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You are watching: Which gases behave most like an ideal gas?


I was wondering i beg your pardon gas would behave many ideally out of $\ceH2,$ $\ceHe,$ $\ceCO2.$ every gasses are in the same condition.

I recognize the answer is one of two people $\ceH2$ or $\ceHe$ since of London dispersion forces. I was wonder which one the was and why.


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In the simplest model, a gas is dubbed ideal once its particles are point-like (no volume) and have no interactions. Actual gases behave prefer ideal gases at low pressure (where the particle volume is neglible contrasted to the complete volume) and high temperature (where condensed phases, i.e. Interatomic or intermolecular interactions space disfavored).

The size-comparison in between helium and also dihydrogen is straightforward: Dihydrogen is larger. Together for the stamin of interparticle interactions, we have the right to compare normal boiling points: Helium"s is 4 Kelvin and also dihydrogen"s is 24 Kelvin.

So this would suggest that helium is "more ideal" together it has the lower boiling point and the smaller size.

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answered Dec 18 "19 at 18:34
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Karsten TheisKarsten Theis
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A straightforward means to advice ideality is to compute the compressibility Z:

$$Z=\fracPV_mRT$$

Z amounts to 1 for an ideal gas, so deviations from this condition serve as a measure up of non-ideality. If you examine a plot the compressibility because that a real gas you will in general notification the presence of 2 regimes: in ~ low press the compressibility is smaller than 1 and also shows a minimum before rising again and also eventually exceeding 1. The deviation from appropriate gas actions at low ns is because of attractive interaction whereas deviations in ~ high ns are because of repulsive excluded volume interactions.

One means to infer ideality is to examine van der Waals gas parameters. The adhering to are the crucial point parameters and vdW parameters because that the gases:

$$\beginarray \hline \textrmcompound&T_c/\puK&P_c/\pubar&V_c/\puL*mol^-1&a/\puL*mol^-1&b/\pubar*L^2*mol^-2 \\ \hline \ceHe &5.195 &2.275&0.0578 &0.0346&0.0237 \\ \ceH_2 &32.938&12.838& 0.065& 0.2465&0.0267 \\ \ceCO_2 &304.14& 73.843&0.094&3.655&0.0428\\ \hline \endarray$$

Since helium has actually the smallest crucial volume $V_c$, it likewise has the the smallest van der Waals parameter $b$ (the covolume) and is expected to have actually the smallest hard-sphere radius. Helium additionally displays the smallest value of the valve der Waals parameter $a$ which shows the strength of attractive interactions, vital at short pressure. This is regular with helium being the the smallest closed covering (inert) monoatomic (spherically symmetric) noble gas, and also nearest amongst the gases to a point particle. If the valve der Waals parameters are supplied to guess the compressibility coefficient (which they might do qualitatively if not quantitatively) using a freely accessible Matlab duty the following figure is obtained at 50 K:

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Clearly helium shows the smallest deviations from Z=1 and also therefore behaves most ideally.

As hinted in a comment, $\ceCO2$ is solid at the temperature and pressure shown in the above figure. In ~ 250 K but it"s habits is still quite non-ideal:

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Interestingly, in ~ 250 K hydrogen is much more ideal 보다 helium (if just slightly so), if the vdW forecast is to be trusted.

An exciting aside is the helium has actually the lowest values of crucial temperature $T_c$ and crucial pressure $P_c$.