Charles' law practical questions help

Hi,

Today we did a practical investigating Charles' Law. There are a couple of practical questions that I didn't understand. I'd really appreciate it if someone could explain.

Thanks

I'm afraid that, to tell you how the graph would change, I would need some idea of what graph you plotted!

More detail of the experiment, and what you plotted please!

We plotted length of trapped air column (in a capillary tube) against water temperature.

So the length of the trapped air is proportional to its volume.

V/T = k is just telling you that, according to Charles' law you should get a straight line graph because V= kT so V and T are proportional to one another.

If k increases, then for a given value of T, V (and hence length of air) will be larger, but T and V must still be proportional so you would observe a graph with a straight line but now with a steeper gradient, the reverse is true for k decreasing.

That answers question 7, just wondering, how did you calculate the volume at absolute zero? did you simply extrapolate the line through the points?

Thanks so much, that's really helpful For Q7. I extrapolated back to the x axis to find an approx value for absolute zero.

Fair enough, Question 9 is more difficult. However, if we can assume that each reading was wrong by the same amount (eg by 2mm), then its easy, it would simply mean that the line was translated upwards by 2mm and so the value at absolute zero would be higher!

Thanks There's just one more question which I can't find the answer to which is why nitrogen and oxygen provide good experimental data but carbon dioxide doesn't for gas law experiments.

That's an interesting question.
What it boils down to is the fact that Charles' law (which is an experimental demonstration of the ideal gas equation) only applies for an ideal gas.
Ideal models always involve simplifying the situation so that it is easily modelled.
For the ideal gas model we assume a few things to simplify the problem.N₂ and O₂ have bonds which aren't at all polar. So no atoms in these molecules have any sort of charge and the interactions between molecules are minimised, that means that the third assumption of the ideal gas model is pretty good and so these are good gases with which to demonstrate ideal gas behaviour.

CO₂ is symmetrical and so overall has no dipole, however each C=O double bond is polarised, this means that the Oxygen atoms have partial negative charge and the Carbon has partial positive charge.
Opposite charges attract and like charges repel and so there is a significantly larger degree of intermolecular interaction in CO₂. This makes it less ideal and so it is a worse choice of gas to demonstrate ideal behaviour.

(This extra intermolecular interaction is also the reason that, at atmospheric pressure, CO₂ stops being a gas below -79^oC, whilst N₂ is still gaseous all the way down to -196^oC)

Thanks for all your help, I really appreciate it. This was a physics practical but I was trying to think it through using more chemistry based ideas.