Question

For which of the following substances is the standard enthalpy of formation equal to zero?

O F₂(g)
O C₃H₈(1)
O CO₂(g)
O H₂O(s)

Answer 1

The standard enthalpy of formation is zero only for diatomic oxygen gas (O2(g)) among the given substances, as it is an element in its most stable form at standard conditions.

Step-by-step explanation:

The standard enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states. For pure elements in their most stable form at 1 atm pressure and 25°C, this value is set to zero.

Thus, for the substances listed: O2(g), C3H8(l), CO2(g), and H2O(s), the standard enthalpy of formation is zero only for diatomic oxygen gas (O2(g)), since it is an element in its standard state. Substances like CO2(g) and H2O(s) are compounds, not elements, and their standard enthalpies of formation are not zero. Propane (C3H8) is also a compound with a non-zero standard enthalpy of formation.