Activity: Heat capacity of N2

Contemporary Challenges 2021 (4 years)
Students sketch the temperature-dependent heat capacity of molecular nitrogen. They apply the equipartition theorem and compute the temperatures at which degrees of freedom “freeze out.”

Follows Quantum mechanical correction to the equipartition theorem.

In the prompt, explain that 50, 500 and 5000 should be equally spaced, for instance. 2, 5, and 10 are also approximately equally spaced.

As it turns out, the heat capacity is about \(\frac52\) for almost the entire temperature range.

For N2 gas molecules in a 10 cm cubic box, the rules of QM dictate the discrete allowed values for:
  1. Translational K.E. in one dimension: \(\left\{ 1\times 10^{-40}\text{ J}, 4\times 10^{-40}\text{ J}, 9\times 10^{-40}\text{ J}, \ldots \right\}\)
  2. Rotational K.E.: \(\left\{ 0\text{ J}, 0.8\times 10^{-22}\text{ J}, 0.8\times 10^{-22}\text{ J}, 0.8\times 10^{-22}\text{ J}, 2.5\times 10^{-22}\text{ J}, \ldots \right\}\)
    The bond length of N2 is 1.1 Å. Its mass is \(m = 14\text{ amu}\approx 2\times 10^{-26}\text{ kg}\). The moment of inertia is thus \begin{align} I &= 2\cdot 2\times 10^{-26}\text{ kg} \left(0.55\times 10^{-10}\text{ m}\right)^2 \\ &\approx 1.2\times 10^{-46}\text{ kg}\cdot\text{m}^2 \end{align} From which we can find the energy eigenvalues: \begin{align} E_{\ell} &= \frac{\hbar^2}{2I}\ell(\ell+1) \\ &= \frac{\left(10^{-34} \text{ J}\cdot\text{s} \right)^2 }{2\cdot 1.2\times 10^{-46}\text{ kg}\cdot\text{m}^2} \ell(\ell+1) \\ &\approx \ell(\ell+1)\cdot 0.4\times 10^{-22}\text{ J} \end{align}
  3. Vibrational energy: \(\left\{ 2.3\times 10^{-20}\text{ J}, 6.9\times 10^{-20}\text{ J}, 11.5\times 10^{-20}\text{ J}, \ldots \right\}\)
    I looked up the experimental vibrational frequency, which is 2358 cm\(^{-1}\).
Sketch a graph of \(\frac{dU}{dT}\) of \(10^{22}\) molecules of N2 gas in the temperature range of 70 K (the temperature at which \(N_2\) becomes liquid at 1 atm of pressure) to 5000 K (at which temperature N2 breaks apart).

(use a logarithmic temperature axis)

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    2. If the engine is to produce 1 GW of electrical power, what minimum volume of water must be processed every second? Note that the specific heat capacity of water \(c_p = 4.2\) Jg\(^{-1}\)K\(^{-1}\) and the density of water is 1 g cm\(^{-3}\), and both are roughly constant over this temperature range.

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    At a power plant that produces 1 GW (\(10^{9} \text{watts}\)) of electricity, the steam turbines take in steam at a temperature of \(500^{o}C\), and the waste energy is expelled into the environment at \(20^{o}C\).

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    2. Suppose you arrange the power plant to expel its waste energy into a chilly mountain river at \(15^oC\). Roughly how much money can you make in a year by installing your improved hardware, if you sell the additional electricity for 10 cents per kilowatt-hour?

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    In carbon monoxide poisoning the CO replaces the \(\textsf{O}_{2}\) adsorbed on hemoglobin (\(\text{Hb}\)) molecules in the blood. To show the effect, consider a model for which each adsorption site on a heme may be vacant or may be occupied either with energy \(\varepsilon_A\) by one molecule \(\textsf{O}_{2}\) or with energy \(\varepsilon_B\) by one molecule CO. Let \(N\) fixed heme sites be in equilibrium with \(\textsf{O}_{2}\) and CO in the gas phases at concentrations such that the activities are \(\lambda(\text{O}_2) = 1\times 10^{-5}\) and \(\lambda(\text{CO}) = 1\times 10^{-7}\), all at body temperature \(37^\circ\text{C}\). Neglect any spin multiplicity factors.

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Learning Outcomes