“The Behavior of Gases”

The Properties of Gases

Gases can expand to fill its container, unlike solids or liquids Compressibility Gases can expand to fill its container, unlike solids or liquids The reverse is also true: They are easily compressed, or squeezed into a smaller volume Compressibility is a measure of how much the volume of matter decreases under pressure

Compressibility This is the idea behind placing “air bags” in automobiles In an accident, the air compresses more than the steering wheel or dash when you strike it The impact forces the gas particles closer together, because there is a lot of empty space between them

Compressibility This empty space makes gases good insulators (example: windows, coats)

Variables that describe a Gas The four variables and their common units: 1. pressure (P) in kilopascals 2. volume (V) in Liters 3. temperature (T) in Kelvin 4. amount (n) in moles The amount of gas, volume, and temperature are factors that affect gas pressure.

1. Amount of Gas When we inflate a balloon, we are adding gas molecules. Increasing the number of gas particles increases the number of collisions thus, the pressure increases If temperature is constant, then doubling the number of particles doubles the pressure

Pressure and the number of molecules are directly related More molecules means more collisions, and… Fewer molecules means fewer collisions. Gases naturally move from areas of high pressure to low pressure, because there is empty space to move into – a spray can is example.

2. Volume of Gas In a smaller container, the molecules have less room to move. The particles hit the sides of the container more often. As volume decreases, pressure increases. (think of a syringe) Thus, volume and pressure are inversely related to each other

3. Temperature of Gas Raising the temperature of a gas increases the pressure, if the volume is held constant. (Temp. and Pres. are directly related) Should you throw an aerosol can into a fire? What could happen? When should your automobile tire pressure be checked?

The Gas Laws

The Gas Laws are mathematical The gas laws will describe HOW gases behave.

Robert Boyle (1627-1691)

#1. Boyle’s Law Gas pressure is inversely proportional to the volume, when temperature is held constant. Equation: P1V1 = P2V2 (T = constant) P V

Jacques Charles (1746-1823)

#2. Charles’ Law The volume of a fixed mass of gas is directly proportional to the Kelvin temperature, when pressure is held constant. V T

Converting Celsius to Kelvin Gas law problems involving temperature will always require that the temperature be in Kelvin. (Remember that no degree sign is shown with the kelvin scale.) Kelvin = C + 273 °C = Kelvin - 273 and

Joseph Louis Gay-Lussac (1778 – 1850)

#3. Gay-Lussac’s Law The pressure and Kelvin temperature of a gas are directly proportional, provided that the volume remains constant. P T

#4. The Combined Gas Law The combined gas law expresses the relationship between pressure, volume and temperature of a fixed amount of gas.

The combined gas law contains all the other gas laws! If the temperature remains constant...

Dalton’s Law of Partial Pressures For a mixture of gases in a container, PTotal = P1 + P2 + P3 + . . . P1 represents the “partial pressure”, or the contribution by that gas.

Ideal Gases

Ideal Gases A gas with particles that are in constant random motion but have no attraction for each other Except at very low temp. and very high pressure, most gases behave as ideal gases

Avogadro’s Principle Equal volumes of gases contain equal numbers of moles at constant temp & pressure true for any gas n1/V1 = n2/V2 V n C. Johannesson

Ideal Gas Law Merges the Combined Gas Law with Avogadro’s Principle: PV=nRT UNIVERSAL GAS CONSTANT R=0.0821 Latm/molK R=8.315 dm3kPa/molK You don’t need to memorize these values!

Ideal Gas Law Problems P = ? atm n = 0.412 mol T = 16°C = 289 K Calculate the pressure in atmospheres of 0.412 mol of He at 16°C & occupying 3.25 L. GIVEN: P = ? atm n = 0.412 mol T = 16°C = 289 K V = 3.25 L R = 0.0821Latm/molK WORK: PV = nRT P(3.25)=(0.412)(0.0821)(289) L mol Latm/molK K P = 3.01 atm

Practice (on sheet…) 1.00 L of a gas at STP is compressed to 473 mL. What is the new pressure of the gas? The temperature inside my refrigerator is about 4oC. If I place a balloon in my fridge that initially has a temperature of 22oc and a volume of 0.5 liters, what will be the volume of the balloon when it is fully cooled by my refrigerator? If I initially have a gas at a pressure of 12 atm, a volume of 23 liters, and a temperature of 200K, and then I raise the pressure to 14 atm and increase the temperature to 300K, what is the new volume of the gas? If I have 4 moles of a gas at a pressure of 5.6 atm and a volume of 12 liters, what is the temperature?

1. 00 L of a gas at STP is compressed to 473 mL 1.00 L of a gas at STP is compressed to 473 mL. What is the new pressure of the gas?

The temperature inside my refrigerator is about 4oC The temperature inside my refrigerator is about 4oC. If I place a balloon in my fridge that initially has a temperature of 22oc and a volume of 0.5 liters, what will be the volume of the balloon when it is fully cooled by my refrigerator?

If I initially have a gas at a pressure of 12 atm, a volume of 23 liters, and a temperature of 200K, and then I raise the pressure to 14 atm and increase the temperature to 300K, what is the new volume of the gas?

If I have 4 moles of a gas at a pressure of 5 If I have 4 moles of a gas at a pressure of 5.6 atm and a volume of 12 liters, what is the temperature?