Question

What are the enthalpies of all elements in their standard states?
${\text{A}}{\text{.}}$ Unity
${\text{B}}{\text{.}}$ Zero
${\text{C}}{\text{.}}$ < 0
${\text{D}}{\text{.}}$ different for each element

Answer 1

Hint- Here, we will proceed by defining the term enthalpy. Then, we will discuss the law of conservation of enthalpy. Finally, we will mention some elements in their standard states along with their enthalpy of formation.

Complete answer:
Enthalpy is defined as the sum of inner energy and the product of a thermodynamic system's pressure and volume. Enthalpy is an energy similar to property or a function of the state. It has the energy dimensions (and is thus expressed in joules units), and its value is entirely determined by the system 's temperature, strain, and composition. The enthalpy H equals the sum of the internal energy E and the product of the system's pressure P and volume V
i.e., H = E + PV
The increase in internal energy is equal to the heat transferred to, minus the work performed by the system, according to the energy conservation law. If the only job done is a volume change at constant pressure, the change in enthalpy is exactly equal to the heat transferred to the system. When energy is needed to be added to a material to change its phase from a liquid to a gas, that amount of energy is called the vaporization enthalpy (or latent heat) and is expressed in joules units per mole. Many phase transitions have similar enthalpy related changes, such as the fusion enthalpy (or latent heat) for changes from a solid to a liquid. As with other energy functions, the determination of absolute enthalpy values is neither convenient nor necessary.
The zero-enthalpy status can be some convenient reference state for each substance.
Any element that exists as aluminum in its standard state exists as Al and oxygen exists as ${{\text{O}}_2}$ having zero enthalpy.
The enthalpy of formation for an element in its elementary state will always be 0, because it takes no energy to form a compound that occurs naturally.
Therefore, the enthalpies of all the elements in their standard states is zero.

Hence, option B is correct.

Note- The standard enthalpy of formation is defined as the change in enthalpy when under the same conditions one mole of a standard state substance (1 atm of pressure and 298.15 K) is formed out of its pure elements. Standard enthalpy of formation is a measure of the energy produced or consumed when one mole of a material is formed from its pure elements under standard conditions.