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Joule-Thomson coefficient for a gas, \(\mu_{j}\) obeying the relation \(p(v - b) = RT\) is

  • A.\(\mu_{j} = \dfrac{c_{p}}{b}\)
  • B.\(\mu_{j} = \dfrac{b}{c_{p}}\)
  • C.\(\mu_{j} = -\dfrac{b}{c_{p}}\)
  • D.\(\mu_{j} = -\dfrac{c_{p}}{b}\)

Answer: C

The Joule-Thomson coefficient is \(\mu_{j} = -\dfrac{b}{c_{p}}\).

In general

\[ \begin{aligned} \mu_{j} &= \left(\frac{\partial T}{\partial p}\right)_{h} \\ &= \frac{1}{c_{p}}\left[T\left(\frac{\partial v}{\partial T}\right)_{p} - v\right] \end{aligned} \]

From \(v = RT/p + b\), \((\partial v/\partial T)_{p} = R/p\), so \(T(R/p) - v = (v - b) - v = -b\), and \(\mu_{j} = -b/c_{p}\).

The negative value means such a gas always warms on throttling. The positive \(b/c_{p}\) has the wrong sign, and \(\pm c_{p}/b\) inverts the expression and does not even have units of K/Pa.

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