doubt about the uncertainty of kinetic theory of gases

[yohai]

i was reading about the kinetic theory of gases and also about its assumptions. while reading, i felt a bit curious that why do we even need to count the  assumptions of ktg and why not just use them for every case

[studiot]

33 minutes ago, yohai said:

i was reading about the kinetic theory of gases and also about its assumptions. while reading, i felt a bit curious that why do we even need to count the  assumptions of ktg and why not just use them for every case

Hello we really need more information to be able to help you.

What do you mean by 'counting the assumptions' and where did that idea come from?

What do you mean by every case ?
What exactly are you using the kinetic theory to model ?

Boyles ,Charles or Avogadros Law, the ideal gas law, or the mechanism of pressure etc. ?

 

[exchemist]

1 hour ago, yohai said:

i was reading about the kinetic theory of gases and also about its assumptions. while reading, i felt a bit curious that why do we even need to count the  assumptions of ktg and why not just use them for every case

Do you mean the assumptions that the molecules engage in perfectly elastic collisions and that the volume they take up can be neglected? These are just the types of simplifying assumption that science theories often resort to, in order to build a simple, idealised model that is easy to work with. It is very often found that by working with simplified models one can get pretty close to predicting behaviour correctly. (Simplifying, in order to see the main thing that is going on, is a very useful way to approach many problems in life). 

What is also interesting is then to investigate the deviations of some real substances from the ideal behaviour predicted by the model. That can allow you to understand the particular extra effects that are responsible. In this example, the effect of the volume taken up by the molecules becomes important at high gas pressures (i.e. when the molecules don't have a lot of room to move around in). Similarly at very low absolute temperatures, the slight "stickiness" between molecules resulting from van de Waals attraction between them can be important.  Both cause deviations from the simple kinetic theory model (and in fact van der Waals himself developed revised equations to account for each of these effects).  

So the basic model explains most of what is observed, while studying the deviations gives you a more complete level of understanding.

Edited October 17 by exchemist

[sethoflagos]

2 hours ago, yohai said:

i was reading about the kinetic theory of gases and also about its assumptions. while reading, i felt a bit curious that why do we even need to count the  assumptions of ktg and why not just use them for every case

Because almost all of the assumptions are only approximations of real world gas behaviour under similar atmospheric conditions to those we are all routinely familiar with at earth's surface. 

For simple, non-critical cases these approximations are often close enough for the differences to be unimportant. 

However, for more complicated, more critical situations, it is necessary to replace the less valid assumptions with empirically grounded correlations specific to the use-case. And to do this you need a thorough working understanding of what those assumptions are in the first place. 

O, yes and in passing - if you don't have that thorough working understanding, you don't even get to judge what counts as simple and non-critical.

Edited October 17 by sethoflagos

[yohai]

15 hours ago, studiot said:

Hello we really need more information to be able to help you.

What do you mean by 'counting the assumptions' and where did that idea come from?

What do you mean by every case ?
What exactly are you using the kinetic theory to model ?

Boyles ,Charles or Avogadros Law, the ideal gas law, or the mechanism of pressure etc. ?

 

like i read that we need to assume:

1. gas molecules are spherical and they follow newtons laws of mechanics.
2. molecules of gas has negligible volume as compared to whole volume of gas.
3. attraction or repulsion doesnt occur between molecules of gases

13 hours ago, sethoflagos said:

Because almost all of the assumptions are only approximations of real world gas behaviour under similar atmospheric conditions to those we are all routinely familiar with at earth's surface. 

For simple, non-critical cases these approximations are often close enough for the differences to be unimportant. 

However, for more complicated, more critical situations, it is necessary to replace the less valid assumptions with empirically grounded correlations specific to the use-case. And to do this you need a thorough working understanding of what those assumptions are in the first place. 

O, yes and in passing - if you don't have that thorough working understanding, you don't even get to judge what counts as simple and non-critical.

sir im a 17 year old student and was just being curious about a topic which still had to be taught in my school.

[swansont]

43 minutes ago, yohai said:

like i read that we need to assume:

1. gas molecules are spherical and they follow newtons laws of mechanics.
2. molecules of gas has negligible volume as compared to whole volume of gas.
3. attraction or repulsion doesnt occur between molecules of gases

These are things you assume in order to develop the mathematical model. As long as these assumptions are reasonable, the model will work well. Models that try to incorporate fewer assumptions are more complicated.

Attraction and repulsion, for example - if these have a small effect the model works. If the system is cold and the molecules don’t have much energy, these become important, so you expect the equations won’t give you the correct behavior. Similarly for high pressure, where the atoms spend a lot of time close to each other, so the short-range interactions become important.

 

[studiot]

2 hours ago, yohai said:

like i read that we need to assume:

1. gas molecules are spherical and they follow newtons laws of mechanics.
2. molecules of gas has negligible volume as compared to whole volume of gas.
3. attraction or repulsion doesnt occur between molecules of gases

sir im a 17 year old student and was just being curious about a topic which still had to be taught in my school.

Thank you for replying is such a prompt and friedly manner.  +1

Yes these are the ones I had in mind as the basic ones that will lead to the mathematical Gas Laws as swansont mentions both as an explanation of what is going on and what the mathematical relationships are.

Since you are keen, this is a good reliable site for answers to all sorts of questions,

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/idegas.html

But if you have any more questions don't hesitate to ask here.

 

Important points for Chemists to note are that these assumptions are not exact for real gases and that some real gases are more ideal than others.

However for many gases they are good enough in everyday temperatures and pressures.

The inert gases (Helium, Argon, Xenon etc) are the nearest to ideal which is to be expected from assumption 3 as their molecules are monatomic.

Polyatomic gases such as hydrogen, oygen and carbon dioxide are more reactive and less ideal.
They are also not as 'spherical' but this assumption has less effect at everyday conditions.
 

Another implied assumtion is that the molecules are all the same, that is we are dealing with a single gas, not a mixture.