Understanding Benzene Saturation with Hydrogen Gas

The question asks about the number of moles of hydrogen gas ($\text{H}_2$) required to fully saturate one mole of benzene ($\text{C}_6\text{H}_6$). Saturation, in the context of hydrocarbons, refers to the addition of hydrogen to break double or triple bonds, converting unsaturated compounds into saturated ones (those containing only single bonds).

Structure of Benzene and Saturation

Benzene is a cyclic aromatic hydrocarbon with the chemical formula $\text{C}_6\text{H}_6$. Its structure consists of a six-membered ring of carbon atoms. Each carbon atom is bonded to one hydrogen atom. The ring structure is characterized by alternating single and double bonds, often represented as a resonant structure or a ring with a circle inside, indicating delocalized pi electrons. To saturate benzene means to convert these double bonds into single bonds by adding hydrogen across each double bond.

A benzene molecule contains three double bonds. Each double bond requires one molecule of $\text{H}_2$ to become a single bond.

Chemical Reaction: Hydrogenation of Benzene

The process of saturating benzene by adding hydrogen is called hydrogenation. This reaction is typically carried out under specific conditions, such as high pressure and temperature, in the presence of a catalyst like nickel (Ni), platinum (Pt), or palladium (Pd). The product of the complete hydrogenation of benzene is cyclohexane ($\text{C}_6\text{H}_{12}$), which is a saturated cyclic hydrocarbon.

The balanced chemical equation for the complete hydrogenation of benzene is:

$\text{C}_6\text{H}_6\text{(l)} + 3\text{H}_2\text{(g)} \xrightarrow{\text{Catalyst}} \text{C}_6\text{H}_{12}\text{(l)}$

From this equation, we can see the stoichiometric ratio between benzene and hydrogen gas.

Determining the Mole Ratio

The balanced chemical equation shows that one mole of benzene ($\text{C}_6\text{H}_6$) reacts with three moles of hydrogen gas ($\text{H}_2$) to produce one mole of cyclohexane ($\text{C}_6\text{H}_{12}$).

Therefore, to saturate one mole of benzene, three moles of hydrogen gas are required.

Summary of Hydrogen Requirements for Benzene

• Benzene molecule: $\text{C}_6\text{H}_6$
• Number of double bonds in benzene: 3
• Hydrogen molecule needed per double bond: 1 $\text{H}_2$
• Total hydrogen molecules needed per benzene molecule: 3 $\text{H}_2$
• Mole ratio: 1 mole benzene : 3 moles $\text{H}_2$

Based on the stoichiometry of the reaction, one mole of benzene requires 3 moles of $\text{H}_2$ for complete saturation.

Revision Table: Benzene Hydrogenation

Additional Information on Benzene Hydrogenation

The hydrogenation of benzene is an exothermic reaction, meaning it releases heat. The reaction is crucial in the petrochemical industry for producing cyclohexane, which is used in the production of nylon.

• Catalysts: Common catalysts are transition metals like Nickel (Ni), Platinum (Pt), Palladium (Pd), or Rhodium (Rh) supported on materials like alumina or carbon. The choice of catalyst affects reaction conditions (temperature, pressure).
• Conditions: The reaction typically requires elevated temperatures (e.g., 150-250 °C) and high pressure (e.g., 20-50 atm or higher) when using catalysts like Nickel. Platinum or Palladium catalysts are more active and can allow the reaction to proceed at lower temperatures and pressures.
• Selectivity: Complete saturation converts all double bonds to single bonds. Partial hydrogenation is difficult to achieve selectively for benzene to produce cyclohexene ($\text{C}_6\text{H}_{10}$), as cyclohexene is hydrogenated faster than benzene.

Understanding the structure of benzene and the concept of saturation through hydrogenation is key to solving this type of stoichiometry problem. The balanced chemical equation provides the necessary mole ratio.