B3 gas laws

assumptions for ideal gas

1. a gas consists of a very large no. of molecules

2. gas molecules are in constant random motion and obey the laws of classical mechanics

3. gas molecules make perfectly elastic collisions with one another and walls of container

4. intermolecular forces are negligible except during time of collision, which is brief compared to time between collisions

5. total volume of molecules is negligible compared to volume of gas

pressure

P = F/A where force is exerted prependicular to the surface

P = 1/3 ρv²

= 1/3 ((N X Mr)/V) v² where v² is mean square speed NOT IN DATA BOOKLET

the change in momentum of particles due to collisions with surface gives rise to pressure in gases REMEMBER DEFINITION

KE of 1 molecule

3/2 kT

derived from U / N

KE of system

= 3/2 NkT = 3/2 nRT = 3/2 pV

ideal gas equation

pV = nRT = NkT

approximate to ideal gas

• low density

• low pressure

• high temperature

so that intermolecular forces negligible

real gas vs ideal gas

real gas

• intermolecular forces not negligible → actual pressure less than ideal

• some volume is occupied by gas molecules → actual volume less than ideal

internal energy

KE + PE of molecules in a system

for ideal gas, assume PE = 0 since so far apart that negligible intermolecular forces

factors that affect potential energy

PE increases when distance increases

However, for ideal gas, molecules are so far apart that they don’t interact

Explain, with reference to the kinetic model of an ideal gas, how an increase in volume of the gas leads to a decrease in pressure

• same no. of particles, collide with larger surface area → frequency of collision decreases

• increased volume, constant velocity → frequency of collision decreases