Chapter 15 IDEAL GASES Flashcards
Definition for one mole
The amount of substance equal to the number if entities in 12g of carbon 12
This number is avagadro, 6.02 x10^23
So 2 mol of water molecules = 12.02 molecules of water
What is the kinetic theory of gases (matter)
How this differnt to kinetic model
Kinetic theory = a model used to describe the BEHAVIOUR OF IDEAL GASES
Kinetic model is just a theory to describe how matter behaves
What are the 5massumptions of an ideal gas under kinetic theory then
1) Atoms of molecules of gas have NEGLIGIBLE INTERMOLECULAR FORCES EXCEPT during collisions
2) they move randomly in all directions in random speeds
3) the time spent between colliding is negligible compared to time spent without colliding
4) they occupy a negligible amount of volume compared to the empty space between them
5) the Collisions of the gases with each other and containers are PERFECTLY ELASTIC
Again
1) the electorsticnforces of attraction between gases are negligible except during Collisons
2) time spent between colliding is nhekigble compared to time spent without colliding
3) volume occupied by the gas is negligible compared to volume of empty space between them
4) Collins between the gases and containers are perfectly elastic and thus no energy is lost
5)gases move in random speeds with random direction
1) time spent between collisions is neg compared to time without
2) volume gas neg comported ti empty space
3) intemrkelcuwr forces neg except during collisions
4) collisons are elastic
5) move in random soeed and random direcjron
Explain them and why they are good
Why is the Colision one good
1) Time spent between (don’t know)
2) random velocities random directions , good as proved by briwnian motion, if there wasn’t random motion, then there would be some sort of net but we can’t see so it’s good assumption
3) intermolecular forces are negligible = good assumption as it shows that 1 mol of any gas occupies same volume at RTP
4) volume occupied is neg vimsored ti empty space, good as it shows that gases can be compressed
5) collisions are elastic and so no energy is lost IS GOOD as we don’t know where energy would be lost anyways !
Using newtons laws, how can we show that gases extend pressure in a container
Using assumption that the collisons with the walls are perfectly elastic, it means that the speed of the particle is not changed when it collides but only velocity.
The force exerted on the particle = to the change of its momentum/ time, this is -mu -mu = -2mu / time
By newtons 3rd law and due to impulse being conserved, thr psrticle also exerts this force to the container at right angles
The total force exerted by al, particles / area if the container is equal to the PRESSURE
What assumptions now gonna be used in ALL EQUATIONS from now on
1) model gas is ideal gas
2) moles of gas is CONSTANT
For constant temp, volume and pressure what variables are proportional to each other
Pressure prop to 1/ v (temp constant , increase volume moleucles disperse so less pressure)
Pressure prop to temp (volume CONSTSNT ) (increase temp molecules move around more inc pressure)
Volume prop to temp (pressure constant)
Why does increasing the temp increase pressure for given volume (2 marks)!
1) inc temp inc ke which increases velociyt of psrticles = increases change of momentum when bump with wall= harder bump = increases force per area and pressure
2) inc ke means FREQUENCY OF COLLISIONS INCREASE TOO, so more collisions means more force exerted in same time as more collisions with each other and wall
What is boyles law and what main thing tk remember when investigating
Thus why at very low volumes does it breakdown
This si pressure and volume
Here you make volume smaller and watch as pressure increases
- however need ti keep temp CONSTSNT
- so ensure you SLOWLY push and reduced volume as pushing it does work externally onto to the particles which increase their temp
- if you do it slowly, there is time for the particles to disperse extra energy
Thus do it slowly + wait a bit for the temp to go down
2) at low volumes pressure too high snd temps are too hard to control, so investigating it breakdowns the formula
What to do then when doing pressure volume boyles law experiment
Slowly push
Wait until it cools
So temp stays constant
Apparatus needed for boyles law ?
- volume scale
- connected to a pressure gauge
- connedted to a foot pump closed so you can inc pressure
How to find absolute zero value for vs temperature?
Gonns plot different pressures at constant volume due to different temps
- can’t get to 0K
- if you plot I’m degrees, you’ll see it’s prodptimsl + 273 as intercept
If you extrapolate back to when pressure is 0, as that’s defintion for absolute zero, you’ll see the temperature in degrees is -273, which means absolute zero = -273°!
Combining gas laws, what do you get
What’s constant
Also how tk do questions with these rather than fsctor method
PV prop to to t
So PV = kT
K=NR
Anyways use PV/T = constant as it’s hard to do 3 factors changes at once
How to work out what a lower gradient means for the other variable she;plotting any graph
Rearrange PV=NRT
find grsdient , equate that and see why it would be lower
So lower for P against 1/V, wherre grsdient = NRT, for constant moles is temp is lower
How to prove PV=NRT
In these experiments, how to keep pressure, volume, tempersture, moles constant so you can vary other
1) just plot p against 1/v or something and chekc grsdient
2) - pressure constant = put a hole in it so pressure is now ATMOSPHERIC PRESSURE
- for temp, either use ROOM TEMP, or water bath to heat it to a constant temp
- for moles, ensure the plunger is secure and CLOSED so it doesn’t change
- for volume, this is proprtionsl to LENTGH ss surface area constant . As long as continued is RIGID, the volume will be constant !
What is RMS and why do we use it as an average for oartickes speeds
RMS is the root mean sqaure speed
Can’t use average velocity as vector, over large dataset they will sum to 0 and thus average woukdbe 0
Thus to take this into account, sqaure each, find mean, this is c2 hat
Then find sqaure root, this is root mean sqaure speed
Why is RMS and average speed different
Rms is sqaured then mean
Average speed is just ignire vector and average
They won’t be the same!
When do gas law equations no longer work (what conditions)
When temp too low as firces are made (thus ideal gas assumption breakdown)
When pressure too high for same reason
Why is rms used instesd of speed
They are both averages, and normally rms is higher
It just festures in more equations and can be differentiated
What is equation linking PV and rms
What is N (big N?)
PV = 1/3NM c 2 hat
N = total amount of molecules of gas = moles n x a Avogadro
How is speed if particles represented on a Boltzmann distribution (chem ideas)
And at a higher temp?
Just that 0 particles have 0 speed, and that a particle doesn’t have max speed
Y axis = number , x = speed / energy for chem
And at higher temps, the graph shifts down and to the right getting wider
- More particles have higher speeds
- range of speeds increase
How are three averages shown on Boltzmann distribution (mode, average speed, rms)
Mode is the value of the stationary point, average speed to the right and rms even mroe
What is Boltzmann constant
This is R/ Na
Gas constant 8.31/ abagavro = K
How to get a second equation of PV from PV = nRT
N = n x Avogadro
N = N/ Avadgro
Sub this in and yiu get PV = NKT
