KINETIC MOLECULAR THEORY

Transcription

1 KINETIC MOLECULAR THEORY

2 IMPORTANT CHARACTERISTICS OF GASES 1) Gases are highly compressible An external force compresses the gas sample and decreases its volume, removing the external force allows the gas volume to increase. 2) Gases are thermally expandable When a gas sample is heated, its volume increases, and when it is cooled its volume decreases.

3 3) Gases have high viscosity Gases flow much easier than liquids or solids. 4) Most Gases have low densities Gas densities are on the order of grams per liter whereas liquids and solids are grams per cubic cm, 1000 times greater. 5) Gases are infinitely miscible undergo diffusion & effusion

4 KINETIC MOLECULAR THEORY OF GASES The word kinetic refers to motion. Kinetic energy is the energy an object has because of its motion Kinetic Molecular Theory makes assumptions about: Size Motion Energy of gas particles Refers to ideal gases

5 REAL VS IDEAL GASES Ideal gas obeys assumptions of KMT all the time It s a model It doesn t exist Used to approximate the behaviour of real gases Real gases follow KMT most of time, but not at high pressure & low temperature KMT breaks down when molecules are close together

6 KINETIC MOLECULAR THEORY Hypothesis 1: According to the KMT all matter consists of tiny particles that are in constant, random motion Move in a straight line until they collide with other particles or with the walls of the container.

7 COLLISIONS OF GAS PARTICLES

8 Hypothesis 2: Gas particles are much smaller than the distances between them. Most of a gas consists of empty space. Gas consists of small particles that are separated from one another by empty space Most of the volume of a gas consists of empty space Because they are so far apart, there are no attractive or repulsive forces between the gas molecules The motion of one particle is independent of the motion of other particles

9 Hypothesis 3: No kinetic energy is lost when gas particles collide with each other or with the walls of the container (elastic collision) Undergoes elastic collision no kinetic energy is lost when particles collide. The total amount of kinetic energy remains constant. No KE is transformed KE is transferred between particles

10 ELASTIC VS. INELASTIC COLLISIONS v 1 POW v 8 2 elastic collision v 3 v 4 8 inelastic collision

11 Hypothesis 4: All gases have the same average kinetic energy at a given temperature Temperature is a measure of average kinetic energy of particle in a sample of matter. Kinetic energy and temperature are directly related When you increase temperature of a gas it will increase its average kinetic energy Molecules that have the same temperature have the same average kinetic energy Kinetic Energy = ½ mv2; where m = mass and v = velocity

12 For the 3 balloons below at the same temperature According to KMT, K.E. He = K.E. NH 3 = KE O 2 If they have the same kinetic energy, will they have the same velocity?

13 DIFFUSION AND EFFUSION Diffusion and effusion are important gas behaviours explained by KMT. Diffusion describes the movement of one material through another Particles diffuse from an area of high concentration to low concentration Effusion gas escapes through a tiny opening. The heavier the molecule, the slower it will effuse or diffuse

14 DIFFUSION AND EFFUSION

15 DIFFUSION AND EFFUSION

16 GRAHAM'S LAW gaseous particles have high kinetic energy therefore they diffuse/effuse quickly but the rate of diffusion is different for different gases Graham observed that lighter gases effuse faster than heavier gases Graham's Law: The rate of effusion or diffusion is inversely proportional to the square root of its molar mass How was this found?

17 GRAHAM'S LAW According to the KMT of gases when gases are at the same temperature the mean Kinetic energy is the same When comparing two gases (gas 1 and gas 2) we can say: Cancel ½ since it is on both sides Take the square root of both sides

18 Since molar mass(m) is proportional to mass (m), we can replace mass in the formula

19 EXAMPLE 1 Example 1: If I pop a balloon of methane and a balloon of carbon dioxide at the same time which gas will diffuse to the other end of the room more quickly? Methane will diffuse faster as it has a smaller molar mass (CH4 = 16 g/mol while CO2 = 44 g/mol)

20 EXAMPLE 2 Example 2: The rate of diffusion of an unknown gas is four times faster than oxygen gas. Calculate the molar mass of the unknown gas. O2 = 32 g/mol

21 FACTORS AFFECTING EFFUSION AND DIFFUSION 1.Temperature: At a lower temperature the kinetic energy of a gas is lower, so it moves slower (diffusion is slower). 2.Size of the Particles: As size increases velocity of a gas decreases. 3.Medium: Diffusion and effusion is affected by the substance into which it diffuses/effuses. ex. a gas diffuses slower into a liquid or gel then into another gas.