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Unit 9 - Every breath you take

Content

Scheme of work

The atmosphere

The atmosphere is defined as the layers of gas that exist all around the Earth. The atmosphere gradually gets thinner are you go higher up.

The atmosphere consists of mainly five gases, nitrogen, oxygen, argon, water vapour and carbon dioxide. There are also trace gases such as methane and ammonia.

Harmful gases that have been put into the atmosphere through human activity are known as pollutants.

You do not need to know the exact percentages, but you should know that nitrogen occupies most of the total (4/5) and oxygen (1/5) approximately the rest. Carbon dioxide is between 0.02 and 0.04% (why would it vary?).

The physical properties of gases

Like all matter, gases consist of particles. The particles have enough energy to break any forces of attraction between them.

This energy is in the form of motion - gas particles move very fast (about 1000m/s at room temperature on average). This means that gases will expand rapidly to fill all available volume.

The physical properties of gases are the result of the particles being spread out and moving around very fast.

Compressible

Gases can be squeezed into smaller volumes as they are mostly empty space. They are said to be "compressible"

Diffusion

Gases spread out in all directions, as their particles are moving very fast. If there were no air in the way, a gas released into an empty volume would spread out almost instantly. As it is, gases diffuse through air at a speed that depends on the size of the gas particles.

The gas laws

The gas laws are the result of observations made on gases. They tell us how gases behave in terms of their volume, pressure and temperature.

Volume

The volume of a gas is simply the size of its container. This is because gases spread out to fill the whole container.

A cubic box of side 10cm has a total volume of 10 x 10 x 10 = 1000 cm3

Pressure

The concept of pressure is a little more complex to describe. Gas particles collide with each other and with the walls of the container. Every single collision with the walls exerts a force on the wall. The definition of pressure is the total force divided by the area on which it acts.

Pressure = force/area

Determine the pressure when a force of 100N acts on a surface of area 2 m2.

Pressure = force/area

Pressure = 100N/2m2 = 50 Nm-2

The units of pressure are the Newton per metre squared, Nm-2, which is also called the Pascal, Pa.

1 Pa = 1 Nm-2

Investigating the pressure of your footprint

The hypothesis is that boys exert more pressure on the floor than girls because they are heavier on average.

Instructions

  • Trace the outline of your shoe on squared paper.
  • Count the squares. Each small square is 0.25 cm2.
  • Weigh yourself on the balance provided.
  • Divide your mass by the surface area of your shoe in m2 (10000 cm2 = 1 m2)
  • Share your data with four members of the same gender.
  • Compare your five results with the results of a group of the opposite gender.

Worksheet

Atmospheric pressure

The pressure of the atmosphere is caused by the large column of air above our heads being pulled downwards by gravity. There are more air particles near the surface and so the pressure is greater. The atmospheric pressure at sea-level is 100 kPa (1 x 105 Pa).

As we go higher up from sea-level the pressure drops. In Madrid the air pressure is usually about 6% lower than at sea-level, the actual value also depends on the weather conditions.

Mountaineers often need to carry oxygen to breathe, as the air becomes too thin to support normal respiration.

The PhyPhox application can be downloaded onto Android devices to give a measurement of the air pressure at any time.

Air pressure measured at the top of Beret, Pyrinees

In the image the data shows measurement in hPa (hectaPascals), where 10 hPa = 1 KPa. Hence, the air pressure was 76.8 kPa, or about 25% lower than atmospheric pressure.

The relationship between pressure and volume

When pressure is applied to a volume of gas the volume decreases. This kind of relationship is called inverse proportionality. We say that the volume is inversely proportional to the pressure. As the pressure increases the volume decreases, and vice versa.

We can show this using a sealed gas syringe with weights. As we apply weights to the syringe plunger, the force down increases. This causes the plunger to enter the syringe further, reducing the volume.

Mathematically, the relationship is:

Volume = 1/Pressure

Explanation

The reason is that the externally applied force pushes the particles inside the container closer together. This reduces the volume making the available surface that the particles can collide with smaller, so that there are more collisions per unit area.

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The relationship between pressure and temperature

If we increase the temperature of a fixed volume of gas, the pressure increases.

