5. Bell jar: use a high bell jar to prevent the flame from damaging either the delicate membrane of the Oxygen or the Humidity sensor.

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1 Chemistry Sensors: Loggers: Temperature, Oxygen, Humidity Any EASYSENSE Logging time: EasyLog Teacher s notes 01 Burning a fossil fuel Read In this investigation a Humidity, Oxygen and Temperature sensor are used to measure the changes in the air in a bell jar as a candle burns. The amount of air in a bell jar is quite small so students must be prepared to work quickly. To introduce the work, student s ideas about fuels and their understanding of the term should be identified. A list of examples they can give of fuels can be created, you may wish to introduce some unfamiliar fuels e.g. recycled chip fat for cars, animal dung, etc. The difference between fossil fuels and renewable fuels may need to be defined. All fuels that burn in air are rich in carbon, ask pupils to suggest what might be formed when they burn. Using BioFuels does not reduce carbon emissions; it simply holds the increase in check. Demonstrate that carbon dioxide (and water) is produced when methane (a fossil fuel but also one that can be made from breakdown of organic materials) burns. If possible, extend the demonstration by using other fuels containing hydrogen and carbon, e.g. wax, ethanol, wood. Discuss with pupils whether it is likely that carbon dioxide and water could be turned back into fuel (introducing photosynthesis and the carbon cycle). The word equation that describes the burning of a fossil fuel is Apparatus 1. An EASYSENSE logger. Fuel + oxygen produces carbon dioxide + water + energy. 2. A Smart Q Oxygen sensor set to the 0 25% O 2 in air range. 3. A Smart Q Humidity sensor. 4. A Smart Q Temperature sensor. 5. Bell jar: use a high bell jar to prevent the flame from damaging either the delicate membrane of the Oxygen or the Humidity sensor. 6. Blu-Tack or plasticine to make an air tight seal. 7. Tea light / small candle. 8. Petroleum jelly e.g. Vaseline, to seal the base of the bell jar. 9. A sheet of smooth plastic or glass for standing the bell jar on. Set up of the software and logger The experiment uses the EasyLog function to provide a simple click and record session. T01-1(V2)

2 Notes Naked flame. The sensors must be kept away from the candle flame. Light the candle, and place the bell jar gently over it, make sure the flame is to one side of the sensors to prevent direct heat from reaching the sensor. The bell jar will need a small amount of sticky gel such as Vaseline or petroleum jelly to make a good airtight seal between it and the sheeting. The sheeting used to form the base should be heat proof (e.g. glass), it is not likely that the tea light will produce enough heat for the short time it is alight in the experiment to cause damage, but the tea light may be burning for some time during the pre experiment explanations. A large plastic equipment box can be used, but care needs to be taken that the plastic is not burnt or heated to its melt point. A tripod and gauze (sheet of metal) over the candle is required to act as a heat shield. The heat shield may have an affect after the flame extinguished by providing a heat source to drive convection within the container. If a large box is used it will be easier to include a light and carbon dioxide sensor to enable a full range of readings. Heat shield over candle Sensors positioned at a distance from the burning candle Alternative apparatus using a large box Results and analysis Identify and use Add Text to label the set of data that matches each sensor and to mark the point at which the candle flame went out. Indicate how temperature and humidity increase. T01-2(V2)

3 This data came from a spirit burner in a large (80 litre) container. Care needs to be taken when scaling up to this size experiment that the box can be absolutely airtight. In trialling this experiment there was many false starts created by leaky boxes. It is surprising how even small gap can produce unusual results! An example of data collected with a small leak in the container, in this case about the size of a drawing pin head! Data from a candle burning inside a large box (80 litres). No internal ventilation, heat shield above the flame to protect the box lid. The increase in CO 2, humidity and temperature can be seen. Temperature increase has been reduced due to heat uptake by the heat shield. O 2 shows a corresponding decrease. T01-3(V2)

4 The same data with Autoscale applied. The point of flame extinction is quite visible; all measured variables show a sudden change in slope. Data from a candle in burning in large box (80 litres) with internal ventilation provided by a computer case fan. Note there is no real difference from the passive data shown above. Tends to suggest that flame extinction is not due to a build up of CO 2 in the bottom of the box the ventilation will give an even mix of air in the box. Also note that extinction of the flame occurred at the same (or very similar) levels of CO 2 and O 2. Perhaps the stated idea that the flame goes out due to all oxygen being burnt up is not the whole truth? T01-4(V2)

5 When the data was being collected, observation showed, 1. There comes a point when the flame shape and size changes, this appears to be at about 17% oxygen. One assumes at this point that something is limiting the flame. 2. At some point you suddenly get a condensation on the walls of the container, suggesting the dew point has been reached. Additional notes. 1. The Oxygen sensor really measures oxygen percentage from its partial pressure. As water vapour is produced the partial pressure of oxygen will change and give an apparent decrease in oxygen levels even though the number of oxygen particles has remained constant. As humidity falls you will therefore see a rise in oxygen as the partial pressure changes. You can see this effect in the ventilated data. 2. The Carbon dioxide sensor measures the ppm of carbon dioxide, i.e. the number of particles irrespective of the pressure it is a direct measure of quantity of carbon dioxide. 3. Carbon dioxide has a cooling effect on the flame. The flame temperature is important to drive the reaction that converts the solid wax into volatile combustible fuels. If the flame temperature is reduced the supply of fuel is reduced. Perhaps this explains the flame out? 4. The yellow in the candle flame indicates incomplete combustion and production of carbon particulates which are ionised and glow yellow. Incomplete combustion produces carbon monoxide; this also has a significant cooling effect on a flame. 5. Consider the bell jar over water experiment. Chemistry informs us that all gasses occupy the same volume at the same pressure with the same number of particles. Combustion is a well balanced reaction, the O 2 in is equalled by the CO 2 out. The volume in the bell jar should not change, but it does, why? It is not from reduction of oxygen, the decrease in oxygen is compensated for by the production carbon dioxide. Data collected with an oxygen generator in the box. Note how much CO 2 was produced and how the flame still extinguished at the 17% O 2 level. Seems to suggest that lack of oxygen is the critical factor but not the total absence of O 2. T01-5(V2)

6 More to do 1. If you have access to a Light sensor it may be worth while showing how the flame responds to drop in oxygen levels in the bell jar. You may need to provide some form of shading to restrict light from sources other than the flame reaching the sensor. 2. Use a large pipette to remove some of the air to shake with limewater and show that carbon dioxide has been formed. 3. What happens if different fuels are used? 4. Try using a CO 2 scrubber to reduce the CO 2 in the bell jar, does combustion last any longer? Do oxygen levels decrease further? 5. Use hydrogen peroxide and Manganese oxide (v) to produce replacement oxygen. Which appears to me more important the level of CO 2 or the level of O 2 in maintaining the flame? T01-6(V2)