TALC 2010

CHEMISTRY 30S – MODULE 2

GASES AND THE ATMOSPHERE

LESSON 1 Atmosphere

OUTCOMES: The student will be able to:

·  Identify the abundances of the naturally occurring gases in the atmosphere and examine how these abundances have changed over geologic time.

·  Research Canadian and global initiatives to improve air quality.

Hot air balloons, SCUBA diving, and baking all have one thing in common – they involve gases…. oh and so does breathing, and driving a car. Gases are the state of matter that is most affected by changes in surrounding temperature and pressures. We will learn about how gases behave under changing conditions and in some cases how that affects our lives.

Present Composition of the Atmosphere

Earth’s air is composed of two types of gases: permanent and variable. The gases are called permanent because their amounts have not significantly changed in recent history. The permanent gases in the atmosphere by percentage are:

·  Nitrogen 78.1%

·  Oxygen 20.9%

These two gases comprise 99% of the Earth’s lower atmosphere.

Some of the other permanent gases are:

·  Argon 0.9%

·  Neon 0.002%

·  Helium 0.0005%

·  Krypton 0.0001%

·  Hydrogen 0.00005%

As their name suggests, variable gases

·  Water vapor 0 to 4%

·  Carbon Dioxide 0.035%

·  Methane 0.0002%

·  Ozone 0.000004%

The Origins of Earth’s Atmosphere
Scientists believe that before life began on the earth, the composition of the atmosphere was dramatically different than it is today. Billions of years ago the atmosphere consisted mainly of helium, hydrogen, ammonia and methane. It is believed that little free oxygen existed.
Assuming that volcanic eruptions have the same composition as today, it is believed that volcanoes released gases into the developing atmosphere. This volcanic outgassing released nitrogen, carbon dioxide and water vapour. Volcanic eruptions contain about 85% water vapour. This water vapour accumulated in the atmosphere and eventually returned to the Earth in the form of rain. The rain collected and created lakes, rivers and oceans. Nitrogen gas is not very chemically reactive so it continued to accumulate in the atmosphere.
Scientists believe that ultraviolet radiation from the sun penetrated the relatively dense atmosphere and sparked chemical reactions that eventually led to life on Earth. Origin-of-life models have generally proposed that about 1 billion years after the first primitive organisms emerged, blue-green algae appeared on the Earth. These algae converted the existing carbon dioxide and water to free oxygen gas and glucose through the process of photosynthesis.
Another important source of oxygen was the photodecomposition of water vapour by ultraviolet light according to the equation below:
2 H2O(g) à 2 H2(g) + O2(g)
As the amount of free oxygen increased, an ozone layer began to form filtering out ultraviolet radiation and allowing for the development of more complex species.
In previous science courses you may have learned about the nitrogen cycle. This cycle maintains the present amount of nitrogen in the atmosphere. Nitrogen is removed from the atmosphere by nitrogen-fixing bacteria and lightning. The nitrogen is returned to the atmosphere through the decomposition of biological matter.
The Variable Gases
Water on the earth is present as solid, liquid and gas. Water plays several roles in the atmosphere. It distributes heat, provides fresh water for plant and animal life as well as functioning as a greenhouse gas. Greenhouse gases trap heat energy within the atmosphere. Water vapour is present in largest amounts, about 4%, between the tropics and is in its lowest amounts, as low as 0%, in deserts and at the poles.
In previous science courses you likely studied the carbon cycle and learned that carbon dioxide is removed from the atmosphere by photosynthesis and returned to the atmosphere by respiration, decay of biological material, volcanic activity and burning fossil fuels. Some scientists believe that carbon dioxide is a greenhouse gas and increased amount of atmospheric CO2 is responsible for global warming. This is a topic of intense debate among scientists, environmentalists and politicians. Between 1840 and the year 2000, the average amount of atmospheric carbon dioxide steadily increased by 25%. The increase in atmospheric CO2 is believed to be largely due to the increased burning of fossil fuels and deforestation.
Methane is also considered to be a greenhouse gas. Its levels have increased dramatically over the last 200 years due to the increased amounts of rice paddies, grazing animals and landfills.

Lesson Summary

In this lesson we have learned

·  99% of the Earth’s atmosphere is nitrogen and oxygen.

·  Gases present in variable amounts are water vapour, carbon doxide, methane and ozone.

·  The Earth’s early atmosphere was composed of largely helium, hydrogen, ammonia and methane. Volcanoes and the advent of life changed the composition of the atmosphere to where it is today.

LESSON 2 Gases & Pressure

If you listen to the weather reports, you will hear units such as kilopascals and millibars. In this lesson you will learn how these units were developed and what they actually mean.

OUTCOMES:

When you have completed this lesson, you will be able to:

·  Describe the historical development of the measurement of pressure.

·  Describe the various units used to measure pressure.

Defining Gas Pressure
Recall from Module 1 that according to the kinetic molecular theory, gas particles are constantly moving. As they move, they collide with the sides of their container. The force per unit area due to these collisions is called gas pressure.
History of Measuring Pressure
Galileo Galilei (1564-1642) developed the suction pump. He used air to draw underground water up a column, similar to how a syringe draws water. He was perplexed as to why there was a limit to what height the water could be raised. That limit was 32 feet or about 11 metres.
In 1643 Evangelista Torricelli (1608-1647) developed the first barometer. He carried on Galileo’s work by determining the limit to the height with which Galileo’s pump could draw water was due to atmospheric pressure. He inverted a closed-end tube filled with mercury into a pan of mercury at sea level.

