Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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 OnurhanSlide 2 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 Relationship between Force and Pressure

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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. At sea-level and 0  C,

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 4 Two Types of Manometers

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 5 Boyles’ Law Gases 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 OnurhanSlide 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  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 OnurhanSlide 7 Gas Constant

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 8 Flowchart for Stoichiometric Calculations Involving Gases

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 9 Partial Pressures of Components in a Mixture

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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 OnurhanSlide 11 Speed Distribution in Gases

Gases General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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 OnurhanSlide 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 OnurhanSlide 14 Real Gases 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’ Equation