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Question

For an ideal gas, starting from state point 1, two different processes take place. The corresponding final states in these two processes are 2 and 3, lying on same isotherm. If $P$ and $h$ represent pressure and enthalpy, respectively, then which one of the following options is correct?

The correct answer is
$h_2 = h_3$

Ideal Gas Enthalpy Property

For any ideal gas, enthalpy ($h$) is exclusively a function of temperature ($T$). This means $h$ changes only when $T$ changes, and is independent of pressure ($P$) or volume ($V$). We can represent this as $h = f(T)$.

Isotherm Condition Analysis

The question specifies that the final states, point 2 and point 3, lie on the same isotherm.

  • An isotherm represents a process where the temperature remains constant.
  • Therefore, the temperature at state 2 ($T_2$) must be equal to the temperature at state 3 ($T_3$). Mathematically, $T_2 = T_3$.

Deriving Enthalpy Relationship

Based on the properties of an ideal gas:

  • Since $h = f(T)$ for an ideal gas, and we have established that $T_2 = T_3$, the enthalpy values at these two states must be identical.
  • Therefore, $h_2 = h_3$.

This relationship holds true regardless of the pressures ($P_2$ and $P_3$) or volumes at states 2 and 3, as long as they reside on the same isotherm.

The correct option is the one stating $h_2 = h_3$.

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Important Questions from Ideal and Real Gases

  1. A perfect gas at 25°C is heated at constant pressure till its volume is doubled. The final temperature will be-

  2. Which of the following laws states that the volume of a gas is inversely proportional to the pressure of a gas?

  3. The internal energy of a perfect gas does not change during the-

  4. The ratio of specific heat of air at constant pressure to the specific heat of air at constant volume is equal to -

  5. A gas having a negative Joule-Thompson effect (μ < 0), when throttled will

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