Understanding Unsaturated Hydrocarbon Flame Color

The question asks about the color of the flame produced when unsaturated hydrocarbons burn. Understanding the combustion of hydrocarbons is key here.

What are Unsaturated Hydrocarbons?

Unsaturated hydrocarbons are organic compounds that contain carbon-carbon double bonds (like alkenes) or triple bonds (like alkynes). Examples include ethene (\(\text{C}_2\text{H}_4\)) and ethyne (acetylene, \(\text{C}_2\text{H}_2\)). They have fewer hydrogen atoms per carbon atom compared to saturated hydrocarbons (alkanes), which only have single bonds.

Combustion of Hydrocarbons

Combustion is a rapid reaction between a substance and an oxidant, usually oxygen, to produce heat and light. When hydrocarbons burn in sufficient oxygen (complete combustion), they produce carbon dioxide and water:

\(\text{C}_x\text{H}_y + (x + \frac{y}{4})\text{O}_2 \rightarrow x\text{CO}_2 + \frac{y}{2}\text{H}_2\text{O}\)

However, when there is insufficient oxygen (incomplete combustion), carbon monoxide or even elemental carbon (soot) can be produced:

\(\text{C}_x\text{H}_y + (\text{less than } x + \frac{y}{4})\text{O}_2 \rightarrow \text{CO} + \text{H}_2\text{O} + \text{C} + \text{other products}\)

Flame Color and Soot Production

The color and luminosity (brightness) of a flame depend on what is burning and the amount of oxygen available. Soot particles, which are tiny solid particles of unburnt carbon, are formed during incomplete combustion. When these soot particles are heated to high temperatures in the flame, they glow, emitting light. This glowing is responsible for the bright, luminous yellow or orange color of the flame.

Unsaturated hydrocarbons, especially alkynes like ethyne, have a higher percentage of carbon by mass compared to alkanes. For example, ethyne (\(\text{C}_2\text{H}_2\)) has a carbon content of about 92.3%, while ethane (\(\text{C}_2\text{H}_6\)), a saturated hydrocarbon with the same number of carbon atoms, has about 80% carbon. This higher carbon content in unsaturated hydrocarbons means they require more oxygen for complete combustion. In typical burning conditions with limited oxygen supply from the air, unsaturated hydrocarbons are more likely to undergo incomplete combustion, producing significant amounts of soot.

Why Unsaturated Hydrocarbons Burn with a Yellow Flame

Due to their tendency to undergo incomplete combustion and produce soot when burning in air, unsaturated hydrocarbons typically burn with a luminous, yellow flame. This is in contrast to saturated hydrocarbons (like methane or ethane) which, when burned with sufficient oxygen (like in a Bunsen burner with the air hole open), undergo complete combustion and produce a non-luminous, blue flame because less soot is produced.

Therefore, the colour of the flame when unsaturated hydrocarbons burn, especially in air, is typically yellow.

Revision Table: Hydrocarbon Combustion and Flame

Additional Information on Flame Characteristics

• Complete Combustion: Occurs when there is ample oxygen. Products are typically \(\text{CO}_2\) and \(\text{H}_2\text{O}\). Flame is often blue and less luminous (hotter).
• Incomplete Combustion: Occurs when oxygen is limited. Products can include \(\text{CO}\), \(\text{H}_2\text{O}\), and \(\text{C}\) (soot). Flame is often yellow and luminous (cooler than the hottest part of a blue flame).
• Carbon-Hydrogen Ratio: A higher carbon-to-hydrogen ratio in a fuel generally leads to more incomplete combustion and thus more soot formation and a more luminous, yellow flame. Unsaturated hydrocarbons have a higher C:H ratio than saturated ones with the same number of carbon atoms.
• Bunsen Burner: A Bunsen burner allows control over the oxygen supply. With the air hole open, saturated hydrocarbons burn completely with a blue flame. With the air hole closed, incomplete combustion occurs, producing a yellow, sooty flame, even with saturated hydrocarbons.