If we take a sealed container connected to a pressure sensor, the container has a fixed volume. As we increase the temperature the pressure inside the syringe increases. The pressure meter or sensor increases its reading.

Mathematically, the relationship is:

Pressure ∝ Temperature

Explanation

The reason is that the increase in temperature gives the particles inside the container more energy and makes their collisions with the walls of the container more energetic. Also, the particles are moving faster, so there are also more collisions with the walls of the container per unit time.

The relationship between volume and temperature

If we increase the temperature of a fixed volume of gas, the volume increases.

This can be shown using a gas syringe which is free to move to equalise the pressure inside with the atmospheric pressure. If the syring is heated the plunger moves further out, increasing the volume.

Mathematically, the relationship is:

Volume ∝ Temperature

Explanation

The reason is that increased energy as the temperature rises causes the particles to move faster on average. This increases the pressure inside the syringe, which forces the plunger to push out against atmospheric pressure until the pressure outside is equal to the pressure inside.

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Respiration in animals

Respiration is the process by which living organisms obtain energy for their life functions.

There are three fundamental methods that animals use to obtain oxygen for respiration.

Living creatures use oxygen to convert glucose or other food stuffs into energy for homeostasis, growth and locomotion. The by-products (waste) of respiration in animals are carbon dioxide and water.

The actual chemical processes are quite complex, but can be summarised as:

Respiration in animals

Glucose + oxygen → carbon dioxide + water + energy

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

How lungs work

The lungs are bags of air sacs connected to the chest cavity. When the muscles cause the chest cavity to expand the volume inside the lungs increases. We have already seen that when the volume of a gas increases its pressure decreases.

With reduced pressure inside the lungs, the air pressure outside forces air into the lungs, which can then absorb the oxygen into the bloodstream for use in respiration.

Modelling lungs in the laboratory

We can use a bell jar with a rubber surface to model the operation of lungs.

When the rubber sheet is pulled down the volume increases and the reduced pressure inside the bell jar causes the air already in the balloons to expand.

This is not a perfect model, as we are reducing the air pressure in the wrong place, but the effect is similar.

In the body system, the lungs themselves are attached to the walls of the chest and forced to increase in size by the chest muscles and diaphragm (below).

The mechanism of gas exchange

Animals breathe to obtain oxygen and to remove the waste products of respiration, carbon dioxide. Just how this happens is known as the mechanism of gas exchange.

The lungs

The lungs have been discussed in the previous section.

The alveoli

The alveoli (singular: alveolus) are tiny sacs at the ends of the lungs' branches and bronchioles surrounded by blood vessels. They have very large surface area to facilitate the gas exchange.

Oxygen gas leaves the air inside the alveoli and passes into the bloodstream via the blood vessels, while at the same time carbon dioxide passes from the blood stream to the air inside the alveoli.

The red blood cells

The oxygen is bonded to the red blood cells using a complex molecule called haemoglobin. The blood cells then carry oxygen in the blood stream to the cells, where it is used for respiration.

Metal and non-metal oxides

Oxygen, element number 8, is a very reactive substance that can form compounds with almost every element. These compounds are called oxides.

Most oxides are very much more stable than the elements that originally made them. Carbon dioxide, CO2, is the end product from animal respiration and also combustion of organic substances.

The equation of animal respiration

C6H12O6 + O2 → 6CO2 + 6H2O

Combustion of ethanol fuel in a spirit burner

C2H5OH + 3O2 → 2CO2 + 3H2O

The periodic table

The periodic table displays all of the elements in a logical order according to their electronic configuration. This gives an arrangement where the metals are on the left and the non-metals are on the right.

The properties of oxides of metals (metal oxides) are very different to the properties of non-metal oxides.

Metal oxides

Non-metal oxides

Apart from their obvious appearance, oxides of metals and non-metals also behave very differently when dissolved in water.

Non-metal oxides make acids when dissolved in water, whereas metal oxides form bases.

The reaction of sodium oxide with water

Na2O + H2O → 2NaOH

Sodium hydroxide, NaOH, is a very strong base.

The reaction of sulfur dioxide with water

SO2 + H2O → H2SO3

Sulfuric(IV) acid, H2SO3, is an acid.