The height of the column of mercury in the tube (in mmHg) is equal to the atmospheric pressure acting on the mercury in the pan. He determined that the height of mercury supported by atmospheric pressure at sea level is 760 mm or 76 cm. He did the same experiment with water first, but found that the glass tube he had to use was too long and fragile.

Between 1643 and 1645 Otto von Guericke (1602-1686) made a pump that could create a vacuum so strong that a team of sixteen horses could not pull two metal hemispheres apart. Otto von Guericke reasoned that the hemispheres were held together by the mechanical force of the atmospheric pressure rather than the vacuum. The same experiment can be demonstrated by pushing two plungers together.

In 1648, Blaise Pascal (1623-1662) used Torricelli’s “barometer” and traveled up and down a mountain in southern France. He discovered that the pressure of the atmosphere increased as he moved down the mountain. Sometime later the SI unit of pressure, the ‘Pascal’, was named after him. He is also well known for his work in mathematics and inventing the first calculator.

In 1661 Christiaan Huygens (1625-1695) developed the manometer to study the elastic forces in gases. He also developed some of the first practical vacuum pumps. Huygens, however, is better known for his work in mathematics and astronomy.

In 1801, a couple years before publishing his atomic theory, John Dalton (1766-1844) stated that in a mixture of gases the total pressure is equal to the sum of the pressure of each gas, if it were in a container alone. The pressure exerted by each gas is called its partial pressure. This is known as Dalton’s Law of Partial Pressures. Another way of stating this relationship is the total pressure of gases in a container is the sum of the pressures of each gas.

In 1808 Joseph Louis Gay-Lussac (1778-1850) observed the law of combining volumes. He noticed that, for example, two volumes of hydrogen combined with one volume of oxygen to form two volumes of water. He is also well known for his passion for hot air ballooning.

Amadeo Avogardo (1776-1856), after studying the work of Gay-Lussac and others, published what is known as Avogadro’s Hypothesis in 1811. His hypothesis stated that a sample of any gas at the same temperature and pressure will contain the same number of particles. His hypothesis also suggests that the more gas particles, the greater the pressure. Unfortunately, because Avogadro did not do his own experiments, his hypothesis was ignored for about 50 years.

UNITS OF PRESSURE

The atmosphere is a common unit for expressing pressure. Two atmospheres of pressure is equal to twice the pressure of the atmosphere at sea level.

1 atm = 760 mmHg

1 atm = 101.3 kPa

The SI unit for pressure is the Pascal (Pa), named after Blaise Pascal. The Pascal is defined as 1 Newton of force per square metre. This is rather small unit, so when measuring gas pressure we use kiloPascals, which is 1000 Pascals. You can watch an animation in the first module on WebCT to see how the Pascal and the Newton are related.

Millimetres of mercury (mmHg)is not a common unit used outside the laboratory today, however, many barometers found in the home use both mm of mercury as well as another unit like kiloPascals. In the United States, air pressure is often reported in terms of inches of mercury.

The millibar is a meteorological unit of atmospheric pressure. One bar is equal to standard atmospheric pressure or 1 atmosphere.

One other common pressure unit is pounds per square inch (psi). This is an Imperial unit of pressure. This unit is often encountered when filling car and bicycle tires. One kiloPascal is equal to 0.145 psi and one atmosphere is equal to 14.7 psi.

Measuring Pressure
Pressure can be measured using several devices or instruments. Air pressure or atmospheric pressure is measured with a barometer, whereas gas pressure is measured with a monometer. A manometer usually has a bulb or glass container on one end and can be open or closed on the other. A liquid, often mercury, is placed in a U-shaped tube. The pressure is measured by finding the difference in height on both sides of the tube (see figure below).

In the open-ended manometer, the pressure of the gas is related to the height difference, h, of the mercury (or other liquid) in both sides of the U-tube. It is called open-ended because one end is exposed to the gas pressure and the other end is open to the atmosphere. When the pressure of the trapped gas (Pgas) is equal to the atmospheric pressure (Patm) the height on both sides of the U-tube are equal (a in the figure below). When the pressure of the trapped gas is greater than the atmospheric pressure, the height of the liquid in the open side of the U-tube will be greater than the closed side (b in the figure below). The opposite is true if the atmospheric pressure is greater than the gas pressure (c in the figure below).

To calculate the pressure of the trapped gas in (b), when the gas pressure is greater than the atmospheric pressure
Pgas = Patm + h
In (c) above, when atmospheric pressure is greater than gas pressure
Pgas = Patm – h
Example 1. Calculate the gas pressure in kilopascals (kPa) in the manometer shown below.

Solution.
The gas pressure is greater than the atmospheric pressure so we use the equation
Pgas = Patm + h
Pgas = 762 mm + 15 mm = 777 mm Hg
We must convert mmHg to the SI unit kiloPascals.
If 760 mmHg = 101.3 kPa, then
Lesson Summary
In this lesson you have learned,
·  Gas pressure is due to the force of gaseous particles colliding with their container.
·  Evangelista Torricelli invented the barometer to measure atmospheric pressure.
·  Among the units used to describe gas pressure are: millimetres of mercury (mmHg), atmospheres (atm), kilopascals (kPa), millibars and pounds per square inch (psi).
·  Christiaan Huygens developed the manometer to measure the pressure of a sample of a gas.
·  When using a manometer, when gas pressure exceeds air pressure Pgas = Patm + h or when airpressure exceeds the gas pressure Pgas = Patm – h.

CHEM 30S 7 M2 L1