Chapter 4: The Gas Laws In this chapter, we’ll study the gas laws which will allow us to predict the properties of any gas We’ll cover sections 1 through 11 over the next three days

The Nature of Gases Eleven elements are gases under normal conditions Despite the fact that the elements are VERY different, their physical properties in the gas phase are very similar Gases are compressible examples of bulk matter –Gases fill the entirety of whatever closed volume they occupy –Gas molecules are in constant motion

Pressure Pressure = Force per unit area The SI Unit of Pressure is the pascal, Pa 1 Pa = 1 kg·m -1 ·s -2 The pressure of a gas exerted on the walls of a container is a result of the collisions of gas molecules with the surface of the walls.

Pressure

Measuring Pressure The atmosphere of the planet is filled with gas molecules. These molecules exert a force on the surface of the earth We use a barometer to measure the pressure of atmospheric gases. 

Vacuum Gauges A manometer is a device used to measure the pressure inside a vessel We measure the pressure on the atmospheric side and the pressure on the device side –The difference is the pressure in the device

The Gas Laws Studies on the effect of temperature, pressure and volume have been carried out by many scientists, but 4 stand out: 1.Robert Boyle: (1662) Studied the relationship between Volume and Pressure 2.Jacques Charles and Joseph-Louis Gay-Lussac: (1810) Studied the relationship between Volume and temperature while hot air ballooning over Paris 3.Avedeo Avogadro: Helped prove atoms exist by confirming the relationship between Volume and the number of molecules of a gas

Boyle’s Law Took a closed tube with an air bubble on the closed end and poured Hg into the open end –As he added more Hg, the air bubble shrank 

Boyle’s Law Pressure is inversely related to volume. V  1/P PV = constant (at constant n)

Boyle’s Law Pressure is inversely related to volume. V  1/P PV = constant (at constant n and T)

Charles’s Law Charles and Gay- Lussac were balloonists and while trying to improve their balloons, they found that: At constant pressure, the volume of a gas increases with temperature

Charles’s Law We could also state Charles’s Law in terms of Pressure. The Pressure of a sample is directly proportional to the absolute temperature

Charles’s Law Volume  absolute constant n, constant P V=(constant) T Pressure  absolute constant n, constant V P=(constant) T constant n, constant constant n, constant V

Absolute Temperature Note: The absolute temperature is the temperature on the Kelvin scale For the remainder of the semester, whenever you use a temperature, you may need to convert it to the Kelvin scale Many of the constants and relationships that use them will only be valid when temperature is on the absolute, or Kelvin, scale 0  C = K 1 degree C = 1 Kelvin 25  C = K

Charles’s Law If we double the absolute temperature, the volume of the gas doubles OR the pressure of the gas doubles 

Avogadro’s Principle Avogadro found that under the same conditions of temperature and pressure, a given number of gas molecules occupy the same volume regardless of their chemical identity At K and 1 atm, all gases occupy approximately 22.4L V m =Molar volume of gas = V/n V=nV m 

The Gas Laws: Summary 1.Boyle’s Law: P 1 V 1 =P 2 V constant n, constant T 2.Charles’s Law: V 1 /T 1 =V 2 /T constant n, constant P P 1 /T 1 =P 2 /T constant n, constant V 3.Avogadro’s Law: As more molecules are added to a sample of gas at constant temperature and pressure, the volume must increase OR As more molecules are added to a sample of gas at constant temperature and volume, the pressure must increase

Putting it all together: The Ideal Gas Law We can combine the relationships stated in the three laws to create a single equation that will allow us to predict the pressure, volume or temperature of a certain number of moles of gas V= n (constant) P V= n (constant) T P= n (constant) T PV= n (constant) T

The Ideal Gas Law PV=nRT where R=8.314 J/K  mol The ideal gas law is an equation of state, an equation that describes the pressure, volume and temperature of a certain amount of a substance We can use the equation by itself or we can use it to determine the properties of an ideal gas at 2 sets of conditions by using the combined gas law 