How can standard entropies be used to determine absolute entropies?

Standard entropies can be used to determine absolute entropies by using the third law of thermodynamics.

The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero is exactly equal to zero. This provides a reference point for measuring absolute entropies of substances at any temperature. Standard entropy, denoted as S°, is the absolute entropy of a substance at standard conditions (1 atm pressure and a specified temperature, usually 25°C).

To determine the absolute entropy of a substance at a given temperature, one can use standard entropies of substances at 25°C and then apply the heat capacity to adjust for the difference in temperature. The heat capacity, or the amount of heat required to raise the temperature of a substance by one degree, is used to calculate the change in entropy as the temperature changes from 25°C to the desired temperature.

The standard entropy values for many substances have been experimentally determined and are available in tables. These values can be used in calculations involving entropy changes in chemical reactions. For example, the standard entropy change for a reaction can be calculated by subtracting the sum of the standard entropies of the reactants from the sum of the standard entropies of the products.

In summary, standard entropies provide a convenient way to determine absolute entropies of substances at any temperature. By using the third law of thermodynamics as a reference point and applying the heat capacity to adjust for temperature differences, one can calculate the absolute entropy of a substance. This is a fundamental concept in thermodynamics and is crucial for understanding the spontaneity of chemical reactions.