CH0002 - Lecture 8
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What is Boyle's Law?
For a fixed mass of gas at constant temperature, the volume is inversely proportional to the pressure.
That means that, for example, if you double the pressure, you will halve the volume. If you increase the pressure 10 times, the volume will decrease 10 times.
V ∝ 1/P
If a flash containing an ideal gas at a pressure of 5 atm is connected to a second flask of volume 300 cm3 the pressure drops to 3 atm
What is the volume of the first flask?
What is Charles' Law?
For a fixed mass of gas at constant pressure, the volume is directly proportional to the kelvin temperature.
That means, for example, that if you double the kelvin temperature from, say to 300 K to 600 K, at constant pressure, the volume of a fixed mass of the gas will double as well.
What is Avogadro's principle?
The volume of a sample of gas at given temperature and pressure is proportional to the number of gas molecules in the sample
V ∝ n
(Independent of which gas it is)
V ∝ n
V = constant x n
Volume of 1 mole N2 = Volume of 1 mole of CO2
What is the volume occupied by 1 mole of a gas?
Calculate the volume occupied by 10g CO2
What is the Ideal Gas Law?
What are the 3 assumptions for the Ideal Gas Law?
1. Gases consist of small particles which have negligible volume
2. Gas molecules are in constant rapid motion, undergoing elastic collisions with each other and their container.
3. Molecules do not interact with each other except during collisions.
What experimental evidence is there in favour of the ideal gas law assumptions?
Evidence for these assumptions:
1 : Compressibility of gases
2 : Brownian motion
3 : Gases expand to fill their containe
What experimental evidence is there against the ideal gas law assumptions?
Evidence against these assumptions:
1 : Molecules have finite mass
2 : Attractive interactions exists between molecules measured pressure < “ideal” pressure
3 : Volume of molecules is not negligible
Why is the measured pressure , "Ideal" pressure (at low to moderate pressure)?
Pressure is due to collisions with container walls.
Attractive dispersion forces exist between molecules
Molecules near the walls are attracted to those in the bulk, therfore:
Pmeasured < Pideal
How do gasses deviate at moderate pressure?
What is the distribution of energies?
The average kinetic energy of particles within a gas depends on the temperature, but there is a distribution of kinetic energies amongst the gas particles.
What is the maxwell-boltzmann distribution?
Because of the key role of activation energy in deciding whether a collision will result in a reaction, it would obviously be useful to know what sort of proportion of the particles present have high enough energies to react when they collide.In any system, the particles present will have a very wide range of energies.
For gases, this can be shown on a graph called the Maxwell-Boltzmann Distribution which is a plot of the number of particles having each particular energy.
The area under the curve is a measure of the total number of particles present.
How does temperature affect reaction rate?
As you increase the temperature the rate of reaction increases. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10°C rise in temperature.
You have to be careful not to take this too literally. It doesn't apply to all reactions. Even where it is approximately true, it may be that the rate doubles every 9°C or 11°C or whatever. The number of degrees needed to double the rate will also change gradually as the temperature increases.
Particles can only react when they collide. If you heat a substance, the particles move faster and so collide more frequently. That will speed up the rate of reaction.
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