Asked Dec 9, 2019

Whereas all pure elements in their standard states are defined as having a standard molar enthalpy of formation equal to zero, all elements will have non-zero values of their standard mplar entropies. Provide a reasoned explanation for why this is so. 


Expert Answer

Step 1

A standard enthalpy is defined as the amount of energy released or gained when 1 mole of a substance is made from the elements under standard conditions of temperature and pressure. It is expressed as Ho (J/mol).

From the definition, we can see that it takes no energy to produce an element from an element. For example, it takes no energy to make O2 from O2 , or carbon from carbon. As a result, the standard molar enthalpy of formation of an element in its standard state is always zero.

Step 2

Entropy is defined as a measure of randomness of a system. It is expressed as So (J/mol.K).

Randomness of a system tends to decrease with increasing temperature. Therefore, the standard molar entropy depends on temperature.  

Any substance at ...

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Chemical Thermodynamics

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