Question

Standard enthalpies of formation are relative values. What are they relative to?

a) Absolute zero
b) Standard temperature and pressure (STP)
c) Standard state
d) Avogadro's number

Answer 1

Standard enthalpies of formation are relative to the standard state of elements at 1 atm and 25°C (298 K), where the most stable form of an element is assigned an enthalpy of zero for uniformity and comparison purposes.

Step-by-step explanation:

Standard enthalpies of formation are relative to the standard state of the elements involved. The standard conditions for tabulating these values are a pressure of 1 atmosphere (atm) for all gases, and a concentration of 1 M for all species in solution. Pure substances must be in their most stable form under these conditions, with the standard temperature assumed to be 25°C (298 K) unless stated otherwise.

By definition, the standard enthalpy of formation for any element in its most stable form at 1 atm and 298 K is set to be zero. For example, oxygen's most stable form at standard conditions is O₂ (g), not ozone or atomic oxygen, and thus has an enthalpy of formation of zero.

These benchmarks provide a reference point, much like sea level is used for measuring heights. It allows for consistency and comparison of enthalpy changes across different substances and reactions.