How much energy is available?
As scientists explore energy supplies, geothermal sources look very appealing. The natural geysers that exist in some parts of the world could possibly be harnessed to provide power for many purposes. The change in energy content and the release of energy caused by steam condensing to liquid can help fill some of our growing energy needs.
All molecular motion ceases at absolute zero (0 K). Therefore, the entropy of a pure crystalline substance at absolute zero is defined to be equal to zero. As the temperature of the substance increases, its entropy increases because of an increase in molecular motion. The absolute or standard entropy of substances can be measured. The symbol for entropy is
The knowledge of the absolute entropies of substances allows us to calculate the entropy change
The entropy change for the vaporization of water is positive because the gas state has higher entropy than the liquid state.
In general, the entropy change for a reaction can be determined if the standard entropies of each substance are known. The equation below can be applied.
The standard entropy change is equal to the sum of all the standard entropies of the products minus the sum of all the standard entropies of the reactants. The symbol “
The entropy change for this reaction is highly negative because three gaseous molecules are being converted into two liquid molecules. According to the drive towards higher entropy, the formation of water from hydrogen and oxygen is an unfavorable reaction. In this case, the reaction is highly exothermic and the drive towards a decrease in energy allows the reaction to occur.
- Calculations of change in entropy using standard entropy are described.
Read the material at the link below and do the problems:
- When is the entropy of any material at its lowest?
- In the reaction involving the formation of water from hydrogen and oxygen, why is the entropy value negative?
- Why would diamond have a lower standard entropy value than graphite?