Question

At absolute zero, the entropy of a pure crystal is zero. This is (a) first law of thermodynamics (b) second law of thermodynamics (c) third law of thermodynamics (d) zeroth law of thermodynamics.

Short answer

(c) third law of thermodynamics.

Step-by-step solution

Identifying the Correct Law of Thermodynamics

The statement given in the exercise relates to the entropy of a pure crystalline substance at absolute zero temperature. The law of thermodynamics that states the entropy of a pure crystal at absolute zero is zero is known as the third law of thermodynamics.

Eliminating Incorrect Options

Knowing that the third law of thermodynamics is related to the entropy of a pure crystal at absolute zero, we can eliminate options (a) first law of thermodynamics, (b) second law of thermodynamics, and (d) zeroth law of thermodynamics as they cover different aspects of thermodynamic systems.

Selecting the Correct Answer

With the information provided, we can determine that the correct answer is (c) third law of thermodynamics.

Key concepts

Entropy

Entropy, often described as the measure of disorder or randomness in a system, is a fundamental concept in thermodynamics and statistical mechanics. The more ways a system can be arranged without changing its energy, the higher its entropy. It is denoted by the symbol 'S' and measured in units of joules per kelvin (J/K).

In practical terms, imagine a room full of scattered toys. If the toys are randomly spread out, we can say the room has high entropy. On the other hand, if the toys are neatly organized, the entropy is low. As entropy increases, more energy becomes spread out and less can be used to do work. The second law of thermodynamics tells us that the entropy of the universe tends to increase over time, which means that systems tend to move from order to disorder.

This relates to the textbook exercise question as the third law of thermodynamics provides information on what happens to entropy at absolute zero temperature. It essentially states that at absolute zero (0 Kelvin), the entropy of a perfect crystal is exactly zero, as the atoms are completely ordered and there is only one possible way to arrange them.

Absolute Zero

Absolute zero represents the theoretical temperature at which substances possess no thermal energy. Measured as 0 Kelvin (K), -273.15 degrees Celsius, or -459.67 degrees Fahrenheit, absolute zero is the point on the temperature scale where the particles constituting matter have minimal vibrational motion, meaning they are as still as quantum mechanics allows.

Reaching absolute zero is a challenge in the physical world because of quantum effects that provide particles with zero-point energy, meaning they always have some level of motion. This makes a state of true absolute zero unattainable. However, scientists have managed to cool substances to temperatures fractionally above absolute zero, allowing them to study quantum phenomena that occur at these extreme conditions.

In the context of the exercise, the concept of absolute zero is relevant because the third law of thermodynamics addresses the behavior of entropy at this limit, declaring that the entropy of a perfectly ordered crystal lattice falls to zero at absolute zero, indicating a system of complete order and no disorder.

Thermodynamic Laws

The thermodynamic laws are a set of principles that describe how energy moves and changes form. They are essential for understanding not just physics, but also engineering, chemistry, and even life itself. These laws can be broken down as follows:

• The zeroth law of thermodynamics establishes that if two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
• The first law, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transformed from one form to another.
• The second law introduces the concept of entropy, affirming that the total entropy of an isolated system can never decrease over time.
• The third law, as described in the exercise, asserts that the entropy of a pure crystal at absolute zero is zero, which implies a perfectly ordered state.

Each of these laws prescribes a specific aspect of thermodynamic processes and together, they form a comprehensive framework for understanding the flow and transformation of energy in the universe.