Attacking the TEKS: Gases

TEKS(4) (C) compare solids, liquids, and gases in terms of compressibility,

structure, shape, and volume (9) Science concepts. The student understands the principles of ideal

gas behavior, kinetic molecular theory, and the conditions that influence the behavior of gases. The student is expected to:

(A) describe and calculate the relations between volume, pressure,

number of moles, and temperature for an ideal gas as described by Boyle's law, Charles' law, Avogadro's law, Dalton's law of partial pressure, and the ideal gas law;

(B) perform stoichiometric calculations, including determination of

mass and volume relationships between reactants and products for reactions involving gases; and

(C) describe the postulates of kinetic molecular theory.

College Readiness StandardsI. Properties and behavior of gases, liquids, and solids

1. Understand the behavior of matter in its various states: solid, liquid, and gas.

a. Describe how gas pressure is affected by volume, temperature, and the addition of gas.

b. Describe the behavior of solids, liquids, and gases under changes in pressure.

3. Understand principles of ideal gas behavior and kinetic molecular theory. a. Use the kinetic molecular theory to explain how gas pressure is affected by

volume, temperature, and the addition of gas. b. Distinguish between real and ideal gas behavior, and identify the criteria in

the kinetic molecular theory that conflict with the properties of real gases.

4. Apply the concept of partial pressures in a mixture of gases. a. Use Dalton’s Law to determine the partial pressure of a gas in a mixture of

gases.

The Basics

Time Frame: 4.5 – 90 minute periods or7 – 50 minute periods

Consistent Mistakes:– Not converting to Kelvin– Trying to use 22.4 L = 1 mol but not at STP– Big difficulties with algebra

Engage

Could be as simple as having everyone inflate a balloon and tie it off and discuss what happened to the balloon as you added air to it; squeezing a full balloon seems to push back at you; filling up a balloon too much has consequences, etc. Introduce the 4 factors for gases (P V n T) and see how many you can relate to the balloon experience – what factors do all of the balloons share.

Kinetic Molecular Theory

• Temperature (K) related to average kinetic energy (AKE)

• Samples at the same temperature have the same AKE – higher mass with lower velocity

• Pressure – what is it, how is it measured, conversion between units

• Explain gas law relationships in terms of the KMT

An alternative to memorizing formulas

Discuss the gas relationship, – which factors are being kept constant – which are varying in each case– what the relationship looks like graphically

(direct or inverse).

For example, in Boyle’s Law, T and n are kept constant and when V ↓, P ↑ which is an inverse relationship.

Gas Law ProblemsRather than learning all of the laws separately, we use:

nT

PV

nT

PV

Variable Initial Final

P

V

n

T

Prediction: ______________________

•Work 6 examples

•10 problems of mixed types

•HW quiz

Derive the ideal gas law

Variable Data ConversionsP 104.5 kPa 101.325 = 1.03

atmV ????

n 2.5 mol O2

T 25oC + 273 = 298 K

Convert all data to the units of R•Work 2-3 examples

•10 problems of mixed types

•HW quiz

What is the volume, in dm3, of 2.5 moles of oxygen gas measured at 25°C and a pressure of 104.5 kPa?

Gas Stoichiometry

1 mole = 22.4 liters ONLY at STP!!

volumemolarmassmolarD STP @

P

dRTMM

Density of gases @ STP

Calculate the density of chlorine @STP.

/

/STP

molar mass g molD

molar volume L mol

70.90 /3.17 /

22.4 /STP

g molD g L

L mol

What is the density (g/L) of NH3, at 800.0 mmHg and 25oC?

P

dRTMM

RT

PMMd

)(

g/L0.73K)98(0.0821)(2

atm)5g/mol)(1.0(17d

Lab Experiences

Do a stations lab where students explore different gas relationships. Have them determine:– the factors that are being kept constant– what factor is changing– what factor is affected– the relationship (whose Law it is)Explanations should utilize the KMT.

Station #1 Marshmallow in a Syringe Take the stopper off the end of the syringe. Take

out the plunger and place a miniature marshmallow in the syringe. Replace the plunger and push it down to the marshmallow without squishing the marshmallow. Replace the stopper on the end of the syringe. Pull the plunger back to 60 cc.

What changes occurred to the air in the syringe?P V = n R T

Lab Experiences

Some sort of lab (molar mass of butane; molar volume of hydrogen, etc.) which requires students to collect a gas by water displacement. The analysis should require temperature conversion, pressure conversion, subtracting water vapor pressure, combined gas law, and the ideal gas law. Some stoichiometry is great too.

Dalton’s Law of Partial Pressures

5 + = CO2

30 ● = N2

15 ○ = O2

P atm = P gas + P water vapor

Compare the rate of effusion of two common gases, helium and oxygen.

1

2

2

1

MM

MM

rate

rate

8.2224

32

2

O

He

rate

rate

PAP only

Gases Review

4. Select the appropriate graph for each relationship.

_____Moles and volume

_____Temperature(K) and pressure

_____Pressure and volume

_____Temperature(K) and volume

_____Moles and pressure

6. For the following situations, indicate whether the pressure of the enclosed gas will increase or decrease.

A. The number of particles of an enclosed gas increases while volume and temperature are held constant.

B. A fixed amount of gas is cooled while the volume is held constant.C. The volume of a container holding 1 mole of nitrogen gas is

reduced at constant temperature.D. A mixture of enclosed gases in a 1 liter container at 2 atm is

allowed to expand to 2 liters.

Gases TestDiffusion between two gases occurs most rapidly if the gas particles are at a

a) high temperature and are less massive.b) low temperature and are more massive.c) low temperature and are less massive.d) high temperature and are more massive.

If the temperature of a container of gas remains constant, how could the pressure of the gas increase?

a) The mass of the gas molecules increases.b) The diffusion of the gas molecules increases.c) The size of the container increases. d) The number of gas molecules increases.

torr1520

torr760XmL30. torr1520

mL30.

torr760X

The pressure on 30. mL of a gas increases from 760 torr to 1520 torr at constant temperature.

The new volume is given by

torr760

torr1520XmL30. torr760

mL30.

torr1520X

PAP

style

Each of the following examples gives a change in volume, temperature, amount, or pressure of a gas sample. Indicate whether the other variable mentioned would increase or decrease. If a variable is not mentioned, assume it is constant.

• Additional gas is added to a soccer ball. The pressure -

• An inflated balloon is placed in a refrigerator. The volume -

• A piston in an engine compresses the gas. The volume -

• The volume of an inflated balloon increases when the amount

of gas in the balloon -

• A person sits on an air mattress. The pressure inside of the

mattress -