GASES AND LIQUIDS Chapter 12

Characteristics of gases The properties of gases can be explained by a model called the kinetic molecular theory. This theory is based on some major assumptions about a theoretical gas often called as an ideal gas. An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly eleastic and in which there are no intermolecular attractive forces. The equation for ideal gas law: PV=nRT n = number of moles V= volume; T= temperature in K. R = universal gas constant = 8.3145 J/mol K

PROPERTIES OF GASES Gases are fluids: They flow just as liquids do. Gases transmit and exert pressure equally in all directions. Gas molecules are in constant motion and frequently collide with one another and with the walls of the container. Gases have low density: Compared with liquids and solids gases have a very low density. Gases are highly compressible: If you fill a syringe with liquid and try to push the plunger in when the opening is plugged . Nothing happens. It takes enormous pressure to reduce the volume of a liquid or a solid. However if there is gas or mixtures

of gases, such as air, in the syringe, it does not take much pressure to move the plunger down and compress the gases. Gases completely fill a container and exert pressure equally in all directions: A solid has a distinctive shape and volume. A liquid has a distinctive volume but assumes the shape of its container. In contrast, a gas fills the entire container and exerts pressure on the container in all directions. This pressure created is because of the molecules colliding with each other.

The temperature of a gas determines the average kinetic energy of its particles: The kinetic energy of a gas molecule can be expressed using the kinetic energy equation that relates kinetic energy to one half the mass, m,multiplied by the square of the speed,v, KE = ½ mv²

What is pressure? It is the force on a surface divided by the area of that surface. Pressure= force/area In SI unit force is measured in newtons (N) and the unit of area is (m²)Therefore the unit of pressure is Pascal(Pa). 1 pascal= 1N/m²

What is STP? STP is Standard conditions of temperature and pressure. To study the effects of changing temperature and pressure on a gas, it is useful to have a standard for comparison. Scientists have specified a set of standard conditions called standard temperature and pressure or STP. STP= 0⁰C and 1 atm

Home work Define the terms: A) Newtons B) Pascal C)Pressure

THE GAS LAWS Boyle’s Law: Boyle's law describes the relationship between the product, pressure and volume within a closed system as constant when temperature remains at a fixed measure; both entities remain inversely proportional.The law was named for chemist and physicist, Robert Boyle who published the original law in 1662. The law itself can be defined as: P1V1=P2V2 “ For a fixed amount of gas kept at a fixed temperature, P and V are inversely proportional (while one increases, the other decreases). ”

Class Practice Page 435 sample problem 12A

Dalton’s Law Of Partial Pressure: The total pressure in a gas mixture is the sum of the partial pressures of the individual components, each behaving as the other gases were absent. Ptotal = PA+PB +Pc Ptotal is the total pressure of the mixture. PA,PB and Pc are the partial pressures of the mixture’s component gases, A,B, and C. If A, B, and C are the components of a gas mixture, then the mole fraction of gas A is represented mathematically as mol fraction= mol A/mol A+ B+C

Charles Law: The volume of a sample of gas at constant pressure is directly proportional to the absolute temperature. V/T=k V1/T1=V2/T2

Class Practice A sample of gas occupies 24m3 at 175 K. What volume would the gas occupy at 400K?

Avogadro’s Law: Avogadro proposed that equal volumes of different gases under the same conditions of temperature and pressure have the same number of molecules.

Gay Lussac’s law: At constant temperature and pressure, gases react in volume proportions that are whole numbers . For example, consider the formation of gaseous hydrogen chloride from the reaction of hydrogen gas and chlorine gas. One volume of chlorine reacts with one volume of hydrogen to form two volumes of hydrogen chloride.

Effusion and Diffusion The passage of a gas through a small opening, called effusion. Effusion occurs when there is a small leak in the tire. The air in the tire effuses through the hole. At constant temperature and pressure the rate of effusion of a gas is inversely proportional to the square root of the molar mass, M. This is stated mathematically when comparing the speed of effusion,v, of two gases, A and B, vA/vB =√MB/MA

Class Practice Page 444 Sample 5,6,7,8

How do the gas laws fit together? The gas laws for a given quantity of gas that is changed from one set of P, V, or T conditions to another. A sample of carbon dioxide with a mass of 0.250 g is placed in a 350 ml container at 400K. What is the pressure exerted by the gas?

Class practice Page 447 Practice samples do all.

A weather balloon containing helium with a volume of 410 L rises in the atmosphere and is cooled from 27⁰C to -27⁰C. The pressure on the gas is reduced from 110 kPa to 25 kPa. What is the volume of the gas at lower temperature and pressure?

Home work Page 449 2 and 4.

Gas Stoichiometry How many liters of hydrogen gas will be produced at 280 K and 96 kPa if 40 g of sodium react with excess water according to the following equation? 2Na(s) + 2H202NaOH(aq)+H2(g)

Class practice Page 451 10,11, 13.

Phase Diagram What is phase? Any part of a system that has uniform composition and properties. A phase diagram is a graph that shows the temperatures and pressures at which a substance exists in different phases. The phases are in equilibrium with each other along the lines of the diagram.

Normal boiling point: The temperature at which a substance boils at 1,0000 atm pressure. Critical point: The temperature and pressure at and above which the properties of the vapor phase of a substance cannot be distinguished from those of the liquid phase. Sublimation: A change in state where a solid is changed into a gas without going through the liquid state. Triple point: the temperature and pressure at which three phases of a substance exist in equilibrium.

Home work Page 463 47, 49,52 and 53. Test prep all