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CHEMISTRY. IT’S A GAS!! Chapter 13. Units of Pressure. 1atm = 101.3 kPa = 760 torr = 760 mm Hg = 14.7 psi Measured with a barometer or manometer to measure pressure less than atmospheric. Assumptions About Properties of Gases – Kinetic Theory p.475-477.

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Chapter 13

Units of pressure
Units of Pressure

  • 1atm = 101.3 kPa = 760 torr = 760 mm Hg = 14.7 psi

  • Measured with a barometer

  • or manometer to measure pressure less than atmospheric

Assumptions about properties of gases kinetic theory p 475 477
Assumptions About Properties of Gases – Kinetic Theory p.475-477

  • Small particles with lots of space between-insignificant volume

  • Compressible

  • High kinetic energy – random motion and straight paths

  • Elastic collisions

  • No attractive or repulsive forces between particles You know what they say about assuming!! More on that later. 

Variables that describe gases
Variables That Describe Gases p.475-477

  • Pressure (P) – kPa, atm, Torr, mm Hg, psi, bars or mbars

  • Volume (V)– liters

  • Temperature (T)– Kelvin scale

  • Moles (n) – number of moles

Gas laws
Gas Laws p.475-477

  • Allow us to predict behavior of gases under specific conditions.

  • Help us understand everyday applications of gases

  • Why do we have to check air pressure in tires?

  • Why did my balloon shrink in the cold?

  • Why does warm soda fizz so much?

Factors affecting gas pressure
Factors Affecting Gas Pressure p.475-477

  • Amount of gas – number of particles/moles

  • Volume – space gas takes up

  • Temperature – increases or decreases kinetic energy of particles resulting in more or fewer collisions

Boyle s law p 447
Boyle’s Law p.447 p.475-477

  • Describes pressure-volume relationship of gases at constant temperature

  • States that for a given mass of gas, as pressure increases, volume decreases and vice versa

  • What kind of relationship is this?

  • How can it be expressed mathematically?

  • P1V1 = P2V2

Charles law p 451
Charles’ Law p.451 p.475-477

  • Describes the relationship between volume and temperature at constant pressure.

  • States that for a fixed mass of gas, as temperature increases, volume increases

  • What kind of relationship is this?

  • How can this be expressed mathematically?

  • V1/T1 = V2/T2

Gay lussac s law
Gay-Lussac’s Law p.475-477

  • Describes relationship between temperature and pressure with constant volume

  • States that the pressure of a gas will increase with increased temperature and vice versa

  • What kind of relationship is this?

  • How can it be expressed mathematically?

  • P1/T1 = P2/T2

Combined gas law p 464
Combined Gas Law p. 464 p.475-477

  • Combines Boyle’s, Charles’ and Gay-Lussac

  • Allows for calculations where none of the variables is constant

  • P1V1/T1 = P2V2/T2

Avogadro s hypothesis law p 455
Avogadro’s p.475-477Hypothesis (Law) p.455

  • Equal volumes of gases at the same temperature and pressure contain equal numbers of particles

  • Gases at constant pressure and temperature have volume that is directly proportional to number of moles.

  • V1/n1 = V2/n2

  • One mole (6.02 x 1023 particles) of any gas at STP occupies a volume of 22.4 L

Ideal gas law p 458
Ideal Gas Law p.458 p.475-477

  • Incorporates the fourth variable of gases – number of moles

  • When “n” is included in combined gas law a constant is recognized and symbolized as “R” with value of 0.08206 Latm/Kmol

  • “R” is known as the ideal gas constant

  • New mathematical expression PV=nRT

Ideal gas p 458
Ideal Gas p.458 p.475-477

  • One that follows all gas laws at all conditions of pressure and temperature

  • No such gas – however, real gases behave as ideal gases under most conditions of pressure and temperature

  • Real gases can be liquefied and sometimes solidified – ideal gases cannot

Real vs ideal p 478
Real vs Ideal p.478 p.475-477

  • Kinetic theory assumptions – no attraction among particles and no volume

  • Assumptions are incorrect!

  • If true, would not be able to liquefy or solidify gases

  • Gases definitely have volume

Dalton s law of partial pressures p 464
Dalton’s Law of Partial Pressures p.475-477p. 464

  • States at constant volume and temperature the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures

  • Expressed mathematically

    P1 + P2 + P3…= Ptotal

Graham s law
Graham’s Law p.475-477

  • Describes diffusion and effusion of gas molecules

  • States that rate of effusion is inversely proportional to the square root of the gases’ molar masses

  • Also found to be true for diffusion of gases

  • Simply put, means small gas molecules move faster than heavier gas molecules

  • Can be expressed mathematically