Question

which statement about enthalpy is true?

enthalpy is a state function because its change depends only on initial and final conditions.

the enthalpy of formation for a pure element in its standard state is always positive.

enthalpy is not a state function because its change depends on the identities of the reactants and products.

the enthalpy of formation for a pure element in its standard state is always negative.

Explanation:

Analyze the definition of state functions

A state function is a property whose value depends only on the current state of the system, not on the path taken to reach that state. Since enthalpy change (\(\Delta H\)) depends solely on the initial and final states of a system, enthalpy is classified as a state function.

Evaluate standard enthalpy of formation

Using the Standard Enthalpy of Formation concept, we recall that the standard enthalpy of formation (\(\Delta H_f^\circ\)) of a pure element in its reference standard state is defined as exactly zero, not positive or negative. This rules out the second and fourth options.

Evaluate the remaining options

The third option states that enthalpy is not a state function, which contradicts the established thermodynamic definition of enthalpy as a state function. Therefore, the first statement is the only true statement.

Answer:

• (A) Enthalpy is a state function because its change depends only on initial and final conditions. (Correct answer)
• (B) The enthalpy of formation for a pure element in its standard state is always positive.
• (C) Enthalpy is not a state function because its change depends on the identities of the reactants and products.
• (D) The enthalpy of formation for a pure element in its standard state is always negative.