Ch. 5 Gases The particles are free to move (fluid).

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Ch. 5 Gases The particles are free to move (fluid).
Gases are defined by: The particles are free to move (fluid). The particles are far apart. The particles move very fast. The particles have no attraction nor repulsion for each other. The fact that they have no definite shape or volume – they take the shape of the container. The fact that they are compressible.

Variables Gases have four measurable variables. Pressure (P) of a gas.
Measured in a variety of units like mmHg, atm, Pascals, torr, bars, and psi. Volume (V) of a gas, Measured in mL or L. Temperature (T) of a gas. Measured in Celsius or Kelvin. For all formulas, Kelvin temperatures must be used. Quantity of a gas. Measured in grams or moles (n). Formulas will require moles.

Pressure Pressure is the force per unit area.
As the molecules of a gas collide with the walls, they exert a force.

Atmospheric Pressure The column of air above us exerts a force due to the gravitational force. Atmospheric pressure has two main variables. What are they?

Atmospheric Pressure A barometer is used to measure pressure.
A dish is filled with liquid mercury. A glass tube that has been evacuated of all air molecules is then placed into the pool of mercury.

Atmospheric Pressure The height of the mercury in the tube over the pool of mercury is measured with a ruler. In weather, it is reported as inches of mercury – inHg. We will measure ours in millimeters of mercury – mmHg. On an average day, at sea level – the column would measure 760mmHg = 1 atmosphere (atm).

Atmospheric Pressure As altitude increases, the pressure decreases.
Denver, CO Death Valley, CA

Empirical Gas Laws Look at the relationship between two of the four variables. Boyle’s, Charles’, and Gay-Lussac’s Laws. Direct (linear) relationship – as one variable increase, so does the other. Inverse relationship – as one variable increase, the other decreases.

Boyle’s Law Allows P and V to vary, while n and T are constant.
Boyle found that as P , the V  and vice versa. What type of relationship – Direct or Inverse? P x V = constant Formula: P1V1 = P2V2 LEP #1

Graph

Charles’ Law Formula: LEP #2
Allows V and T to vary, while n and P are constant. Charles’ found that as V , the T . What type of relationship – Direct or Inverse? Formula: LEP #2

Graph

Gay-Lussac’s Law Allows P and T to vary, while n and V are constant.
Gay-Lussac found that as P , the T . Direct or Inverse? Formula? LEP #3

Combined Gas Law All three empirical gas laws can be merged into a single formula. This means that two of the three variables can be changed. LEP #4

The quantity of a gas (moles) and volume are proportional. STP = standard temperature (0oC) and pressure (1 atm). At STP, one mole of any gas will occupy 22.4L. LEP #5

Ideal Gas Law The relationship between ALL four variables is found in the ideal gas law. Formula: PV = nRT. R is called the gas law constant and equals L atm / mol K. Because of the units, each of the variables must be in the same units! LEP #6

Ideal Gas Law Other derived versions of the ideal gas law can be useful. Since moles = grams (g) / molar mass (MW),

Ideal Gas Law Density is grams / volume, so we can include this in our formula as well. LEP #7 LEP #8

Stoichiometry and Gases
Many reactions generate a gas as a product. Grams of solid or liquid must be converted to moles of gas. Ex) 2 KClO3(s)  2 KCl(s) + 3 O2(g) In this example, when 2 moles of KClO3 decompose, 3 moles of O2 gas are generated. LEP #8

Explosives Explosives like dynamite, TNT, nitroglycerin, etc. are compounds that produce mostly gases when allowed to react. Unlike hydrocarbons, most explosives _________ rather than burn. A ____________ is a very rapid chemical reaction using oxygen that is contained in the material rather than in the air.

Explosives

TNT 2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C

ANFO Ammonium nitrate / fuel oil is commonly used in mining.
It was also used in the bombing of the Murrah building in Oklahoma City. LEP #10

Dalton’s Law Many gases we encounter are mixtures of two or more substances. The total pressure exerted by a mixture is equal to the partial pressures of each of the gases. Ptotal = Pa + Pb + Pc + … The partial pressure of a gas is proportional to its mole fraction. Pa = Ptotal ca. LEP #11, #12

Kinetic Molecular Theory
In the study of gases, five postulates describing them were published by Rudolf Clausius in 1857. Gases consist of large numbers of molecules that are in continuous, random motion. Pressure is uniform on any surface.

Kinetic Molecular Theory
The volume occupied by the molecules is negligible when compared to the volume of the container. A 5.0L container is assumed to be the volume even though the gas molecules must occupy some of that space. Attractive and repulsive forces between gas molecules are negligible.

Kinetic Molecular Theory
Collisions of gas molecules are elastic. Energy can be transferred between molecules, but cannot be lost. The average kinetic energy of a gas is proportional to the temperature of the gas.

KE of a Gas

Root-mean-square Velocity
This is an approximation of the average speed of a gas molecule. LEP #13

Real Gases All gases deviate from “ideal” behavior under different conditions. At higher pressures, gas molecules do occupy a portion of the volume.

Real Gases At low temperatures, gas molecules can have attractive forces for each other. Both of these factors can be corrected for in the van der Waals equation.

Real Gases

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