SCH3U, STRAND F:GASES AND ATMOSPHERIC CHEMISTRY

By Farah Farah and Vanessa Poehlmann

GAS LAWS

Minds On!The Mysterious Shrinking Balloon

Inflate a balloon and tie off the end tightly.

Use a string to measure its circumference.

Take turns constantly submerging the balloon in ice-cold water for approximately 15 minutes.

Measure the circumference again. Compare the two measurements. Why did the balloon

shrink?

Investigation done in pairs

A class discussion or a think-pair-share can follow the activity to develop hypothesis.

The answer is explained by GAS LAWS: The gas inside the balloon

compressed when the overall temperature of the balloon was decreased in the cold water.

Therefore, the circumference of the balloon decreased!

Why Did the Balloon Shrink?

Background Knowledge:Physical Characteristics of Gases

• Gases assume the volume and shape of their containers.

• Gases are the most compressible state of matter.

• Gases will mix evenly and completely when confined to the same container.

• Gases have much lower densities than liquids and solids.

Variables That Affect Gases

• Volume (V) – the size of the container that holds the gas in liters (L).

• Temperature (T) – the speed or kinetic energy of the particles in degrees Kelvin (oC +273)

• Pressure (P) – The outward push of gas particles on their container in atmospheres (atm) or

millimeters of mercury (mm Hg) or pounds/square inch (psi)

*Think of pressure as the number of collisions between gas particles and their container.

• Moles (n) – the amount of gas.

Units of Pressure

Pressure =

1 pascal (Pa) = 1 N/m2

1 atm = 760 mmHg = 760 torr1 atm = 101,325 Pa

Force

Background Knowledge: Pressure

Area How does pressure affect

our lives? Give some examples of

pressure from our every day life.

Have you ever felt your ears “pop” in an airplane or when driving over a hilly road?

This can be explained with pressure and the influence of gravity.

sTP – Standard Temperature Pressure

The behavior of a gas depends on its temperature and the pressure at which the gas is held.

So far we have only dealt with gases at STP.

The values for Standard Temperature and Pressure are:

273o kelvin and 1 atm

Kinetic-Molecular Theory (KMT) describes the behavior of gases

• A gas consists of very small particles.

• The distances between gas particles are relatively large.

• Gas particles are in constant, random motion.

• Collisions between gas particles are perfectly elastic.

• Average KE of particles depends only on the temperature of the gas.

• There is no attractive force between particles of a gas.

The Gas Laws

With all of that background information, we can begin to discuss the gas laws:

Boyle’s LawCharles’ LawGay-Lussac’s LawThe Combined Gas LawThe Ideal Gas Law

Boyle’s Law• Describes the Pressure-Volume Relationship.• The pressure and volume of a sample of gas at constant

temperature are inversely proportional to each other.(As one goes up, the other goes down)

P1V1 =P2V2

If 3 of the variables are known, the fourth can be calculated.

Constant temperatureConstant amount of gas

Charles’ LawDescribes the temperature-volume relationship.

At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature.

2

2

1

1

TV

TV

If 3 of the variables are known, the fourth can be calculated

Constant pressureConstant amount of gas

Gay-Lussac’s LawDescribes the Temperature-Pressure Relationship.

If a volume of a sample of gas remains constant, the temperature is directly proportional to its pressure.

If 3 of the variables are known, the fourth can be calculated

Constant volumeConstant amount of gas

2

2

1

1

TP

TP

The Combined Gas Law

If more than one variable changes, a different equation is needed to analyze the behavior of the gas.

2

22

1

11

TVP

TVP

5 of the variables must be known to calculate the 6th.

The Ideal Gas LawDescribes the physical behavior of an ideal gas in terms of the pressure, volume, temperature and the number of moles of gas.

Ideal gas: a gas as described by the kinetic-molecular theory.

BUT . . . All gases “real gases”

FORTUNATELY, they behave like ideal gases under most conditions.

Only at very low temperatures and very high pressures do real gases show significant non-ideal behavior.

We assume that most gases are close to ideal and that the ideal gas equation applies.

Ideal Gas Equation

P-pressureV-volumen-number of moles of gasR-ideal gas constant (universal gas constant) 0.0821 atm .L /mol.K

or 62.396 torr .L/mol.KT-temperature

PnRTV

Lesson SequenceLesson 1: Physical Characteristics of Gases & Pressure

Lesson 2: Kinetic Molecular Theory & S.T.P

Lesson 3: Boyle’s Law & Charles’ Law

Lesson 4: Gay- Lussac’s Law & Combined Gas Law

Lesson 5: Ideal Gas Law

Curriculum ExpectationsLesson 1: Physical Characteristics of Gases & Pressure

F2.1, F2.2

Lesson 2: Kinetic Molecular Theory & S.T.PF3.2, F3.3

Lesson 3: Boyle’s Law & Charles’ LawF2.3, F3.5

Lesson 4: Gay- Lussac’s Law & Combined Gas LawF2.3, F3.5

Lesson 5: Ideal Gas LawF2.1, F3.5

NOTE: The Gas Laws are a very large topic and encompass almost all of Strand F in SCH3U

Laboratory Experiment:Alka-Seltzer and the Ideal Gas Law

When Alka Seltzer reacts with water, CO2 gas is produced.

In this lab, students will collect the gas given off from this reaction by covering a flask with a balloon.

Using the mass difference, students will determine the mass lost by the process, and thus the mass of CO2 produced.

Students then use the ideal gas law to calculate the number of moles of gas produced, and from this, the molar mass of CO2.

Discussion questions follow the lab, including sources of error.

