The Gas Laws. Learning about the special behavior of gases

Transcription

1 The Gas Laws Learning about the special behavior of gases

2 The States of Matter What are the 3 states of matter that chemists work with? Solids, liquids, and gases We will explain the behavior of gases using the Kinetic Molecular Theory

3 The Kinetic Molecular Theory Kinetic - motion Molecules are in motion! Solids - a bit, Liquids - a bit more, Gases - a whole bunch! Kinetic Energy - the energy an object contains because it is in motion Kinetic Molecular Theory - atoms in all forms are in constant motion. We will focus on gases only and use the kinetic theory to explain their behavior.

4 3 Basic Assumptions of the Kinetic Theory 1. A gas is composed of particles, usually molecules or atoms. The atoms occupy a negligible volume compared to their container! No attractive or repulsive forces between molecules.

5 Second Assumption The particles in a gas move rapidly in constant random motion. The molecules travel in straight lines and move independently of each other. They change direction only when they rebound from collisions with one another or with other objects.

6 Third Assumption All collisions are perfectly elastic perfectly elastic means that energy is transferred from one particle to another during collisions, but the total energy remains the same.

7 Question to Ponder: If we opened a perfume bottle In Washington D.C., the gas molecules should reach Mexico City in 90 minutes... Why don t they ever make it? They collide with each other, other particles or objects!!

8 Random Walk The aimless path molecules take as they diffuse from areas of higher concentration to areas of lower concentration. What are the 3 variables that effect the random walk of gas molecules? Temperature, Pressure, and Volume!!

9 Pressure Gas Pressure - the result of simultaneous collisions of billions and billions of gas particles on an object. Vacuum - absence of particles, no pressure empty space caused by removing gas molecules Atmospheric Pressure - results from the collisions of air molecules with objects Barometers - measure air pressure, both mercury and aneroid types.

10 Units of Pressure Pressure can be measured in 3 different units: Pascal - the SI unit of pressure kpa is Standard Pressure mmhg - (millimeters of mercury) - one mm of Hg is the pressure needed to support a column of mercury 1 mm high. 760 mm Hg is standard pressure ATM - atmosphere - the average atmospheric pressure at sea level. 1 ATM is standard pressure

11 Remember these conversions: 1 ATM = 760 mmhg = kpa These are the values of standard pressure. Standard Temperature = 0 Celsius or 273 Kelvin (Kelvin = Celsius) Let s work some example conversion problems...

12 Kinetic Energy and Temperature Kinetic energy - the energy an object contains because of its motion. KE = 1/2 (mass)(velocity) 2 Gas molecules have kinetic energy because. They Move!!

13 What happens? What happens to the amount of kinetic energy when molecules are heated? It increases! The average kinetic energy of the particles of a substance is proportional to the KELVIN temperature of the substance. Particles of all substances at the same temp have the same kinetic energy

14 Absolute Zero The temperature at which the motion of particles ceases 0 Kelvin or -273 Celsius is it s value Remember, the particles in a substance at 200 K have twice the average kinetic energy of the particles in a substance at 100K.

15 Chapter 12 - The Behavior of Gases How does adding molecules into a container, keeping volume and temperature constant, effect the pressure inside the container? More molecules results in an increased pressure. WHY? More molecules will strike the side of the container more often = higher pressure

16 Flip side - How does removing particles, keeping volume and temp constant, affect the pressure? Lowers it WHY? Fewer particles = fewer collisions against the side of the container

17 The effect of Changing the Size of the Container: What happens to the pressure inside of a container when we reduce the volume, keeping temp constant? Increases because shorter distance traveled before a collision against the side What happens if we increase the volume? Lower pressure because greater distance traveled means fewer collisions

18 The Effect of Heating or Cooling a Gas How does raising the temperature affect pressure if volume is constant? INCREASE = increased kinetic energy = molecules move faster and strike the side more often. How does lowering the temperature affect pressure if volume is constant? DECREASE - lowered KE = move slower =fewer collisions

19 Questions 5-9, pg 332 Read and answer questions here

20 The Gas Laws - Boyle s Law State Boyle s Law in words: When temperature and the number of particles are held constant, pressure and volume are inversely related Equation: P1V1 = P2V2 or P x V = k Graph Boyle s Law:

21 Charles s Law - Temperature vs Volume State Charles s Law in Words: When pressure and amount of gas is held constant, Kelvin temperature and volume are directly related. Show this law in a formula: V1/T1 = V2/T2 What would the graph for this look like?

22 Gay-Lussac s Law: Temperature and Pressure State Gay-Lussac s Law in words: When the amount of gas and the volume of the gas are held constant, Kelvin temperature and pressure are directly related. Express this in a formula: P1/T1 = P2/T2 What would the graph look like?

23 Dalton s Law of Partial Pressures Define Partial Pressure - The contribution each gas in a mixture makes to the total pressure is the partial pressure exerted by that gas Dalton s Law - At constant volume and temp, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures. **We use this in the lab when we collect a gas over water ( called water displacement)

24 Combined Gas Law: There really is no need to remember 3 different equations A single expression, called the combined gas law, combines the three gas laws, only holding the amount of gas constant.. P1 x V1 / T1 = P2 x V2 / T2

25 Avogardro s Hypothesis Gas molecules are different sizes However, equal volumes of gases at the same temp and pressure contain equal numbers of particles. Model this idea using a diagram: What volume does one mole of any gas occupy at STP?