15. Ideal gases Flashcards
What is Amount of Substance
Indicates the number of elementary entities (normally atoms or molecules) within a given sample of substance
Define One Mole
The amount of substance that contains as many elementary entities as there are atoms in 0.012kg (12g) of carbon-12
Define Avogadros constant
6.02x10^23, the number of atoms in 0.012kg (12g) of carbon-12
What is the Kinetic Theory of Matter
A model used to describe the behaviour of the atoms or molecules in an ideal gas
What is Ideal Gas
A model of a gas including assumptions that simplify the behaviour of real gases
What are Gas laws
The laws governing the behaviour of ideal gases
What is the Maxwell-Boltzmann distribution
The distribution of the speeds of particles in a gas
What are all the Equations i need for this topic?
(12)
N = n x NA
Number of particles = number of mols x Avogadro’s constant
Pressure (Pa) is inversely proportional to volume (m3)
pV = constant
Pressure (Pa) is directly proportional to temperature (K)
p/T = constant
pV/T = constant
p1V1/T1 = p2V2/T2
pV = nRT
pV = NkT
k = R/NA
pV = 1/3Nmc2
1/2mc2 = 3/2kT
Ek = 3/2kT
Ek is directly proportional to Temperature (K)
What is the Kinetic theory of matter?
A model used to describe the behaviour of the atoms or molecules in an ideal gas
What are the assumptions made about ideal gases and why?
Real gases have complex behaviour, so assumptions are made about the atoms or molecules in an ideal gas:
- Random - The gas contains a very large number of atoms or molecules moving in random directions with random speeds
- Attraction - Electrostatic forces between atoms or molecules are negligible except during collisions
- Volume - The atoms or molecules of the gas occupy a negligible volume compared with the volume of the gas
- Elastic - The collisions of atoms or molecules with each other and the container walls are perfectly elastic (no kinetic energy is lost)
- Duration - The time of collisions between the atoms or molecules is negligible compared to the time between the collisions
Explain, in terms of the behaviour of its molecules, how a gas exerts a pressure on the walls of its container [4 marks]
A change in momentum occurs when molecules collide with the walls of container
Hence, walls exert a force on the molecule (by Newtons 2nd law)
The total force exerted by the molecules on the wall is equal to the total force exerted by the wall on the molecules (by Newtons 3rd law)
pressure = total force on wall / area of wall
What are Gas laws and list them
Gas laws area the laws governing the behaviour of ideal gases
A few simple gas laws can describe the relationship between the temperature, pressure, and volume of an ideal gas:
- If the temperature and mass of an gas remain constant, then the pressure of an ideal gas is inversely proportional to its volume
-> p ~ 1/V or pV = constant (Boyles law) - If the volume and mass of a gas remain constant, the the pressure of an ideal gas is directly proportional to its absolute temperature (K)
-> p ~ T or p/T = constant - pV/T = constant or p1V1/T1 = p2V2/T2
What is the equation state of an ideal gas
For 1 mol of an ideal gas, the constant in pV/T = constant is R.
For n moles of gas the equation becomes:
pV/T = nR or pV = nRT
This relationship is called the equation of state of an ideal gas
Why do we use the measure root mean squared speed (r.m.s speed)?
and how do we calculate it?
If we calculate the average velocity of the particles in a gas, because velocity is a vector quantity and the particles in a gas move in random directions, at different speeds, the average would be 0ms-1
So in order to describe the typical motion of particles inside a gas, we use a different measure, the r.m.s. speed
- square the velocity of each particle
- find the mean of the squares
- square root it
What is the Boltzmann constant?
k = R/NA
k = 8.31 / (6.02x10^23)
(value of k is in data book)
What is the Boltzmann constant?
k = R/NA
k = 8.31 / (6.02x10^23)
(value of k is in data book)
Derive 1/2mc2 = 3/2kT
- pV = 1/3Nmc^2 and pV = NkT
- So, 1/3Nmc^2 = NkT
- Simplify: 1/3mc^2 = kT
- Rearrange: mc^2 = 3kT
- Divide by 2: 1/2mc^2 = 3/2kT
Tell me about particle speeds at different temperatures
At a given temperature the atoms or molecules in different gases have the same average kinetic energy.
However, as the particles have different masses their r.m.s speeds will be different.
What do you know about the internal energy of an ideal gas?
The internal energy of a gas is the sum of the kinetic and potential energies of the particles inside the gas.
One of the assumptions of an ideal gas states that the electrostatic forces between particles in the gas are negligible except during collisions.
-> this means that there is no electrical potential energy in an ideal gas. All the internal energy is in the form of kinetic energy of the particles.
Doubling the temperature of an ideal gas doubles the average kinetic energy of the particles inside the gas -> and therefore also doubles its internal energy.
Tell me about the practical investigating pV = constant (Boyles law)
Draw and label apparatus.
This practical investigates how changing the volume of a gas affects the pressure of the gas.
If the pressure of a pressuried gas is slowly reduced, its volume increases.
-> The gas must be in a sealed tube to ensure the amount of gas inside the tube remains fixed.
Draw graph.
Tell me about the practical investigating p/T = constant
Draw and label apparatus.
The temperature of the water bath is increased and the resulting increase in pressure of the gas inside the sealed vessel is recorded.
This practical is used to determine absolute zero through investigating how the temperature of a gas affects its pressure.
At absolute zero the particles are not moving (the internal energy is at its minimum) so the pressure of the gas must be zero.
Plotting a line of pressure against temperature in celcius form the experimental results gives a line that can be extrapolated back to a point where the pressure is zero and therefore gives us a celcius value for absolute zero.
