Properties of Gases - Chp 10

Gas

a substance that has no well-defined boundaries but diffuses rapidly to fill any container in which it is placed

How to convert Celsius to Kelvin?

Temperature can be converted from the Celsius scale to the Kelvin scale by adding 273

Standard pressure

1x10⁵ Pa/100 kPa (the same as normal atmospheric pressure)

Standard temperature

273K (ie the freezing point of water)

Boyle's Law

states that, at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure

Boyle's Law expressed mathematically

pV = k
P₁V₁ = P₂V₂

Charles' Law

states that, at constant pressure, the volume of a fixed mass of gas is directly proportional to its temperature measured on the Kelvin scale

Combined Gas Law (General Gas Law)

V/T = k
P₁V₁ = P₂V₂
T₁ T₂

Gay-Lussac's Law of Combining Volumes

states that, in a reaction between gases, the volumes of the reacting gases and the volumes of any gaseous products are in the ratio of small whole numbers provided the volumes are measured at the same temperature and pressure

Avogadro's Law

states that equal volumes of gases contain equal numbers of molecules under the same conditions of the temperature and pressure

What is temperature?

• a measure of the degree of hotness of an object
• celcius scale aka centrigrade scale
• kelvin scale aka absolute scale

What are the two fixed points of the celcius scale?

• 0 degrees celcius
• 100 degrees celcius

What is absolute zero?

at the zero mark of the Kelvin scale a gas would occupy zero volume it could be cooled indefinitely without becoming a solid of liquid (theory not physically possible)

What is pressure measured in?

• newtons per metre squared / N m⁻²
• pascal / Pa

Convert cubic centimetres to cubic metres

X x 10⁻⁶

Kinetic theory of gases origin

• developed in order to explain many of the gas laws
• scientists imagine all matter as being made up of tiny particles
• in a solid the particles can only vibrate about a fixed position
• in a liquid the particles have larger amounts of energy and can move freely around eachother, but do not have complete freedom as there are forces of attraction between the particles
• gas particles have sufficient energy to almost completely overcome the forces of attraction between the particles in the gas
• only valid for ideal gases

Brownian motion

• random movement of tiny particles suspended in a liquid or gas
• Robert Brown dropped pollen grains in water, he observed the tiny pollen grains were being continuously bombarded by the moving water molecules, resulting in continuous irregular motion

What are the assumptions of the kinetic theory of gases?

1- gaseous particles are in continuous rapid, random motion, colliding with each other and with the wall of their container (they move in straight lines until they hit something and bounce off in new directions)
2- no attractive or repulsive forces between the molecules of a gas
3- gases are made up of particles whose diameters are negligible compared to the distance between them
4- average kinetic energy of the particles is proportional to the Kelvin temperature
5- all collisions are elastic

What are elastic collisions?

means there is no loss of kinetic energy in these collisions although there may be a transfer of energy between colliding particles

Limitations of the kinetic theory of gases

low temperatures - attractive forces such as van der Waals' forces, dipole-dipole forces or hydrogen bonding affect particles as they are moving slowly
high pressures - diameters not negligible compared to distances between particles

When are gases most like ideal gases?

high temperature - when the molecules are moving rapidly, preventing the forces between molecules from operating
low pressure - the molecules are widely spread
non-polar molecules come closer to ideal gas behaviour due to the lesser degree of attractive forces

Ideal gas

one that perfectly obeys all the assumptions of the kinetic theory of gases under all conditions of temperature and pressure

Real gas

differ from ideal gases because forces of attraction and repulsion do exist between the molecules, and the volume of the molecules is not negligible

Universal gas constant

R = 8.31 J mol⁻¹ K⁻¹

Equation of state for an ideal gas/ ideal gas equation

pV = nRT
p - Pa
V - M³
n - mol
R - J mol⁻¹ K⁻¹
T - K

Convert cubic metres to litres

X x 10⁻³

Convert cubic metres to cubic centimetres

X x 10⁻⁴

Convert kPa to Pa

X x 10⁻³

Apparatus required

• conical flask
• aluminium foil
• rubber band
• barometer
• graduated cylinder
• hot plate
• mass balance
• beaker
• retort stand

Procedure

• find the mass of a dry conical flask, rubber band, and aluminium foil

• pour 10cm³ of the volatile liquid using a graduated cylinder

• put foil over the mouth of the conical flask and secure with band, make a pin hole in the top

• clamp the conical flask in the retort stand and submerge the most of the flask in boiling water in the beaker, leave for 8 minutes

• the liquid is now vapourised, measure temperature of water with a thermometer (should be 373K) this is the temperature of the liquid.

• remove flask from boiling water and allow to cool at room temperature

• dry outside of the flask, reweigh, and subtract original mass to find mass of the volatile liquid

• fill flask to brim with water, the pour into graduated cylinder to find the volume of the volatile liquid

• record atmospheric pressure using barometer