Carbon dioxide

Carbon dioxide is a very important non-metallic oxide. It is involved in respiration in both animals and plants, and it is involved in the carbon cycle (see below). Carbon dioxide is a weakly acidic gas that dissolves in water forming a slightly acidic solution, called carbonic acid.

The oceans dissolve a large amount of the carbon dioxide in the air.

When gases dissolve in water, their solubility decreases as the temperature increases. Warming global ocean temperature releases more carbon dioxide into the air to the benefit of plants, but to the detriment of sea creatures, that need the carbon dioxide to construct their shells and bony growths.

Another major issue is that rising carbon dioxide levels increase the temperature of the earth via the greenhouse effect. This has many consequences.

The greenhouse effect

The Greenhouse effect was covered in MYP9 - this is just a summary.

Atmospheric pollution

Pollution refers to gases and microparticulates that have been produced in the air due to human activity and which are harmful to living creatures.

The main pollutants are:

This is not intended as a complete list, but it shows the most important pollutants.

PM2.5 microparticulates

These are tiny particles of carbon produced from incomplete combustion, that measure up to 2.5 micrometres (microns) = 2.5 x 10-6 m in diameter. These particles are so small that they spend a long time in the air and get ingested into the body through normal breathing. Once in the lungs they are a focus for infection and a possible source of cancers. Microparticles are responsible for more deaths per year than any other type of pollution.

May countries have cleaned up their air compared to 50 years ago, but the problem of microparticles still exists.

Sulfur dioxide

This is a toxic gas produce by both volcanic activity (natural) and burning fossil fuels (anthropogenic). It has the chemical formula SO2. Sulfur dioxide is one of the pollutant gases responsible for acid deposition (rain, snow, dust) that can destroy forests and acidify lakes making them uninhabitable.

Nitrogen oxides

These are actually a group of chemicals all of which contain nitrogen and oxygen. They are given a general formula NOx. They are acidic gases that cause lung disease and contribute to other environmental problems.

Ozone

Ozone, O3 is an essential gas, but only in the upper stratosphere, where it protects the Earth from harmful ultraviolet light from the sun. At ground level it is a poisonous gas responsible for lung problems and illness. It is created by electric discharges, such as occur in internal combustion engines in cars.

The carbon cycle

The carbon cycle explains how carbon moves around our ecosystem. It is used by animals in the form of carbohydrates for energy. It is used by plants in the form of carbon dioxide for energy and growth. It exists in the atmosphere and also dissolved in the oceans. It is tied up in rocks as inorganic carbonates, and as fossil fuels underground.

The whole ecosystem is balanced and has been for millions of years. It is only in the past couple of hundred years that human activity has been disturbing this balance, with devastating consequences for the ecosystem.

The rate of chemical reactions

Chemical reactions are processes that cause new substances to be formed. The rate of a chemical reaction is how fast this happens.

Some reactions are very fast (think explosions) and some reactions take place over millions of years.

If we wish to measure how fast a chemical reaction happens it needs to be neither too fast nor too slow.

We also have to decide what to measure. If we want to know how fast a car travels then we just need to measure how far it goes over a period of time.

To measure how far a chemical reaction has gone we need to have something visible that tells us how much of a reactant has been used up, or how much product has been formed.

Reactions that produce gases

Fortunately, there are some reactions that make this measurement easy because the reaction produced a gas. Gases are easy to collect and measure in gas syringes, or over water. We can simply measure the volume of gas produced per unit time.

Reactions that produce dense gases

Reactions that produce carbon dioxide, which is a relatively dense gas can be monitored by measuring the total mass of the reactions vessel. As the carbon dioxide leaves the mass goes down.

It is possible to do the reaction in a container, such as a conical flask, on the top-pan electronic balance and record the mass.

Problem!

There is a problem measuring the rate of a chemical reaction in that the rate actually changes as the reactants are used up. This is because there are increasingly fewer reactant particles to collide.

Chemical reactions are fastest (highest rate) at the beginning of a reaction and from there they get progressively slower.

Finding the rate from graphs of gas produced against time

The definition of rate is change of amount of reactant or product/time

This is given as the gradient (slope) of the tangent to any point on the curve of mass loss against time.

Tangent = a line that just touches a curve and is perpendicular to the radius of the curve at that point.

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