Boiling points are also indicative of a substance's physical properties, and chemists can use them to identify or characterize a substance.

Temperature Conversion

Temperature conversion is a process used in science and daily life to translate a temperature reading from one scale to another. The most common temperature scales are Celsius, Fahrenheit, and Kelvin. Each scale has its own uses and range of convenience:

Celsius and Fahrenheit

These are used for everyday temperatures. Celsius is commonly used around the world and in scientific contexts, while Fahrenheit is primarily used in the United States for everyday applications.

Kelvin

The Kelvin scale is crucial for scientific measurements, as it is the SI base unit for temperature and does not have negative values, representing absolute thermodynamic temperature.

As demonstrated in the exercise, converting temperature from Celsius to Kelvin is straightforward - just add 273.15 to the Celsius value. This step acknowledges the principle that the increments in both scales are the same size, so you're effectively adjusting for the different starting points of each scale (0°C equals 273.15 K).

Kelvin Scale

The Kelvin scale is a thermodynamic temperature scale named after William Thomson, who was also known as Lord Kelvin. This scale is one of the cornerstones of physics and chemistry since it is used to define the absolute scale of temperature.

The zero point on the Kelvin scale, 0 K, is known as absolute zero, which is theoretically the coldest possible temperature, where all molecular motion stops. The Kelvin scale is preferred in scientific work because, unlike the Celsius or Fahrenheit scales, it is an absolute scale with no negative numbers. This makes it particularly useful for scientific calculations and studies of thermodynamics.

• The Kelvin scale is used when it is important to treat all temperatures on an equal footing and when dealing with very high temperatures – such as those in astrophysics.
• It is also key in studying the properties of gases, where the relationship between pressure, volume, and temperature is critical.

Remembering that the difference between the freezing point of water on the Celsius scale and the Kelvin scale is always 273.15 provides a quick reference for conversion between these temperature scales. This value is known as the Kelvin to Celsius constant.