Standard State & Enthalpy Explained in 1 Minute! ⚡ | JEE Main Chemistry Concept

 

❓ Question

Which of the following statements is correct?

1️⃣ $\Delta_f H^\circ_{298}$ is zero for O(g)
2️⃣ The standard state of a pure gas is the pure gas at a pressure of 1 bar and temperature 273 K
3️⃣ The term standard state implies that the temperature is 0 °C
4️⃣ $\Delta_f H^\circ_{500}$ is zero for O₂(g)

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🖼️ Question Image

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✍️ Short Solution

Let’s analyze each statement carefully using thermodynamic definitions.

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🔹 Step 1 — Recall definition of standard enthalpy of formation (ΔfH°)

The standard enthalpy of formation is the enthalpy change when 1 mol of a substance is formed from its constituent elements in their most stable states at 1 bar pressure and a specified temperature (usually 298 K).

By convention:

$${\mathrm{\Delta }}_f{H}^{\circ }=0\text{for all elements in their standard states at 298 K.}$$

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🔹 Step 2 — Check each statement

(1) $\Delta_f H^\circ_{298}$is zero for O(g)

❌ Incorrect.
The standard state of oxygen is O₂(g), not atomic oxygen.
To form O(g), you must break the O=O bond:

$$\frac{1}{2}\text{O}_2(g) → \text{O}(g)$$

That requires energy (bond dissociation enthalpy ≠ 0).
Hence, $\Delta_f H^\circ_{298}$ for O(g) ≠ 0.

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(2) Standard state of a pure gas is the pure gas at 1 bar and 273 K

❌ Incorrect.
The correct definition:
Standard state of a pure gas = the pure gas at 1 bar pressure, at a specified temperature (usually 298 K).
Temperature is not fixed at 273 K; it’s a variable reference, commonly 298 K.

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(3) The term “standard state” implies that the temperature is 0 °C

❌ Incorrect again.
Standard state does not fix the temperature at 0 °C; it’s usually 25 °C (298 K) in most data tables.

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(4) $\Delta_f H^\circ_{500}$ is zero for O₂(g)

✅ Correct.
Even at 500 K, O₂(g) remains the standard state of oxygen.
The convention that ΔfH° = 0 for elements in their standard states holds at any temperature.

Hence, this is the correct statement.

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🧮 Image Solution

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✅ Conclusion & Video Solution

✅ Final Answer:

$$\boxed{{\displaystyle (4){\mathrm{\Delta }}_f{H}_{500}^{\circ }\text{ is zero for }{O}_2(g)}}$$

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📘 Concept Recap:

• Standard enthalpy of formation (ΔfH°) for any element in its most stable form = 0.

• Standard state → 1 bar pressure and specified T (often 298 K), not fixed at 273 K.

• Atomic forms like O(g) or N(g) are not standard states — they require bond dissociation.