Which one of the following carbon compounds will not give a sooty flame?
Hexane
Understanding Sooty vs. Non-Sooty Flames
When carbon compounds burn, they undergo combustion. The type of flame produced depends largely on the completeness of the combustion, which is influenced by the structure of the compound and the availability of oxygen. A flame is considered 'sooty' when it is yellow and smoky. This indicates incomplete combustion, where not all the carbon is converted to carbon dioxide. Instead, some carbon remains as fine particles (soot), which glow in the flame, making it luminous and sooty.
Conversely, a non-sooty or clean flame is typically blue and is a sign of complete combustion, where the carbon compound reacts fully with oxygen to produce carbon dioxide and water. Saturated hydrocarbons, especially those with a lower carbon-to-hydrogen ratio, tend to undergo complete combustion more easily and produce non-sooty flames. Aromatic hydrocarbons and unsaturated hydrocarbons generally have a higher carbon-to-hydrogen ratio and are more prone to incomplete combustion, resulting in sooty flames.
Analyzing the Given Carbon Compounds
Let's examine each of the given carbon compounds:
- Benzene: The chemical formula is $\text{C}_6\text{H}_6$. Benzene is an aromatic hydrocarbon. Aromatic compounds have a relatively high proportion of carbon atoms compared to hydrogen atoms. Due to the presence of double bonds within the ring structure and the high carbon-to-hydrogen ratio, benzene typically undergoes incomplete combustion in air, producing a sooty flame.
- Hexane: The chemical formula is $\text{C}_6\text{H}_{14}$. Hexane is a saturated aliphatic hydrocarbon (specifically, an alkane). Saturated hydrocarbons have only single bonds between carbon atoms and contain the maximum possible number of hydrogen atoms for the given number of carbon atoms. This gives them a lower carbon-to-hydrogen ratio compared to aromatic or unsaturated hydrocarbons with a similar number of carbon atoms. Alkanes like hexane tend to burn completely in the presence of sufficient oxygen, resulting in a clean, non-sooty flame.
- Naphthalene: The chemical formula is $\text{C}_{10}\text{H}_8$. Naphthalene is a polycyclic aromatic hydrocarbon (it consists of two fused benzene rings). Like benzene, it is an aromatic compound with a high carbon-to-hydrogen ratio. Naphthalene is known to burn with a very sooty flame due to incomplete combustion.
- Anthracene: The chemical formula is $\text{C}_{14}\text{H}_{10}$. Anthracene is also a polycyclic aromatic hydrocarbon (consisting of three fused benzene rings). It has an even higher carbon-to-hydrogen ratio than naphthalene or benzene. Anthracene burns with a very sooty flame, characteristic of the incomplete combustion of large aromatic molecules.
Comparing Combustion Properties
We can compare the carbon-to-hydrogen ratio (by number of atoms) for these compounds:
| Compound | Formula | C:H Ratio (atoms) | Type | Expected Flame |
|---|---|---|---|---|
| Benzene | $\text{C}_6\text{H}_6$ | 6:6 or 1:1 | Aromatic | Sooty |
| Hexane | $\text{C}_6\text{H}_{14}$ | 6:14 or 3:7 | Saturated Aliphatic | Non-sooty |
| Naphthalene | $\text{C}_{10}\text{H}_8$ | 10:8 or 5:4 | Aromatic | Sooty |
| Anthracene | $\text{C}_{14}\text{H}_{10}$ | 14:10 or 7:5 | Aromatic | Sooty |
As you can see from the table, Hexane has the lowest carbon-to-hydrogen ratio among the given options (3:7 or approximately 0.43:1), while the aromatic compounds have significantly higher ratios (1:1 for Benzene, 5:4 or 1.25:1 for Naphthalene, and 7:5 or 1.4:1 for Anthracene). A lower carbon-to-hydrogen ratio makes complete combustion more likely, resulting in a non-sooty flame.
Conclusion on Sooty Flames
Based on the analysis of their structures and carbon-to-hydrogen ratios, the aromatic hydrocarbons (Benzene, Naphthalene, and Anthracene) are expected to produce sooty flames due to incomplete combustion. Hexane, being a saturated aliphatic hydrocarbon with a lower carbon-to-hydrogen ratio, is expected to undergo more complete combustion and thus produce a non-sooty flame.
Revision Table: Carbon Compounds and Flames
| Compound Type | Structure Characteristic | Carbon:Hydrogen Ratio | Combustion Type (typically in air) | Flame Type |
|---|---|---|---|---|
| Saturated Aliphatic (Alkanes) | Single bonds, maximum H atoms | Lower ratio | More complete | Non-sooty (clean, blue) |
| Unsaturated Aliphatic (Alkenes, Alkynes) | Double or triple bonds | Higher ratio than alkanes | More incomplete | Sooty |
| Aromatic Hydrocarbons | Ring structure with delocalized electrons | High ratio | Highly incomplete | Very sooty (luminous, yellow) |
Additional Information: Complete vs. Incomplete Combustion
Combustion is a chemical process where a substance reacts rapidly with oxygen, producing heat and light. For hydrocarbons, the ideal combustion reaction (complete combustion) is:
$\text{Hydrocarbon} + \text{Excess Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Energy}$
For example, complete combustion of hexane ($\text{C}_6\text{H}_{14}$):
$\text{C}_6\text{H}_{14(l)} + \frac{19}{2}\text{O}_{2(g)} \rightarrow 6\text{CO}_{2(g)} + 7\text{H}_2\text{O}_{(g)} + \text{Energy}$
When there is insufficient oxygen, incomplete combustion occurs. This leads to the formation of carbon monoxide ($\text{CO}$) or even elemental carbon (soot, $\text{C}$), in addition to carbon dioxide and water:
$\text{Hydrocarbon} + \text{Limited Oxygen} \rightarrow \text{Carbon Monoxide} + \text{Water} + \text{Energy}$ (or)
$\text{Hydrocarbon} + \text{Very Limited Oxygen} \rightarrow \text{Soot (C)} + \text{Water} + \text{Energy}$
Aromatic and highly unsaturated hydrocarbons require more oxygen for complete combustion due to their higher carbon content relative to hydrogen. If sufficient oxygen isn't available (as is often the case when burning in open air), incomplete combustion is more likely, leading to soot formation and a sooty flame. Saturated hydrocarbons require less oxygen per carbon atom, making complete combustion easier under similar conditions.
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?
Directions: The following 05 (Five) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the answers to these items using the code given below:
Statement I: Petroleum is a mixture of many different hydrocarbons of different densities.
Statement II: The grade of petroleum depends mainly on the relative proportion of the different hydrocarbons.
