The question asks about the enthalpies of elements in their standard states. To understand the correct answer, we need to delve into the concept of standard enthalpy.
Concept of Standard Enthalpy:
- Standard enthalpy refers to the enthalpy change when one mole of a substance is formed from its elements in their most stable forms under standard conditions (1 atm pressure and 298 K temperature).
- The convention in thermochemistry is to assign a standard enthalpy of zero to all pure elements in their most stable physical forms at 1 atm and 298 K. These physical states are often called the 'reference state' or 'standard state'.
Explanation of Options:
- Unity: This implies that the enthalpies of elements in their standard states are 1, which is incorrect.
- Zero: As explained, the standard enthalpy of pure elements in their standard states is defined to be zero for consistency in thermodynamic calculations.
- 0: Numerically the same as the previous option 'zero', thus also correct.
- Different for each element: This would mean that each element has a unique standard enthalpy, which contradicts the established convention of having a zero enthalpy in their standard states.
Given these explanations, the correct answer to the question is that the enthalpies of all elements in their standard states are zero.
Conclusion: Hence, the statement "The enthalpies of all elements in their standard states are zero" aligns with the established principles of thermochemistry.