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# Assertion: The enthalpy of formation of gaseous oxygen molecules at 298 K and under a pressure of one atmosphere is zero.Reason: The entropy of formation of gaseous oxygen molecules under the same conditions is zero.(A) If both (A) and (R ) are correct, and (R ) is the correct explanation for (A).(B) If the both (A) and (R ) are correct, but(R ) is not a correct explanation for (A).(C) If (A) is correct, but (R ) is incorrect.(D) If (A) is incorrect, but (R ) is correct.

Last updated date: 21st Jul 2024
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Hint :The standard reaction enthalpy for forming a compound from its elements (atoms or molecules) in their most stable reference states at the chosen temperature (298.15K) and pressure (1bar) is the enthalpy of formation.

The standard enthalpy of formation is a measurement of how much energy is emitted or absorbed when one mole of a material is made from its pure elements under standard conditions.
The standard reaction enthalpy for forming a compound from its elements (atoms or molecules) in their most stable reference states at the chosen temperature (298.15K) and pressure (1bar) is the enthalpy of formation.
Oxygen is a chemical element with the atomic number 8 and the symbol O. It is a highly reactive nonmetal and an oxidising agent that readily forms oxides with most elements and other compounds. It belongs to the chalcogen group in the periodic table.
${O_2}(g) \to {O_2}(g)$
The enthalpy of formation of gaseous oxygen molecules at 298 K and under a pressure of one atmosphere is zero.
$\Delta S = 0$
The standard enthalpy of formation is expressed in kilojoule per mole, but it can also be expressed in kilocalorie per mole, joule per mole, or kilocalorie per gram.
Since there is no change in their composition, all elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero.
The formation reaction takes place at constant pressure and temperature. The standard formation enthalpy or reaction heat is a function of temperature since the pressure of the standard formation reaction is fixed at 1 bar.
Hence, option B is correct.

Note :
The formation reaction takes place at constant pressure and temperature. The standard formation enthalpy or reaction heat is a function of temperature since the pressure of the standard formation reaction is fixed at 1 bar.