1 © 2006 brooks/cole - thomson importance of gases airbags fill with n 2 gas in an accident.airbags...

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© 2006 Brooks/Cole - Thomson

Importance of Gases

• Airbags fill with N2 gas in an accident. • Gas is generated by the decomposition of

sodium azide, NaN3.• 2 NaN3 ---> 2 Na + 3 N2

•How much sodium azide is required to fill an air bag?

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Hot Air Balloons —How Do They Work?

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THREE STATES

OF MATTER

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General Properties of

Gases• There is a lot of “free” space in a gas.

• Gases can be expanded infinitely.

• Gases occupy containers uniformly and completely.

• Gases diffuse and mix rapidly.

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Properties of Gases

Gas properties can be

modeled using math. Model depends on—

• V = volume of the gas (L)• T = temperature (K)• n = amount (moles)• P = pressure

(atmospheres)

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PressurePressure of air is

measured with a BAROMETER (developed by Torricelli in 1643)

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PressureHg rises in tube until

force of Hg (down) balances the force of atmosphere (pushing up into the cylinder).

P of Hg pushing down related to

• Hg density (13.55 g/mL)

• column height

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Pressure

Column height measures P of atmosphere

• 1 standard atm= 760 mm Hg

= 29.9 inches Hg= about 34 feet of

waterSI unit is PASCAL,

Pa, where 1 atm = 101.325 kPa

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IDEAL GAS LAW

Brings together gas properties.

Can be derived from experiment and theory.

P V = n R T

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Boyle’s LawIf n and T are constant,

thenPV = nRT = kP = k (1/V) = nRTThis means, for

example, that P goes up as V goes down. They are inversely proportional.

For changing pressure or volume of a confined gas, PV=PV

Robert Boyle (1627-1691). Son of Earl of Cork, Ireland.

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Charles’s Law

If n and P are constant, then

V = (nR/P)T = kTV and T are directly

related.For changing

temperature or volume of a confined gas,

V/T=V/T

Jacques Charles (1746-1823). Isolated boron and studied gases. Balloonist.

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Charles’s original balloon

Modern long-distance balloon

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Charles’s Law

Balloons immersed in liquid N2 (at -196 ˚C) will shrink as the air cools (and is liquefied).

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Avogadro’s Hypothesis

Equal volumes of gases at the same T and P have the same number of molecules.

V = n (RT/P) = knV and n are directly related.For changing moles of a confined

gas, V/n=V/n

twice as many molecules

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KINETIC MOLECULAR THEORY(KMT)

Theory used to explain gas laws. KMT assumptions are

• Gases consist of molecules in constant, random motion.

• P arises from collisions with container walls.

• No attractive or repulsive forces between molecules. Collisions elastic.

• Volume of molecules is negligible.

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Kinetic Molecular Theory

Because we assume molecules are in motion, they have a kinetic energy.

KE = (1/2)(mass)(speed)2

At the same T, all gases have the same average KE.

As T goes up for a gas, KE also increases — and so

does speed.

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Using KMT to Understand Gas Laws

Recall that KMT assumptions are• Gases consist of molecules in

constant, random motion.• P arises from collisions with container

walls.• No attractive or repulsive forces

between molecules. Collisions elastic.• Volume of molecules is negligible.

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Deviations from Ideal Gas Law

• Real molecules have

volume.

• There are intermolecular forces.– Otherwise a gas could

not become a liquid.

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Deviations from Ideal Gas Law

Account for volume of molecules and intermolecular forces with VAN DER WAALS’s EQUATION.

Measured V = V(ideal)Measured P

intermol. forcesvol. correction

J. van der Waals, 1837-1923, Professor of Physics, Amsterdam. Nobel Prize 1910.

nRTV - nbV2

n2aP + ----- )(

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Combining the Gas Laws

• V proportional to 1/P• V prop. to T• V prop. to n• Therefore, V prop. to nT/P• V = 22.4 L for 1.00 mol when

– Standard pressure and temperature (STP)

– T = 273 K– P = 1.00 atm

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Using PV = nRTHow much N2 is req’d to fill a small room

with a volume of 960 cubic feet (27,000 L) to P = 745 mm Hg at 25 oC?

R = 0.082057 L•atm/K•molSolution1. Get all data into proper units V = 27,000 L T = 25 oC + 273 = 298 K P = 745 mm Hg (1 atm/760 mm Hg)

= 0.98 atm

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Using PV = nRTHow much N2 is req’d to fill a small room with a volume of 960

cubic feet (27,000 L) to P = 745 mm Hg at 25 oC?

R = 0.082057 L•atm/K•mol

Solution2. Now calc. n = PV / RT

n = 1.1 x 103 mol (or about 30 kg of gas)

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Gases and Stoichiometry

2 H2O2(liq) ---> 2 H2O(g) + O2(g)

Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the pressure of O2 at 25 oC? Of H2O?

Bombardier beetle uses decomposition of hydrogen peroxide to defend itself.

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2 H2O2(liq) ---> 2 H2O(g) + O2(g)


Solution

Strategy: Calculate moles of H2O2 and then moles of O2 and H2O. Finally, calc. P from n, R, T, and V.

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2 H2O2(liq) ---> 2 H2O(g) + O2(g)


Solution

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2 H2O2(liq) ---> 2 H2O(g) + O2(g)


Solution

P of O2 = 0.16 atm

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GAS DENSITYScreen 12.5

PV = nRT

d and M proportional

and density (d) = m/V

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The Disaster of Lake Nyos

• Lake Nyos in Cameroon was a volcanic lake.

• CO2 built up in the lake and was released explosively on August 21, 1986.

• 1700 people and hundreds of animals died.

1 © 2006 brooks/cole - thomson importance of gases airbags fill with n 2 gas in an accident.airbags...

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