Question

Interpreting Concepts Absolute enthalpy cannot be determined; only change in energy can be measured. However, absolute entropy can be determined. Explain why an absolute entropy can be determined.

Short answer

Absolute entropy can be determined based on the third law of thermodynamics using a defined zero-point reference like absolute zero.

Step-by-step solution

- Understanding Enthalpy and Entropy

First, it is essential to understand the concepts of enthalpy and entropy. Enthalpy is a measure of the total energy of a thermodynamic system, while entropy is a measure of the disorder or randomness in a system.

- Absolute Enthalpy

Absolute enthalpy cannot be determined because it depends on an arbitrary reference point. The internal energy of a system and the work done by the system are both relative measurements requiring a reference to compare changes.

- Absolute Entropy

Absolute entropy can be determined based on the third law of thermodynamics. This law states that the entropy of a perfect crystal at absolute zero temperature is exactly zero. From this reference point, the entropy of any substance at any temperature can be calculated.

- Calculating Entropy

The absolute entropy of a substance can be determined through calorimetric measurements and integration of heat capacities from absolute zero to the temperature of interest. This cumulative approach allows the calculation of total entropy as a sum of incremental changes.

- Summary

Although absolute enthalpy cannot be determined due to its dependence on a relative reference point, absolute entropy can be measured from a defined zero-point reference (absolute zero), making it possible to calculate entropy directly.

Key concepts

thermodynamic system

A thermodynamic system is any quantity of matter or a region in space chosen for study. Everything outside the system is considered the surroundings. Thermodynamic systems can be classified into three types based on exchanges with the surroundings:

• Isolated Systems: No exchange of energy or matter.
• Closed Systems: Exchange energy but not matter.
• Open Systems: Exchange both energy and matter.

Understanding the limits and interactions within a thermodynamic system is crucial for studying properties like enthalpy and entropy.

third law of thermodynamics

The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero temperature is exactly zero. This law provides a baseline from which absolute entropies can be calculated. Because entropy measures disorder, a perfectly ordered system (perfect crystal) has zero entropy at 0 Kelvin. Using this absolute reference point enables scientists to determine the entropy of substances at any temperature by measuring the changes from this baseline. This concept helps understand why absolute entropy, unlike enthalpy, can be determined.

calorimetric measurements

Calorimetric measurements are techniques used to measure the heat exchanged in physical and chemical processes. By measuring the heat capacities and integrating these values from absolute zero to the temperature of interest, the absolute entropy of a substance can be determined. This process involves:

• Heating the substance incrementally.
• Recording the heat addition at each step.
• Calculating the entropy increase for each temperature increment.

Summing these incremental changes provides the total entropy, allowing for precise measurements essential in thermodynamic studies.

absolute zero

Absolute zero is the lowest possible temperature, 0 Kelvin or -273.15°C. At this point, the particles in a substance have minimum thermal motion. The third law of thermodynamics states that a perfect crystal at absolute zero has zero entropy. This zero-point allows scientists and engineers to measure entropy from a defined reference. In theory, absolute zero is unattainable; however, approaching this limit helps in studying quantum phenomena and fundamental thermodynamic properties.

enthalpy vs. entropy

Enthalpy and entropy are both thermodynamic properties but they describe different aspects of a system:

• Enthalpy refers to the total heat content, involving energy and system work. It can only be measured in terms of changes from a reference state, not as an absolute value.
• Entropy is a measure of disorder. With the third law of thermodynamics, we can establish an absolute baseline (zero entropy) at 0 Kelvin, making it possible to calculate absolute entropies.

Understanding the distinct roles of enthalpy and entropy helps in analyzing energy exchanges and the feasibility of thermodynamic processes.