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Chapter 15-
Ideal gases
- An ideal gas is a theoretical model that includes
assumptions to simplify the behaviour of real gases
- There are 6 main assumptions
- 1) Fixed volume
- 2) Large number of atoms/molecules
with random directions + speed
- 3) Volume of atoms in
gas<<<volume of gas
- 4) All collisions are perfectly elastic
- 5) Time taken for collisions are
negligible
- 6) No electrostatic interaction
- 1) Fixed volume
- These assumtions explain how atoms/molecules in
an ideal gas cause pressure
- Pressure arises from force due to
atoms/molecules change in momentum as
they bounce of container walls F=Δp/Δt
- Pressure arises from force due to
atoms/molecules change in momentum as
they bounce of container walls F=Δp/Δt
- There are 6 main assumptions
- Mass- "Amount of matter"
- Kg
- Kg
- Moles- "The amount of substance that contains as many
elementary entities as there are atoms in 12g of carbon-12"
- mol
- (N) Number of atoms or molecules= (n) Number of
moles * (NA) Avagadro's constant
- Avagadro's constant=6.02*10^23
- Avagadro's constant=6.02*10^23
- mol
- Molar mass- "The mass of one mole of a substance"
- (m) Mass of substance= (n) Number of moles * (M) Molar mass
- (m) Mass of substance= (n) Number of moles * (M) Molar mass
- Gas laws
- 1) When temperature & mass of gas remain
constant then... pV=constant
- Boyle's law
- Boyle's law
- 2) When volume & mass of a gas
remain constant then...
p/T=constant
- This relationship allows us to estimate absolute zero
- This relationship allows us to estimate absolute zero
- Combining the gas laws 1 & 2 gives
(pV)/T=constant
- Equation of state of an
ideal gas
- pV=nRT
- Molar gas constant (8.31JK^-1mol^-1
- pV=nRT
- 1) When temperature & mass of gas remain
constant then... pV=constant
- Average velocity of particles in a gas =0ms^-1
- To describe the motion of particles inside
the gas we use the root mean square
speed (r.m.s speed)
- Maxwell-Boltzmann distribution
- "The distribution of the speed of particles in a gas"
- Changing the temperature,
changes the distribution
- The hotter the gas becomes,
the greater the range of
speeds and the distribution
becomes more spread out
- The hotter the gas becomes,
the greater the range of
speeds and the distribution
becomes more spread out
- "The distribution of the speed of particles in a gas"
- (k) Boltzmann constant= (R) Molar gas constant /
(NA) Avagadro's constant
- pV=NkT
- Kinetic
energy=(3/2)kT
- Kinetic energy∝Temperature
- At a given temperature, atoms
and molecules in different gases
have the same kinetic energy.
However because their masses
are different, their r.m.s speeds
will be different
- For an ideal gas all the internal
energy is completely in the
form of kinetic energy
- For an ideal gas all the internal
energy is completely in the
form of kinetic energy
- At a given temperature, atoms
and molecules in different gases
have the same kinetic energy.
However because their masses
are different, their r.m.s speeds
will be different
- Kinetic energy∝Temperature
- pV=NkT
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