Carbon monoxide

Carbon monoxide kills around 50 people in the UK each year, with a further 150 requiring hospital treatment. These are preventable deaths. Educating students about the causes of CO poisoning and how to prevent it may help to reduce the number of incidents. The way the poison works is an interesting piece of blood biochemistry which is accessible to most students and adds an extra dimension to the activity.

There are two available worksheets:

• ’Carbon monoxide – the silent killer’ is a comprehension exercise. This exercise requires learners to have prior knowledge of balancing equations and reversible reactions.
• ’Carbon monoxide alert’ is a research exercise that requires students to make a poster or leaflet to present the information they have found.

Carbon monoxide – the silent killer

Formation

When hydrocarbon fuels are burnt in a good supply of air, carbon dioxide and water are formed:

Equation 1 CH₄ + O₂ → CO₂ + H₂O (unbalanced equation)

However, when the air supply is restricted for some reason and there is not enough oxygen for this reaction to take place, carbon monoxide forms instead of carbon dioxide. As the name suggests, carbon monoxide has one oxygen atom for every carbon atom.

Equation 2 CH₄ + O₂ → CO + H₂O (unbalanced equation)

Carbon monoxide is colourless, odourless and tasteless. It gives no indication of its presence, which makes it very dangerous. It is formed in car engines and can also be produced by faulty gas cookers, water heaters and fires, or when a hydrocarbon-burning appliance is used in a room with inadequate ventilation. It is present in cigarette smoke.

Equation 3 Hb(aq) + 4O₂(g) ⇌ Hb(O₂)₄(aq)

The forward reaction takes place in the lungs, the back reaction at cells around the body that are low in oxygen. Carbon monoxide is toxic because it interferes with this process. It is better at binding to haemoglobin than oxygen is – about 200 times better. This means that, even if there is far more oxygen present than carbon monoxide, the carbon monoxide is more likely to bind to the haemoglobin. This forms a compound called carboxyhaemoglobin:

Equation 4 Hb(aq) + CO(g) → HbCO(aq)

Since the carbon monoxide sticks so tightly, there are fewer places for oxygen to bind to haemoglobin when carbon monoxide is present. But carbon monoxide does something else as well. It changes the other three oxygen binding sites on the haemoglobin molecule so that they can pick up oxygen but do not release it when they get to the body cells. This means that the haemoglobin stops acting as the body’s oxygen delivery system.

Questions

1. Why is carbon monoxide so difficult to detect?
2. Copy and balance Equations 1 and 2.
3. How do these equations show that carbon monoxide is formed when not enough oxygen is present?
4. Where in your house might carbon monoxide be formed?
5. Why would breathing carbon monoxide make you feel fatigued (tired)?
6. Look at Equation 3. What does the ⇌ symbol mean?
7. In Equation 4, the symbol → is used instead of ⇌ to show that a different type of reaction is taking place. Explain the difference between the two types of reaction and why this difference makes carbon monoxide toxic.
8. Carbon monoxide is toxic in a second way. Explain how in your own words.
9. Cigarette smoke is harmful to unborn babies in many ways. Explain why the carbon monoxide in cigarette smoke is worse for the baby than for its mother

Answers

1. CO is colourless, odourless and tasteless. We cannot sense it without special equipment.

• 2CH₄ + 2 O₂→ CO₂ + 2 H₂ O 
• CH₄ + 1 ¹/₂ O₂→ CO + 2 H₂ O or
• 2 CH₄ + 3 O₂→ 2 CO + 4 H₂ O

3. The first equation uses two molecules of oxygen for every molecule of methane. The second uses only 1.5 molecules of oxygen for every molecule of methane. When less oxygen is present, carbon monoxide is formed.
4. Answers could include any gas appliances (eg gas cooker, fire, central heating boiler), wood- or paraffin-burning appliances and cigarette smoke. Electric appliances do not produce CO.
5. Carbon monoxide prevents oxygen getting to the cells in your body. This makes you feel tired.
6. ⇌ means the reaction is reversible.
7. The reaction represented by Equation 4 is not reversible. This means that once carbon monoxide has bonded to your haemoglobin it does not come off easily and so prevents the haemoglobin from carrying oxygen as it should. If there is no CO present, haemoglobin binds oxygen and releases it again easily when it reaches cells that need it.
8. Carbon monoxide changes the haemoglobin and prevents it from giving up any of the oxygen it is carrying to the body cells. In other worlds, it stops the reaction represented by Equation 3 from being reversible.
9. Unborn babies’ haemoglobin binds even more carbon monoxide than the mother’s. This means that the baby suffers more at low CO levels than its mother. (It can also alleviate the mother’s symptoms as the baby removes CO from her blood.)

Carbon monoxide alert

You are going to do some research to find out about this deadly gas and then present your findings as a warning poster or leaflet.

• What you need to find out
• Where and when carbon monoxide is formed
• What effects this poisonous gas can have – what symptoms people should look out for
• Who is most at risk
• How carbon monoxide poisoning can be cured
• How carbon monoxide poisoning can be prevented.

Your leaflet or poster

Your leaflet or poster should be aimed at one of these groups of people:

• Students
• The elderly
• Families with young children.