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You are watching: Which of the following gases is expected to behave most ideally

I was wondering which gas would behave actually a lot of ideally out of $ceH2,$ $ceHe,$ $ceCO2.$ All gasses are in the exact same problem.

I know the answer is either $ceH2$ or $ceHe$ because of London dispersion forces. I was wondering which one it was and why.



In the simplest model, a gas is referred to as best when its pposts are point-choose (no volume) and have no interactions. Real gases behave actually favor appropriate gases at low pressure (where the particle volume is neglible compared to the full volume) and also high temperature (where condensed phases, i.e. interatomic or intermolecular interactions are disfavored).

The size-compariboy between helium and also dihydrogen is straightforward: Dihydrogen is larger. As for the strength of interpshort article interactions, we deserve to compare normal boiling points: Helium"s is 4 Kelvin and also dihydrogen"s is 24 Kelvin.

So this would certainly imply that helium is "even more ideal" as it has the reduced boiling allude and also the smaller dimension.

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answered Dec 18 "19 at 18:34

Karsten TheisKarsten Theis
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A straightforward means to evaluate ideality is to compute the compressibility Z:


Z equals 1 for a suitable gas, so deviations from this condition serve as a measure of non-ideality. If you study a plot of compressibility for a actual gas you will certainly in basic notice the existence of two regimes: at low pressure the compressibility is smaller than 1 and also reflects a minimum before climbing aobtain and eventually exceeding 1. The deviation from right gas habits at low P is as a result of attrenergetic interactions whereas deviations at high P are as a result of repulsive excluded volume interactions.

One means to infer ideality is to examine van der Waals gas parameters. The following are the critical suggest parameters and also vdW parameters for the gases:

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

Because helium has the smallest instrumental volume $V_c$, it likewise has actually the smallest van der Waals parameter $b$ (the covolume) and is supposed to have the smallest hard-spright here radius. Helium also display screens the smallest worth of the van der Waals parameter $a$ which mirrors the toughness of attrenergetic interactions, necessary at low pressure. This is constant via helium being the smallest closed shell (inert) monoatomic (spherically symmetric) noble gas, and also nearest among the gases to a point pshort article. If the van der Waals parameters are used to predict the compressibility coreliable (which they can carry out qualitatively if not quantitatively) making use of a easily easily accessible Matlab attribute the adhering to number is obtained at 50 K:


Clbeforehand helium reflects the smallest deviations from Z=1 and also therefore behaves a lot of ideally.

As hinted in a comment, $ceCO2$ is solid at the temperature and also push shown in the above number. At 250 K but it"s behavior is still quite non-ideal:


Interestingly, at 250 K hydrogen is more appropriate than helium (if just slightly so), if the vdW prediction is to be trusted.

An exciting aside is that helium has the lowest worths of critical temperature $T_c$ and also important pressure $P_c$.