Chapter 2 p Behavior of Gases

The behavior of gases refers to the way gases react to different conditions. The behavior of gases refers to the way gases react to different conditions. The 4 conditions we will now look at are: The 4 conditions we will now look at are: 1) Compressibility 1) Compressibility 2) Expansion 2) Expansion 3) Diffusion 3) Diffusion 4) Pressure 4) Pressure

Compressibility Gases can expand to fill their container, unlike solids or liquids The reverse is also true: They are easily compressed, or squeezed into a smaller volume.

Expansion Gases do not have a definite shape or volume, so they can expand to fit the volume available to them.

Diffusion Gas particles constantly collide with each other but these collisions are random. When a gas is introduced into a container, it will diffuse (mix) through out the container even if there are other gases present. This can be a slow process.

Pressure Gases exert pressure on the objects they come in contact with. Gases exert pressure on the objects they come in contact with. To calculate the pressure use the following formula: To calculate the pressure use the following formula: P = F A P= pressure in pascals (Pa), F is force in Newtons(N), A is the area (m 2 ) P= pressure in pascals (Pa), F is force in Newtons(N), A is the area (m 2 )

Ideal Gases don’t exist, because:   Molecules do take up space   There are attractive forces between particles

Ideal Gases There are no gases for which this is true (acting “ideal”); however Real gases behave this way at a) high temperature, and b) low pressure.

Real Gases behave like Ideal Gases... When the molecules are far apart. The molecules do not take up as big a percentage of the space We can ignore the particle volume. This is at low pressure

Real Gases behave like Ideal Gases… When molecules are moving fast This is at high temperature Collisions are harder and faster. Molecules are not next to each other very long. Attractive forces can’t play a role.

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. 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) The molecules hit the walls harder, and more frequently! Should you throw an aerosol can into a fire? What could happen?

Absolute zero is ° C You can’t get colder than that, no matter how hard you try

Boyle’s Law Equation: P 1 V 1 = P 2 V 2 (T = constant) Gas pressure is inversely proportional to the volume, when temperature is held constant.

Graph of Boyle’s Law Boyle’s Law says the pressure is inverse to the volume. Note that when the volume goes up, the pressure goes down

Practice Question A chemist collects 20.0 ml of a gas at a pressure of 250 kPa. What will be the volume of the sample of gas if the pressure in increased to 400kPa. A chemist collects 20.0 ml of a gas at a pressure of 250 kPa. What will be the volume of the sample of gas if the pressure in increased to 400kPa. P 1 V 1 = P 2 V 2 P 1 V 1 = P 2 V 2 (250)*(20.0) = (400) * V ml = V ml = V 2

Charles’s Law The volume of a fixed mass of gas is directly proportional to the Kelvin temperature, when pressure is held constant.

Temperature vs. Volume Graph Volume (mL) Temperature (  C) – 273

If a volume vs. temperature graph is plotted for gases, most lines can be interpolated so that when volume is 0 the temperature is -273  C. If a volume vs. temperature graph is plotted for gases, most lines can be interpolated so that when volume is 0 the temperature is -273  C. Naturally, gases don’t really reach a 0 volume, but the spaces between molecules approach 0. Naturally, gases don’t really reach a 0 volume, but the spaces between molecules approach 0. At this point all molecular movement stops. At this point all molecular movement stops. –273  C is known as “absolute zero” (no E K ) –273  C is known as “absolute zero” (no E K ) Lord Kelvin suggested that a reasonable Lord Kelvin suggested that a reasonable temperature scale should start at a true zero value. The Kelvin Temperature Scale

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

Practice Question A sample of gas occupies 3.5 L at 300 K. What volume will it occupy at 200 K? A sample of gas occupies 3.5 L at 300 K. What volume will it occupy at 200 K? V 1 = 3.5 L, T 1 = 300K, V 2 = ?, T 2 = 200K V 1 = 3.5 L, T 1 = 300K, V 2 = ?, T 2 = 200K Using Charles’ law: V 1 /T 1 = V 2 /T L / 300 K = V 2 / 200 K 3.5 L / 300 K = V 2 / 200 K V 2 = (3.5 L/300 K) x (200 K) = 2.3 L

Gay-Lussac’s Law The pressure and Kelvin temperature of a gas are directly proportional, provided that the volume remains constant. How does a pressure cooker affect the time needed to cook food?

Equivalent Pressures In most of our Chemistry questions, pressure will be in kPa( kilopascals). In most of our Chemistry questions, pressure will be in kPa( kilopascals). But there are other units of measurement for pressure. This is how they are equivalent: But there are other units of measurement for pressure. This is how they are equivalent: kPa = 760 mm Hg = 1.00 atm = 14.7 psi Did you know? The weight of a postage stamp exerts a pressure of about one Pascal on the surface of the envelope.

Avogadro’s Law Amedeo Avogadro stated that equal volumes of different gases, under the same temperature and pressure conditions, will have the same number of moles. Amedeo Avogadro stated that equal volumes of different gases, under the same temperature and pressure conditions, will have the same number of moles. Did you know? Did you know? In honour of his discoveries, the number of units present in one mole is called Avogadro’s number. (N A )

Combining the gas laws So far we have seen these gas laws: So far we have seen these gas laws: Jacques CharlesRobert Boyle P1V1P1V1P1V1P1V1= P2V2P2V2P2V2P2V2 V1V1V1V1 T1T1T1T1 = V2V2V2V2 T2T2T2T2 These are all subsets of a more encompassing law: the combined gas law P1P1P1P1 T1T1T1T1 = P2P2P2P2 T2T2T2T2 Joseph Louis Gay-Lussac

STP (Standard Temperature and Pressure) You will hear me mention STP in chemistry problems, at STP: You will hear me mention STP in chemistry problems, at STP: Temperature: 0 o C Pressure: kPa Molar volume of any gas: 22.4 L If you ever hear about SATP (Standard ambient temperature and pressure) Temperature: 25 o C Pressure: kPa Molar Volume of any gas: 24.5 L

The Ideal Gas Law Equation: P x V = n x R x T Pressure times Volume equals the number of moles (n) times the Ideal Gas Constant (R) times the Temperature in Kelvin. R = 8.31 (L x kPa) / (mol x K)

1. How many grams of Cl 2 (g) can be stored in a 10.0 L container at 1000 kPa and 30°C? PV = nRT (8.31 kPaL/Kmol)(303 K) (1000 kPa)(10.0 L) = n = 3.97 mol P= 1000 kPa, V= 10.0 L, T= 303 K 3.97 mol x 70.9 g/mol = 282 g 2. At 150°C and 100 kPa, 1.00 L of a compound has a mass of g. Calculate molar mass. PV = nRT (8.31 kPaL/Kmol)(423 K) (100 kPa)(1.00 L) = n = mol P= 100 kPa, V= 1.00 L, T= 423 K g/mol = g / mol = 88.1 g/mol

Dalton’s Law of Partial Pressures For a mixture of gases in a container, P Total = P 1 + P 2 + P P 1 represents the “partial pressure” of gas 1... Dalton’s Law is particularly useful in calculating the pressure of gases collected over water.

If the first three containers are all put into the fourth, we can find the pressure in that container by adding up the pressure in the first 3: 2 atm + 1 atm + 3 atm = 6 atm

Diffusion: describes the mixing of gases. The rate of diffusion is the rate of gas mixing. Molecules move from areas of high concentration to low concentration.

Effusion: a gas escapes through a tiny hole in its container. -Think of a nail in your car tire… Diffusion and effusion are explained by the next gas law: Graham’s