Ch 12 Gases

1. Ch 12 Gases

2. Properties of gases • Unique due to distance between particles • Gases are fluids(can flow) due to separation, can pass each other. • Low density due to lg distance of separation (most volume empty space) • High compressibility due to lg separation. Apply pressure push gas molecules closer together and decrease volume. • Completely fill a container, gas in constant motion and expand to fill a container.

3. Gas pressure • Collision of gas molecules with container, also causes air pressure. Force per unit area of surface. • P=F/A (units 1N/m2 = Pascal Pa) • Measuring pressure: • Atmospheric pressure measured by a barometer • Atm exert pressure on the surface of Hg in an evacuated column. • Pressure units (pg 420 table 1) • 1 atm = 101 kPa = 760 mm Hg = 760 torr • At sea level 760 mm Hg = 1 atm. STP 1 atm & 0oC • Convert 740. Mm Hg to atm, torr and kPa

4. Kinetic-molecular theory • Behavior of a physical system depend on the combine actions of the molecules constituting the system. • For gases; • 1. Gas particles are in constant rapid motion • 2. Gas particles are very far apart relative to their size. • => explain fluidity and compressibility • Collisions perfectly elastic, total energy conserved (low attractive intermolecular forces) • Gas temperature proportional to ave. KE • Increase temp (add heat) increase motion, not all gas are moving at the same speed. (pg 422 figure 8)

5. Gas laws • Measurable properties of gases • Pressure, temperature, volume and moles • Look at pairs of properties to determine relationship (direct or indirect). • P/V relationship, Robert Boyle/Boyle’s law • Fixed amount of gas at constant temperature the volume of a gas increases as the pressure decreases (indirect relationship) • P1V1 = P2V2: complete the following • A 2.5L of a gas at 110.0 kPa is expanded to 4.0L, what is the new pressure? • 650 mL of a gas is stored in acylinder with a moveable piston at 225 kPa. What would the volume of the gas at 545 kPa?

6. Temperature/volume relationship • Jacques Charles/Charles law: • Gases volume is directly proportional the Kelvin temperature at conts. P&n. • V1/T1 = V2/T2: complete the following • A balloon with a volume of 15.5 L is inflated in a room at 20.0oC and then taken outside with temperature of 7.0oC, what is the new volume? • If the original temperature of a 62.2L sample of a gas is 150oC, what is the final temperature of the gas if the new volume is 24.4L

7. Temperature/pressure relationship • Joseph Gay-lussac/Gay-Lussac’s law: • Pressure of a gas at a constant volume is directly proportional to the absolute (Kelvin) temperature • P1/T1=P2/T2: complete the following: • The pressure in a tire is 101 kPa at 10.0oC, what will the pressure of the tire at 45.0oC? • If a gas at 600oC exerts a pressure of 1515 kPa, at what temperature will the gas exert 151.5 kPa of pressure?

8. Volume/mole relationship • Avogadro: equal volume of all gases under the same conditions have the same number of particles. • Avogadro’s law; equal volume of gases at the same temperature and pressure contains equal # of molecules. • Gas volume is directly proportional to # of moles of gas at the same T&P • V1/n1=V2/n2, at STP 1 mol gas = 22.4L

9. Ideal gas law • Combination of Boyle’s, Charlie's, Gay-Lussac’s and Avogadro’s law. Assume gases behave ideally (no attractive forces and volume) • Ideal gas: imaginary gas whose particles are infinitely small and do not interact with each other. • By combining 4 laws • PV, V/T, P/T, & V/n = ideal gas law • PV=nRT, P (kPa, mm Hg or atm), V (L), n (mol) and T (k) • R=proportionality constant=8.314 L kPa/mol K • = 0.0821 L mmHg/mol K = 62.4 Latm/mol K

10. Ideal/real gases • Real gases behave ideally at rm.temp and atm pressure. • Low vol (attractive forces ignore), as volume decreases increase particle attraction ==> deviation. • At high P volume of particles close to total volume => volume higher than calculated (pg 434 figure 16) • Complete the following: • How many moles of gas are in a balloon that has a volume of 15.9L at a pressure 149 kPa and 28oC? • What is the volume of 4.35 moles of a gas at a pressure of 85.6 kPa and 26.0oC?

11. Gas behavior & chemical formulas • Diffusion: movement of particles from regions of higher density to regions of lower density. • Gases diffuse through each other at s very rapid rate. • High [ ] to low [ ] => homogeneous mixture • Low mass diffuse faster than higher mass => entropy increase

12. Effusion • Passage of gas under pressure through a tiny opening. • Thomas Graham: • At constant T&P the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass (M) => Graham law of diffusion • Va/Vb = Mb/Ma low MM travels faster than heavier particles. Calculate the following: • The average VO2 = 480m/s at rm T, how fast would HCN travel at rm T • The average VCO2 = 409m/s at rm T, what is the MM of a gas whose average Vx = 322 m/s?

13. Gas Rx => chemical formulas • Gay-Lussac’s law of combining volume: • Volume of gases involved in chemical change can be represented by the ratio of small whole numbers (coeff. of balanced equation) • 1 vol H gas + 1 vol Cl gas => 2 vol HCl gas • H and Cl has to be diatomic H2 and Cl2 • Cannizzaro used this relationship to correctly write H2O for water (Dalton had OH formula for water)

14. Dalton’s law of partial pressure • Mixture of gases, each gas exerts pressure as if alone => partial pressure of gas • Dalton’s law of partial pressure: • Total pressure of mixture of gases is equal to the sum of the partial pressure of the components gases • Pt = Pa + Pb + Pc +… • Gas Stoichiometry: • Ideal gas law relates moles with other gas variables (TP&V) • Gas volume corresponds to mole ratio. • Ratio of gas volume same as mole ratio in a balanced equations.

15. Practice • In the combustion reaction of 149g of propane with excess oxygen, what volume of CO2 is produced at STP? • C3H8 + 5O2 --> 3CO2 + 4H2O • How much potassium chlorate (decomposed) will be necessary to produce 125mL of O2(g) at 700oC and a pressure of 98.6 kPa?