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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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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 Pressuresp. 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

- 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

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