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Chemistry Question 52 – JEE-MAIN 2025

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The correct statement amongst the following is :

The standard state for a substance refers to its pure form at a defined pressure (1 bar for gases) or concentration (1 M for solutions), but it does not fix the temperature.

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Ninja Strategy•Recall Standard Definitions

Correctly recalling the definitions of 'standard state' and 'standard enthalpy of formation' for elements in their most stable forms is key to identifying the correct statement.

Step 1: Analyze the definition of Standard State✦ Active

The standard state for a substance is defined as its pure form at 1 bar pressure for gases, 1 M concentration for solutions, and for pure solids/liquids, it's the pure substance at 1 bar. The temperature is not part of the definition of the standard state itself; rather, standard thermodynamic values are reported at a specified temperature (e.g., 298 K). Therefore, options 1 and 2 are incorrect as they incorrectly fix the temperature for the standard state definition.

Step 2: Analyze the definition of Standard Enthalpy of Formation○ Expand

The standard enthalpy of formation (ΔfH0) of an element in its most stable reference state at a specified temperature and 1 bar pressure is defined as zero. For oxygen, the most stable reference state is diatomic oxygen gas, O2(g). Atomic oxygen, O(g), is not the most stable reference state; its formation requires energy. Thus, option 3 is incorrect because ΔfH2980 for O(g) is not zero.

Step 3: Evaluate Option 4○ Expand

Option 4 states that ΔfH5000 is zero for O2(g). Since O2(g) is the most stable elemental form of oxygen at 500 K and 1 bar pressure, its standard enthalpy of formation at this temperature is indeed defined as zero. This statement is correct.

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