What is the colour of the flame when alkenes are burnt?
Yellow
Understanding Alkene Combustion Flame Colour
When hydrocarbons burn, the colour of the flame depends largely on how completely the combustion reaction occurs. Combustion can be complete or incomplete, depending on the availability of oxygen.
Complete vs. Incomplete Combustion
- Complete Combustion: Occurs when there is sufficient oxygen. Hydrocarbons react completely to form carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)). A complete combustion flame is typically hot and appears blue or non-luminous because there are very few unburnt carbon particles.
- Incomplete Combustion: Occurs when there is insufficient oxygen. Hydrocarbons do not react completely, leading to the formation of carbon monoxide (\(\text{CO}\)), water (\(\text{H}_2\text{O}\)), and soot (unburnt carbon particles, C). An incomplete combustion flame contains hot, glowing soot particles, which emit light and make the flame luminous, often appearing yellow or orange.
Combustion of Alkenes
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (\(\text{C}=\text{C}\)). They have a general formula \(C_nH_{2n}\). Compared to saturated hydrocarbons (alkanes) with the same number of carbon atoms, alkenes have a higher carbon-to-hydrogen ratio.
For example, consider Ethene (an alkene, \(C_2H_4\)) and Ethane (an alkane, \(C_2H_6\)). Ethene has a higher percentage of carbon by mass compared to Ethane.
Due to this higher carbon content, alkenes tend to undergo incomplete combustion more readily than alkanes, especially when burnt in air (which has a limited oxygen supply). The incomplete combustion produces a significant amount of soot (unburnt carbon particles).
Why Alkenes Burn with a Yellow Flame
The hot soot particles produced during the incomplete combustion of alkenes are heated to incandescence by the heat of the reaction. These glowing carbon particles emit light, which makes the flame luminous and gives it a characteristic yellow or orange colour. This type of flame is often described as 'sooty' or 'luminous'.
While small alkenes might burn with a relatively cleaner flame under ideal conditions, typically, when alkenes are burnt in air, the combustion is incomplete, resulting in a sooty, yellow flame. This is a distinguishing characteristic of the combustion of many unsaturated hydrocarbons compared to saturated ones.
Summary of Flame Colours
The colour of the flame during hydrocarbon combustion provides an indication of the completeness of the reaction:
| Flame Colour | Type of Combustion | Presence of Soot |
|---|---|---|
| Blue | Complete Combustion (sufficient oxygen) | Minimal/None |
| Yellow/Orange | Incomplete Combustion (insufficient oxygen) | Significant (glowing particles) |
Therefore, the combustion of alkenes typically results in a yellow flame due to incomplete combustion and the formation of hot soot particles.
Revision Table: Hydrocarbon Combustion
| Hydrocarbon Type | Carbon Content | Tendency for Incomplete Combustion (in air) | Typical Flame Colour (in air) |
|---|---|---|---|
| Alkanes (Saturated) | Lower % C | Less | Often blue (or less sooty) |
| Alkenes (Unsaturated) | Higher % C | More | Often yellow/sooty |
| Alkynes (More Unsaturated) | Even Higher % C | Even More | Very sooty, bright yellow/orange |
Additional Information: Factors Affecting Flame Colour
- Oxygen Supply: More oxygen leads to more complete combustion and a bluer flame. Limited oxygen leads to incomplete combustion and a yellower, sooty flame.
- Type of Hydrocarbon: Unsaturated hydrocarbons (alkenes, alkynes) and those with more carbon atoms per molecule are more likely to produce sooty, yellow flames due to their higher carbon content.
- Mixing of Fuel and Oxygen: Efficient mixing allows for better access to oxygen for all fuel molecules, promoting complete combustion.
Understanding the relationship between the structure of hydrocarbons and their combustion properties is key to predicting flame characteristics.
Important Questions from Hydrocarbons
The number of structural isomers of pentane is
C 4H 8belongs to the homologous series of
- Refining of petroleum is carried out using which one of the following techniques?
What is the number of mole(s) of H 2(g) required to saturate one mole benzene?
Which one of the following carbon compounds will not give a sooty flame?
