3.2 Modelling a gas See pages in your textbook

Pressure (same slide as in Temperature notes)  Force acting on a surface per unit area  Units  N·m -2 or pascal (Pa)  Standard air pressure (sea level) x 10 5 Pa =101.3 kPa

What causes pressure for a sample of gas? (same slide as in Temperature notes)  Elastic collisions between the gas atoms or molecules and the walls of the container  In these collisions, there is a force exerted to change the momentum of the particle  What can make the pressure increase?  Molecules moving faster  More molecules moving  Making the collisions with the wall of the container happen more frequently

Kinetic Model of an Ideal Gas:  This model extends the assumptions of the Kinetic Theory of Matter in order to relate the macroscopic behavior of an ideal gas to the microscopic behavior of its molecules. 1.Gases consist of tiny particles called atoms (for Noble Gases) or molecules 2.The total number of molecules in any sample of a gas is extremely large* 3.The molecules are in constant random motion *see slide about Avogadro’s number

Kinetic Model of an Ideal Gas: 4.The range of the intermolecular forces is small compared to the average separation of the molecules 5.The size of the particles is relatively small compared with the distance between them 6.Collisions of short duration occur between molecules and the walls of the container and the collisions are perfectly elastic 7.No forces act between particles except when they collide, and hence particles move in straight lines 8.Between collisions the molecules will obey Newton’s Laws of Motion

Ideal gas vs. Real gas  Ideal Gas:  One that obeys all gas laws (which we will discuss in a few minutes) under all pressures and temperatures.  Real Gas:  Under certain pressure and temperature combinations, real gases will condense and become a liquid  HOWEVER, ALL real gases will behave as an ideal gas through a certain range of pressures and temperatures (the range depends on the gas itself)

Post-Lab—Journal Entry (#2) (your pre-lab sketches and your raw data is entry #1)  Tape your Pressure/Volume graph into your journal  What is the best fitting curve for your data?  Describe what this tells us about the effect changing volume has on the pressure of a gas.  According to the kinetic theory of matter/Kinetic model of an ideal gas, why did the pressure change?

Boyle’s Law Increasing Temperature

Boyle’s Law: Calculations

Charles’ Law

Charles’ Law: Calculations

Pressure Law (third gas law)  For a gas of fixed mass and volume, the pressure is directly proportional to the absolute temperature  The line on this graph is called an ISOCHORE (same volume)  Smaller volume, steeper slope

Pressure Law Calculations

Combining the Gas Laws:

Practice Problem  A mL container of gas, originally at 345 kPa and 25.0°C is heated to 45.0°C, causing the gas to expand to 275 mL. What is the new pressure in the container?

But what IS that constant?

Equation of State of an Ideal Gas