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SCH3U, Strand f: Gases and atmospheric chemistry

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Gas laws

SCH3U, Strand f:Gases and atmospheric chemistry

By Farah Farah and Vanessa Poehlmann

- Investigation done in pairs

- Inflate a balloon and tie off the end tightly.
- Use a string to measure it’s 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?

- 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!

- Gases assume the volume and shape of their containers.

Pressure =

Force

- 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.

Area

Units of Pressure

1 pascal (Pa) = 1 N/m2

1 atm = 760 mmHg = 760 torr

1 atm = 101,325 Pa

- 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.

- 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

- 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.

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

Boyle’s Law

Charles’ Law

Gay-Lussac’s Law

The Combined Gas Law

The Ideal Gas 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 temperature

Constant amount of gas

Describes the temperature-volume relationship.

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

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

Constant pressure

Constant amount of gas

Describes 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 volume

Constant amount of gas

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

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

Describes 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.

P-pressure

V-volume

n-number of moles of gas

R-ideal gas constant (universal gas constant) 0.0821 atm.L /mol.K

or 62.396 torr .L/mol.K

T-temperature

- Lesson 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

- Lesson 1: Physical Characteristics of Gases & Pressure
- F2.1, F2.2
- Lesson 2: Kinetic Molecular Theory & S.T.P
- F3.2, F3.3
- Lesson 3: Boyle’s Law & Charles’ Law
- F2.3, F3.5
- Lesson 4: Gay- Lussac’s Law & Combined Gas Law
- F2.3, F3.5
- Lesson 5: Ideal Gas Law
- F2.1, F3.5

NOTE: The Gas Laws are a very large topic and encompass almost all of

Strand F in SCH3U

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 CO2produced.

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 is included in handout.

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.htmlTo 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.

When 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.

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.

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

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!

- Inquiry 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.

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.

- 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.

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

- 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.

- The 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)

- Accommodations 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.

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 Diver

http://www.youtube.com/watch?v=sNOXFiJ4IDU

Air Pressure Demo with Potty Plungers

http://www.youtube.com/watch?v=fguD27Q2ijg

Gizmo – Boyle’s Law and Charles’ Law

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

SCUBA Science Acivity

http://www.pbs.org/wnet/secrets/previous_seasons/lessons/lp_andreadoria.html