Chapter 13
Gases
- Gases flow like liquids and are fluids.
- The difference between a gas and a liquid is the distance between the molecules.
- Barometer- a common instrument to measure air pressure
- Pressure conversion
- 1 atm= 760 mm Hg
- 1 atm= 760 torr
- 1 atm = 14.7 psi
- 1 atm= 101,325 Pa
- Atmospheric pressure results from the mass of the air being pulled toward the center of the earth by gravity.
- aka weight of the air
- The Irish scientist Robert Boyle( 1627-1691) was one of the first scientist to carefully study gases.
- Using a J-shape tube closed at one end. He saw that there was a relationship between pressure and volume
Boyle's Law
- Pressure and volume are inversely proportional. If one doubles, the other decreases by one-half.
- PV = PV
- Temperature must be consistent
- The French physicist Jacques Charles( 1746- 1823) showed that the volume of a given amount of gas at constant pressure will increase with the temperature of the gas.
- A 1.5 L sample of Freon-12 had a pressure of 56 torr. If the pressure is changed to 150 torr, what is the new volume if the temperature is constant?
- Equation : PiVi=PfVf
- You are solving for Vf since we are looking for the new pressure.
- Pi= 56 torr Pf= 150
- Vi=1.5 L Vf =??
Solve for Vf
- Temperature is proportional to volume.
- As temperature increase volume increase.
- As temperature decrease volume decreases
- Vi/Ti=Vf/Tf
- A 2.0 L sample of air is collected at 298 K and then cooled to 278K. The pressure is held constant.
- Does the volume increase or decrease
- The temperature decrease from 298 K to 278 K. So if temperature decreases the volume will also decrease.
Combined gas Law
- Assumption:Number of moles constant
- This law combines both Boyle’s law and Charles’s Law
- PiVi/Ti=PfVf/Tf
Avogadro's Law
- There is a relationship between the volume of a gas and the number of molecules presents in the gas sample.
- the number of moles is proportional to volume.
- As number of moles increase volume increase.
- As number of moles decrease volume decreases.
- Vi/ni=Vf/nf
Ideal Gas Law
- PV=nRT
- R= universal gas constant
- 0.08206 L*atm/(mol *K)
- ideal gas law was derived from the observation of boyle’s, Charles’s and Avogradro’s.
- Assumption: the gas must be ideal.
What is an Ideal Gas?
- Kinetic molecular theory of gases is a relatively simple model that attempts to explain the behavior of an ideal gas.
- 1. Gases consist of tiny particles (atoms or molecules)
- 2. These particles are so small, compared with the distances between them, that the volume(size) of the individual particles can be assumed to be zero.
- 3. the particles are in constant random motion, colliding with the walls of the container. These collision with the walls cause the pressure exerted by the gas.
- 4. The particles are assumed not to attract or to repel each other
- 5. The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature of the gas
Daltons Law of Partial Pressure
- Many important gases contain a mixture of components.
- John Dalton was one of the first scientist to studied mixtures of gases
- His observation became Dalton’s law of partial pressures.
- For a mixture of gases in a container, the total pressure exerted is the sum of the partial pressures of the gases present
Remember:
- Pressure of a gas is not based on the element of the gas but the number of molecules of the gas.
Absolute zero is the lower limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value, taken as 0
STP- Definition: STP corresponds to 273 K (0° Celsius) and 1 atm pressure. STP is often used for measuring gas density and volume.
Gas Stoichiometry- The quantitative relationship of reactants and products is called stoichiometry. Stoichiometric problems require you to calculate the amounts of reactants required for certain amounts of products, or amounts of products produced from certain amounts of reactants. If, in a chemical reaction, one or more reactants or products are gases, gas laws must be considered for the calculation.
The Kinetic Molecular Theory is used to explain the behavior of gases and is based upon the following postulates: