1GASKIMIA FISIK I BAB INKIM613 4SKS/4JS

HEMAT KERTAS DAN TINTA!!!

JANGAN CETAK BERWARNA

2Importance 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

3THREE STATES OF MATTER

4General Properties of Gases

There is a lot of free space in a gas.

Gases can be expanded infinitely.

Gases fill containers uniformly and completely.

Gases diffuse and mix rapidly.

5Properties of Gases

Gas properties can be modeled using math. Model that depends on

V = volume of the gas (L)

T = temperature (K)

ALL temperatures MUST be in Kelvin!!! No Exceptions!

n = amount (moles)

P = pressure(atmospheres)

6PressurePressure is defined

as a force per unit area ( P = F/A )

The SI unit of pressure is the pascal (Pa)

7PressurePressure of air is measured

with a BAROMETER(developed by Torricelli in 1643)

Hg rises in tube until force of Hg (down) balances the force of atmosphere (pushing up). (Just like a straw in a soft drink)

P of Hg pushing down related to

Hg density

column height

8PressurePressure of liquid is

measured with an assist of mass and volume or density of liquid

m = mass

V = volume

d = density

P = g x h x d (1.1)

9Pressure

10PressureColumn height measures

Pressure of atmosphere

1 standard atmosphere (atm) *

= 760 mm Hg (or torr) *

= 29.92 inches Hg *

= 14.7 pounds/in2 (psi)*Termodynamics only

= 101.3 kPa (SI unit is PASCAL)

= about 34 feet of water!

* Termodynamics only

* Memorize these!

11

Pressure Conversions

A. What is 475 mm Hg expressed in atm?

1 atm

760 mm Hg

B. The pressure of a tire is measured as 29.4 psi.

What is this pressure in mm Hg?

760 mm Hg

14.7 psi = 1.52 x 103 mm Hg

= 0.625 atm475 mm Hg x

29.4 psi x

12

Pressure Conversions

A. What is 2 atm expressed in torr?

B. The pressure of a tire is measured as 32.0 psi.

What is this pressure in kPa?

13

Learning Check

A gas has a volume of 675 mL at 35C and 0.850 atm pressure. What is the temperature in C when the gas has a volume of 0.315 L and a pressure of 802 mm Hg?

14

One More Practice Problem

A balloon has a volume of 785 mL on a fall day when the temperature is 21C. In the winter, the gas cools to 0C. What is the new volume of the balloon?

15And now, we pause for this

commercial message from STP

OK, so its really not THIS kind of STP

STP in chemistry stands for

Standard Temperature and

Pressure

Standard Pressure =

1 bar (or an

equivalent, 105 Pa)

Standard

Temperature = 0 deg

C (273 K)

STP allows us to

compare amounts of

gases between different

pressures and

temperatures

16

Try This One

A sample of neon gas used in a neon sign has a volume of 15 L at STP. What is the volume (L) of the neon gas at 2.0 atm and 25C?

17

IDEAL GAS LAW

Brings together gas properties.

Can be derived from experiment and theory.

BE SURE YOU KNOW THIS EQUATION!

P V = n R T

18

Using PV = nRTP = Pressure

V = Volume

T = Temperature

N = number of moles

R is a constant, called the Ideal Gas Constant

Instead of learning a different value for R for all the possible unit combinations, we can just memorizeone value and convert the units to match R.

R = 0.0821L atm

Mol K

19

Using PV = nRTHow much N2 is required to fill a small room

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

Solution

1. 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

And we always know R, 0.0821 L atm / mol K

20

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

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

Solution

2. Now plug in those values and solve for the unknown.

PV = nRT

n = (0.98 atm)(2.7 x 10 4 L)

(0.0821 L atm/K mol)(298 K)

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

RT RT

21

Learning Check

Dinitrogen monoxide (N2O), laughing gas, is used by dentists as an anesthetic. If 2.86 mol of gas occupies a 20.0 L tank at 23C, what is the pressure (mm Hg) in the tank in the dentist office?

22

Learning Check

A 5.0 L cylinder contains oxygen gas at 20.0C and 735 mm Hg. How many grams of oxygen are in the cylinder?

23Mixtures of Gases

(Non-reactive)

The % of gases in air Partial pressure (STP)

78.08% N2 593.4 mm Hg

20.95% O2 159.2 mm Hg

0.94% Ar 7.1 mm Hg

0.03% CO2 0.2 mm Hg

PAIR = PN + PO + PAr + PCO = 760 mm Hg2 2 2

Total Pressure 760 mm Hg

Gases in the air

24Daltons Law of Partial Pressures

25Daltons Law of Partial Pressures

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

0.32 atm 0.16 atm

26

Daltons Law

John Dalton

1766-1844

27Health NoteWhen a scuba diver is several

hundred feet under water, the

high pressures cause N2 from

the tank air to dissolve in the

blood. If the diver rises too

fast, the dissolved N2 will form

bubbles in the blood, a

dangerous and painful

condition called "the bends".

