Physical Properties of Gases

1. Physical Properties of Gases The Simple Gas Laws

2. What are the four variables that describe a gas? • Pressure, P • Volume, V • Absolute temperature, T • The quantity of a gas in moles, n Recall the standards, • STP – Standard temperature and pressure • T @ 0C and P @ 101.3 kPa • SATP – Standard ambient temperature and pressure • T @ 25C and P @ 101.3 kPa

3. The Simple Gas Laws Scientists were able to manipulate only two variables at a time, while maintaining the other two constant; this is how they were able to determine the simple gas laws. From the simple gas laws, scientists were able to formulate – the ideal gas law. The ideal gas law can be applies to real gases under most conditions. The behavior of real gases will not follow the model of an ideal gas at temperatures close to the condensation of the gas and at a pressure close to 1 kPa.

4. Boyle’s law The greater the pressure exerted on a gas, the more its volume decreases, as long as the temperature and the quantity of gas remains constant. Boyle’s law – At a constant temperature, the volume occupied by a given quantity of a gas is inversely proportional to the pressure of the gas. • If the pressure of a gas doubles because it is has been compressed, the volume of the gas will decrease by half. • If the volume of a gas is permitted to occupy double its initial volume, then the pressure will decrease by half.

5. Boyle’s law Pressure and volume are inversely proportional: P  The following formula may be used to determine the pressure and volume for a sample of the same gas: P1V1=P2V2 • P in mmHg or kPa • V in mL or L • n and T remain constant

6. Boyle’s law http://www.mentorials.com/high-school-chemistry-matter-properties-of-gases-and-gas-laws.htm

7. How does the Kinetic Theory of Gases help us to explain this phenomenon? Hypothesis 1: The particles of a gas are far apart from one another… • If the volume of a container containing a gas is reduced at a constant temperature, • The particles move closer and closer together, therefore they must travel shorter distances before they collide with one another or the container walls. This results in more frequent collisions, resulting in an increase in pressure. • If the volume is increased the result is fewer collisions, therefore the pressure decreases. Recall,

8. Charles’ law • As temperature increases, the volume a gas occupies increases provided the quantity of a gas is kept constant and a constant pressure is maintained. • The volume of a gas at 273C is double that of its volume at 0C. • One observation that was made was that the volume of any gas is not directly proportional to its temperature: http://butane.chem.uiuc.edu/cyerkes/Chem102AEFa07/Lecture_Notes_102/lecture%2016.htm

9. Charles’ law • Note, from the previousgraph, that the volume of a gas is at zero at -273C. This is the lowest possible temperature and the temperature at which the kinetic energy of all particles is zero. Lord Kelvin called this absolute zero and established the Kelvin scale: 0 K = 273C. • Using the absolute temperature (Kelvin) scale instead of the Celsius scale allows us to see a directly proportional relationship between the temperature of a gas and its volume: http://cnx.org/content/m12598/latest/

10. Charles’ law • Charles’ law: at a constant pressure, the volume occupied by a given quantity of gas is directly proportional to the absolute temperature of the gas. V  T

11. How does the Kinetic Theory of Gases help us to explain this phenomenon? Hypothesis 4: A higher temperature results in an increase in kinetic energy (faster moving particle). • As the temperature increases the gas particles move faster and they collide with each other and the walls of the container more often. This causes an increase in pressure; therefore in order to maintain a constant pressure in the system, the volume must increase.

12. Gay-Lussac’s law • As temperature increases, the pressure of system increases when the volume of a gas and its quantity, in moles, are kept constant; if we double the absolute temperature of a gas, the pressure of the gas doubles (given that the volume remains constant).

13. Gay-Lussac’s law • At a constant volume, the pressure of a given quantity of gas is directly proportional to the absolute temperature of the gas. P  T http://cfbt-us.com/wordpress/?p=1126

14. How does the Kinetic Theory of Gases help us to explain this phenomenon? Hypothesis 4: A higher temperature results in an increase in kinetic energy (faster moving particle). • As the temperature increases the gas particles move faster and they collide with each other and the walls of the container more often; if we maintain the volume of the system, this causes an increase in pressure.

15. Avogadro’s law • At a constant temperature and pressure, chemical reactions between gases always occur in whole number ratios - a fundamental law of chemistry, Gay-Lussac. • Avogadro determined that equal volumes of different gases contain the same number of particles, given that temperature and pressure remain constant.

16. Avogadro’s law Avogadro’s law: Under the same temperature and pressure conditions, the volume of a gas is directly proportional to its quantity expressed in number of moles. V  n http://dwb4.unl.edu/chem/chem869j/chem869jinfofiles/pubchem869j-info012.html

17. How does the Kinetic Theory of Gases help us to explain this phenomenon? Hypothesis 2: The particles of a gas are in constant motion… • If we increase the number of particles of a gas, more collisions will occur between them and the walls of the container. In order to maintain a constant pressure, the volume of the container must increase.

18. Molar volume of gases • The molar volume of a gas is the volume that one mole of the gas occupies. Avogadro’s law states that one mole of a gas contains 6.02 x 1023 particles, so the volume of one mole of all gases is the same regardless of the gas. The size of the particles has no effect on the volume occupied by the gas. • @ STP the molar volume is 24.5 L/mol • @SATO the molar volume is 24.8 L/mol

19. How does the Kinetic Theory of Gases help us to explain this phenomenon? Hypothesis 1: the particles of a gas are infinitely small and the size of a particle is negligible compared to the volume of the container that holds the gas. • Each particle may have its own mass, but their size is negligible. The size of the gas particles does not contribute to the overall volume that the gas occupies because the particles are far apart and there is empty space.

20. Relationship between pressure and quantity(in moles) of gas • When temperature and volume remain constant, the pressure of a gas increases as the quantity of gas increases. Inversely, if the quantity of gas decreases, so does the pressure. • The pressure of a gas is directly proportional to the quantity of a gas (in moles), as long as temperature and volume remain unchanged. Pn

21. How does the Kinetic Theory of Gases help us to explain this phenomenon? Hypothesis 2: The particles of a gas are in constant motion…translational… • As the number of particles increases, so do the number of collisions the particles have with each other and the walls of the container. When there are more collisions on the same surface area, more force is exerted on the surface, therefore pressure increases.