Question

Which one of the following statements best describes the standard enthalpy of formation of yelement?

a. The value of ΔH∘ r (element) depends on temperature.
b. The value of ΔH∘ r (element) is zero only for elements in the solid state.
c. The value of ΔH∘ r (element) is zero for any element in the standard state.
d. The value of ΔH∘ r (element) is zero only at absolute zero temperature.

Answer 1

The standard enthalpy of formation of an element in its standard state is zero. This standard is used to consistently compare the enthalpy changes of formation for various compounds.

Step-by-step explanation:

The correct statement describing the standard enthalpy of formation (∆H°_f) of an element is: The value of (∆H°_f) (element) is zero for any element in the standard state. Standard enthalpies of formation are defined such that the enthalpy change for the formation of 1 mole of a compound from its component elements in their standard states is measured.

For example, for elemental oxygen, the standard state at 25°C and 1 atm is O2 (g). This means that elements like oxygen have a standard enthalpy of formation of zero since they are already in their most stable form. This is essential for calibrating and comparing the enthalpies of formation of different compounds consistently.