eLFH - Kinetic Theory of Gases Flashcards
Definition of kinetic theory of gases
Model of gas behaviour at molecular level that arises from 4 postulates.
Describes characteristics of an imaginary, ideal gas.
Postulate 1 of kinetic theory of gases
Gases consist of a large number of particles - either atoms or molecules
Particles treated as point masses which are very far apart in a gas
Therefore volume of particles in a gas is negligible compared to total volume of the gas
Postulate 2 of kinetic theory of gases
Individual particles move in random directions and at random speeds
Postulate 3 of kinetic theory of gases
Individual particles travel in straight lines between abrupt collisions with other particles, objects or walls of a container
Collisions are perfectly elastic so total kinetic energy does not change during the collision
Postulate 4 of kinetic theory of gases
There are no attractive or repulsive forces between the particles
Would be a breach of postulate 3 if they did as it would involve a loss of kinetic energy and particles would change phase eg to liquid / solid
How do real gases deviate from behaviour of the imaginary ideal gas
Real gas particles occupy a small but finite volume
Gas particles exhibit attractive forces for one another especially when particles are close together eg at low temperatures or high pressures
Therefore real gases are affected by their surrounding conditions
Brownian motion
Provides demonstration that fluids consist of fast moving particles
Particles suspended within a medium (liquid or gas) will move randomly as a result of the random movement of fast moving particles withing the suspension medium
Maxwell-Boltzmann distribution
Mathematical description / graphical illustration of the distribution of probability that any random gas particle will have a given speed
Effect of temperature on Maxwell-Boltzmann distribution and implication on gas particles
As heat energy is added, manifests as increase in kinetic energy of gas particles and average speed increases
Therefore peak of graph shifts to right
Area of curve remains constant as the sum of probability distributions is always one
Therefore temperature is a reflection of average kinetic energy of particles of a gas
Kinetic energy formula
Kinetic energy = 1/2 x mass x speed^2
Implication of kinetic energy formula
Gases with higher molecular weight (and therefore mass) will have slower average speed for a given kinetic energy
Therefore at a given temperature, gases with higher molecular weight will have speed distributions shifted to the left, and lower molecular weights shifted to the right
Force (and Pressure) changes as explained by kinetic theory
(Linked with later flashcard “How increase in pressure is generated according to kinetic theory explanations for a gas within a container”)
Example of particle colliding with wall of container at right angles with speed of “x”
Perfectly elastic collision
Velocity (which is speed with direction) changes from +x to -x (total change of 2x)
Acceleration is rate of change of velocity - therefore particle undergoes acceleration
Force = mass x acceleration (mass remains constant, therefore force increases with increased acceleration)
Pressure is cumulative force generated divided by total area over which that force is applied
How increase in pressure is generated according to kinetic theory explanations for a gas within a container
(Linked with previous flashcard “Force (and Pressure) changes as explained by kinetic theory”)
Pressure is cumulative force generated divided by total area over which that force is applied so increased pressure generated by either:
Increasing frequency of collisions of gas particles with the container walls
OR
Reducing area over which collisions occur
Boyle’s Law
At constant temperature, as volume of gas is decreased, its pressure will increase
Charles’ Law
At constant pressure, the volume of gas varies directly with the absolute temperature