Helium, which is inert, less

dense, and does not dissolve

in the blood, is mixed with O2in scuba tanks used for deep

descents.

28Soal-soal

1. After the gases mix, what is the partial pressure of each gas, and what is the total pressure?

2. What is the mole fraction of each gas in the mixture?

3. Assume that unpolluted air has the composition shown

in Table above

(a) Calculate the number of molecules of N2, of O2, of Ar

and of CO2 in 1.00 L of air at 21C and 1.00 atm.

(b) Calculate the mole fractions of N2, of O2, of Ar and of

CO2 in the air

294. Let us represent gasoline as octane, C8H18. When hydrocarbon fuels burn in the presence of sufficient oxygen, CO2 is formed.

Reaction A: 2C8H18 + 25O2 16CO2 + 18H2O

But when the supply of oxygen is limited, the poisonous gas carbon monoxide, CO, is formed.

Reaction B: 2C8H18 + 17O2 16CO + 18H2O

Any automobile engine, no matter how well tuned, burns its

fuel by some combination of these two reactions. Suppose an automobile engine is running at idle speed in a closed garage with air volume 97.5 m3. This engine burns 95.0% of its fuel by reaction A, and the remainder by reaction B.

(a) How many liters of octane, density 0.702 g/mL, must be burned for the CO to reach a concentration of 2.00 g/m3?

(b) If the engine running at idle speed burns fuel at the rate

of 1.00 gal/h (0.0631 L/min), how long does it take to reach the CO concentration in (a)?

30Collecting a gas over waterGases, since they mix with other gases readily, must be collected in an environment where mixing can not occur. The easiest way to do this is under water because water displaces the air. So when a gas is collected over water, that means the container is filled with water and the gas is bubbled through the water into the container. Thus, the pressure inside the container is from the gas AND the water vapor. Daltons Law of Partial Pressures becomes useful here.

31

Table of Vapor Pressures for Water

32

Solve This!

A student collects some hydrogen gas over water at 20 degrees C and 768 torr. What is the pressure of the H2 gas?

768 torr 17.5 torr = 750.5 torr

33

GAS DENSITY

High

density

Low

density

22.4 L of ANY gas

AT STP = 1 mole

34

Gases and Stoichiometry

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

Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2at STP?

Bombardier beetle

uses decomposition

of hydrogen peroxide

to defend itself.

35

Gases and Stoichiometry2 H2O2 (l) ---> 2 H2O (g) + O2 (g)

Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2 at STP?

Solution

1.1 g H2O2 1 mol H2O2 1 mol O2 22.4 L O2

34 g H2O2 2 mol H2O2 1 mol O2

= 0.36 L O2 at STP

36

Gas Stoichiometry: Practice!

A. What is the volume at STP of 4.00 g of CH4?

B. How many grams of He are present in 8.0 L of gas at

STP?

37

What if its NOT at STP?

1. Do the problem like it was at STP. (V1)

2. Convert from STP (V1, P1, T1) to the stated conditions (P2, T2)

38

Try this one!

How many L of O2 are needed to react 28.0 g NH3 at 24C and 0.950 atm?

4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)

39

CalculationP1 = 0.800 atm V1 = 180 mL T1 = 302 K

P2 = 3.20 atm V2= 90 mL T2 = ??

P1 V1 P2 V2= P1 V1 T2 = P2 V2 T1

T1 T2

T2 =

T2 =

T2 = 604 K - 273 = 331 C

= 604 K

P2 V2 T1

3.20 atm x 90.0 mL x 302 K

P1 V1

0.800 atm x 180.0 mL

40

The KineticMolecular Theory

1. Gases consist of discrete molecules. The individual molecules are very small and are very far apart relative to their own sizes.

2. The gas molecules are in continuous, random, straight-line motion with varying velocities.

3. The collisions between gas molecules and with the walls of the container are elastic; the total energy is conserved during a collision; that is, there is no net energy gain or loss.

4. At any given instant, only a small fraction of the molecules are involved in collisions. Between collisions, the molecules exert no attractive or repulsive forces on one another; instead, each molecule travels in a straight line with a constant velocity.

41Molecular SpeedTotal kinetic energy per mole of gas 3RT /2

Root-mean-square speed

42Distribution of Molecular Speed

43

THE REAL GAS