Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 1

Some Common Properties of Gases

• They all assume the volume and shape of their containers

• They are all compressible, i.e. reduce their volumes considerably under even small pressures

• Gases will mix thoroughly and evenly in a container

• They have much lower densities than solids or liquids

• They move constantly in their containers

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 2

FP

A

If a force of magnitude F is applied to a surface with an area equal to A, then we say that the pressure on surface is P and is given by

Force, F

Surface with Area A

Relationship between Force and Pressure

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 3

A Simple Barometer A barometer is a device for measuring the pressure of the atmosphere.

This figure shows a mercury barometer. The height of mercury in the column is a measure of the pressure of the atmosphere.

The general relationship between pressure, (P) and the height of a liquid column (h) in a barometer is

where d is the density of the liquid and g is the gravitational acceleration.

P dhg

At sea-level and 0C, 760 mmd

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 4

Two Types of Manometers

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 5

Boyles’ LawGases are compressible, i.e. they can easily be compressed (decrease in volume) by applying even moderate pressures to them. Liquids and solids are not easily compressed, i.e. they transmit the force applied to them (by pressure) instead of changing their volume. This compressibility of gases led to an empirical law, known as the Boyle’ Law, which states that the volume of a sample of gas varies inversely with pressure at a given temperature..

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 6

Charles’s Law

Charles was the first scientist to study the behaviour of hot gases. His and later observations indicated a linear relationship between volume and temperature when expressed on the Kelvin scale for a sample of a gas at fixed pressure. Furthermore, plots of volume vs. temperature showed that the volume of a gas would vanish at -273.15 C (0 K, the absolute zero) if the sample remains gaseous. This empirical law is formulated as the volume occupied by any sample of a gas at constant pressure is directly proportional to the absolute temperature. This law is known as Charles’s Law.

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 7

Gas Constant

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 8

Flowchart for Stoichiometric Calculations Involving Gases

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 9

Partial Pressures of Components in a Mixture

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 10

Collecting Gases Over Water

The gas collected in the inverted tube is not pure hydrogen. It inevitably contains some water vapour, picked up while passing through water.

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 11

Speed Distribution in Gases

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 12

Diffusion

The green molecule will eventually make its way into the red zone.

It will take a while to reach a given point, because of

a) its random motion

b) presence of like and unlike molecules on its path will result in collisions, delaying the molecule’s arrival at the required spot

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 13

Effusion

Rate depends on the speed of molecules here, since the concentration and the hole is the same for all of them. Since the temperature is constant, the lightest will effuse more in unit time.

Gases

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 14

Real Gases

InteractionPath

When the concentration is high (low temperatures and high pressures), the free volume in which the molecules move is lowered considerably and interactions are much stronger. This means that the pressure on the walls is lower than when we have an ideal gas (no interaction). The pressure term in the ideal gas equation must be modified to reflect this drop. Also, the volume of the container must be modified to reflect drop in the free volume.

Hence, van der Waals’ Equation2

2( )

n aP V nb nRT

V