Combustion Reactions
Combustion Reactions
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Type of Reaction: Combustion
Type of Reaction: Combustion
Combustion is the scientific word for burning. It requires a fuel (mostly carbon compounds), oxygen (usually from the air) and a source of heat (spark or flame) to ignite the fuel.
It is the amount of oxygen available that determines which type of combustion occurs.
Complete combustion occurs when there is plenty of oxygen available.
Incomplete combustion occurs when there is limited oxygen available.
A note about balanced equations for combination reactions: You should be able to balance equations for complete combustion reaction equations, but NOT incomplete combustion reaction equations (unless they're simple and only have one carbon-containing product).
Combustion of Candle Wax
Combustion of Candle Wax
Most candles are made of paraffin wax. This is a mixture of solid hydrocarbons left over after petroleum oil has been separated into its components. The candle wick (braided cotton fibres) acts like a miniature fuel pump when the candle is burning, drawing melted wax (the fuel) up to the flame for combustion.
When a candle is first lit, heat from the flame melts the wax at the top of the candle. This liquid wax is drawn up through the wick to the tip inside the flame. The heat of the flame turns the liquid wax into a gas or vapour that combines with oxygen (combustion) into the air to produce light and heat.
Pentacosane C₂₅H₅₂ is a key component in candle paraffin wax. I couldn't embed the 3D model here, so to view the structure in 3D visit this link - https://molview.org/?cid=12406
VIDEO QUESTIONS
VIDEO QUESTIONS
What are your observations from the experiment?
... of the candles?
... in the funnel?
... in the test tube?
... in the round bottom flask?
Why are we seeing these observations?
Combustion of LPG Fuels
Combustion of LPG Fuels
Barbecues use cylinders of Liquefied Petropleum Gas (LPG) as a fuel source. LPG contains the gases propane (C₃H₈) and butane (C₄H₁₀) under pressure. When these gases burn, they react with oxygen in the air to produce the heat needed to cook food. Carbon dioxide and water vapour are also produced (complete combustion). If there is not a good air supply around the gas jets, some carbon monoxide may also be produced (incomplete combustion).
Propane molecule
Butane molecule
CHECKPOINT ACTIVITY:
CHECKPOINT ACTIVITY:
Write the word equation and balanced symbol equation for the combustion of propane and ethene (NOT an LPG fuel, but much easier to balance than butane). Note: You don't have to use all of the words / numbers provided.
Testing for Carbon Dioxide and Water
Testing for Carbon Dioxide and Water
Carbon dioxide
Carbon dioxide
Collect the gas products of combustion using a delivery tube and bubble this gas through limewater solution. If carbon dioxide is present, the colourless limewater will turn cloudy or milky white.
Water
Water
Place a piece of blue anhydrous cobalt chloride paper where the combustion products are being collected. Look for any colour change on the paper.
If water is present, the blue anhydrous cobalt chloride paper will turn pink. Anhydrous means "without water", so in the presence of water, the colour will change dramatically.
Complete Combustion
Complete Combustion
When there is a plentiful supply of oxygen, complete combustion occurs. The products of this reaction are carbon dioxide and water (vapour).
Incomplete Combustion
Incomplete Combustion
When there is a limited amount of oxygen present, incomplete combustion occurs. The products of this reaction include carbon monoxide or carbon (soot) and water. This explains why beakers get black bottoms when you use them with the yellow flame.
Incomplete combustion occurs in car engines. If. you look closely at the exhaust pipe of a ar that has the engine running, you will see black soot around on the inside of the pipe and you may see water dripping from the pipe.
Note that there is no laboratory test for carbon monoxide. It is an odourless and colourless gas. Its presence must be inferred using the type of combustion (oxygen available) and the generic equation.
CHECKPOINT ACTIVITY:
CHECKPOINT ACTIVITY:
Are the statements about complete combustion or incomplete combustion? Win by slicing the correct fruit!
Click on the button below to play the other games about Complete vs Incomplete Combustion.
Dangers of Incomplete Combustion
Dangers of Incomplete Combustion
Ideally, when we burn fuels (like natural gas, propane, wood, or kerosene) for heating or cooking, we want them to undergo complete combustion. This occurs when there is an ample and continuous supply of oxygen. In this scenario, the fuel reacts fully with oxygen to produce maximum energy, carbon dioxide and water.
The problem arises when the oxygen supply is limited or insufficient. This leads to incomplete combustion. In this situation, the fuel cannot fully oxidize, and instead of just carbon dioxide and water, it produces a deadly byproduct: carbon monoxide as well as soot, water and less energy.
Adequate ventilation can prevent carbon monoxide poisoning by:
Ensuring enough oxygen supply. Good ventilation ensures a constant supply of fresh air, which provides the enough oxygen for complete combustion. In a well-ventilated space, even if an appliance is running, there's enough oxygen for the fuel to burn cleanly, producing mostly CO₂ and H₂O.
Removing the harmful products of combustion. Ventilation systems (like flues, chimneys, and exhaust pipes) are designed to safely carry away all the combustion products, including CO₂ and any trace amounts of CO that might inevitably form, to the outside environment. This prevents their buildup in enclosed spaces.
Preventing stagnant air. In a poorly ventilated space, the oxygen in the air gets quickly consumed by the burning appliance. As oxygen levels drop, the combustion automatically shifts from complete to incomplete, leading to a rapid increase in CO production. Without adequate ventilation, this CO then accumulates in the room, reaching dangerous concentrations..
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