NOTE: Full laboratory can be found at:http://www.nclark.net/Alka_Seltzer_and_the_Ideal_Gas_Law.doc

Class Activity: SCUBA Science

This activity brings a real-life connection to gas laws by discussing the tragic sinking of the Andrea Doria and the science of SCUBA diving.

Visit this link to view the entire activity:http://www.pbs.org/wnet/secrets/previous_seasons/lessons/lp_andreadoria.html

To begin, students watch video clips of the Andrea Doria, and SCUBA divers that have become fascinated with diving the wreck site.

Students then rotate through 4 exploration stations, as listed on the website.Through these stations they complete the Gas Laws Activities Handout.

The activity is designed to be completed in one class period. This can be done in small groups or as a whole class, if individual computers are not available. Also note that the end of the webpage lists assessment opportunities and other useful links to resources.

Safety Rules for Gas Laws LabsWhen performing the previous lab or activity, the basic lab rules apply, as well as some special considerations for gasses:

No eating or drinking in the lab Use proper safety protection Always clean glassware before you use it to be sure that residues

are cleaned away. Add at least some water first, before adding any liquid or solid solutes.

Read up experiment procedure. Any compressed gas cylinders are to be stored upright and away

from high traffic areas. Never allow a gas cylinder to fall or strike another cylinder violently. Tell the instructor of any accidents immediately. If you smell gas, tell the instructor immediately.

Gizmo: Boyle’s Law and Charles’ Law

http://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=422

This Gizmo allows students to investigate Boyle's Law and Charles' Law by performing experiments where either temperature or pressure are held constant, while other variables change.

It also provides a great visual example, as students can see how the movement of the molecules change with changing variables.

This appeals to visual learners as well as student with spatial multiple intelligence.

Assessment opportunities are included in the student exploration guide.

Potential Student Difficulty: “If I can't see it, who cares?”

Many gases are invisible and odourless, so it is difficult for students to conceptualize their behavior.

It is our challenge as teachers to make the invisible world of gases seem real and relevant to our students!

Techniques to HelpInquiry learning can help to make gasses “real” to students. By setting up various “exploration stations” for students to perform activities related to gas laws, they can formulate and test their own hypothesis about the behaviors of gases.

REMEMBER: Resist the temptation to give students the answers! Let them arrive there themselves!

There are many great demonstrations that bring the gas laws to life. Click on the links below to watch some great demos on youtube.

The Cartesian diver - this demonstration is a classic, but is great for inquiry learning. Why does the diver sink and float?Magdeburg Hemispheres - a demonstration done with household toilet plungers instead of actual Magdeburg Hemispheres.

Misconceptions: Hot air rises?Students are often confused about what really happens when a gas is heated or cooled.

Some students will talk about attractive forces bringing molecules together on cooling, or repellant forces pushing molecules apart on heating.

Some will suggest that there are somehow more molecules in heated gas,causing expansion in a heated balloon.

Techniques to Help

• This analogy can be acted out by a few students, if safety is emphasized.

• To get students thinking about increasing or decreasing particle motion, an analogy can be made to a room full of people (gas molecules) dancing to music (temperature of the gas).

Think of Dancing with the Stars . . . • For waltz music (low temperature), the dancers will be gently swaying, and they

won't bump up against the walls very often, or very hard. • For jive music (high temperature), they'll start dancing faster, bouncing off each

other more, and running into the walls more often and harder. Either pressure will increase or the container will expand.

Practical Applications

• SCUBA diving and decompression sickness• Discuss dissolved gasses in the human

body, and how a quick change in pressure can cause them to come out of solution as bubbles.

• Health Canada Air Quality Index• Discuss pollutants in our air, as a result of mining, smelting, car emissions, and industrial effluents.

Differentiated AssessmentThe culminating task for gas laws is an excellent opportunity for differentiated assessment.

Students must meet the following criteria, for one of the gas laws:• an authentic use of the gas law• a real world application of the law through a

demonstration

Within these criteria, students can submit their culminating task in any form, as long as it is pre-approved by the teacher:• A class demonstration (spatial and musical)• A skit [performed live or taped] (body-kinesthetic)• A written report (linguistic)• A laboratory for other students to complete

(logical/mathematical)

ELL Learners and Students with IEPsAccommodations for ELL learners:• ELL students can be paired with student with strong

language skills to assist them with labs and activities.• Graphical organizers can help ELL students to visualize

the relationship between the different gas laws.• Visual demonstrations such as the Cartesian diver and

Madgeburg spheres should be used as much as possible so ELL students can grasp the concepts visually.

Students with IEPs:• Adhere to the recommendations made on each student’s

IEP.• Handouts can be given for notes.• If students require a quiet area to focus during loud

laboratories or activities, a resource room can be used with alternative opportunities for assessment.

Resources/ReferencesReger, D.L., Goode, S.R., Mercer, E.E. (1993) Chemistry: Principles & Practice. USA: Saunders College Publishing. Note: The above textbook was used for the background information summary of gas laws.

A wealth of information and resources on gas laws:http://www.nclark.net/GasLaws

Gas Laws Alka Seltzer Lab:http://www.nclark.net/Alka_Seltzer_and_the_Ideal_Gas_Law.doc

Decompression Sickness:http://en.wikipedia.org/wiki/Decompression_sickness

Health Canada Air Quality Index:http://www.ec.gc.ca/cas-aqhi/default.asp?lang=en&n=065be995-1

The Cartesian Diverhttp://www.youtube.com/watch?v=sNOXFiJ4IDU

Air Pressure Demo with Potty Plungershttp://www.youtube.com/watch?v=fguD27Q2ijg

Gizmo – Boyle’s Law and Charles’ Lawhttp://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=422

SCUBA Science Activityhttp://www.pbs.org/wnet/secrets/previous_seasons/lessons/lp_andreadoria.html