Question

In the hydrogenation of pheny 1 ethylene \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\) using a platinum catalyst the volume of hydrogen (measured at STP) that reacts with one mole of phenyl ethylene could be (1) \(11.2\) litres (2) \(22.4\) litres (3) \(44.8\) litres (4) 1 litre

Short answer

\(22.4\) liters

Step-by-step solution

Understand the Reaction

In the hydrogenation reaction, phenyl ethylene (C_6H_5CH=CH_2) reacts with hydrogen gas (H_2) to form ethylbenzene (C_6H_5CH_2CH_3). Each molecule of phenyl ethylene requires one molecule of hydrogen gas to saturate the double bond.

Write the Balanced Equation

The balanced chemical equation for the hydrogenation reaction is: C_6H_5CH=CH_2 + H_2 → C_6H_5CH_2CH_3. This shows that 1 mole of phenyl ethylene reacts with 1 mole of hydrogen gas.

Determine Volume of Hydrogen at STP

At Standard Temperature and Pressure (STP), 1 mole of any gas occupies 22.4 liters. Therefore, 1 mole of hydrogen gas (H_2) will occupy 22.4 liters.

Compare Options

Considering that the reaction requires 1 mole of hydrogen gas for 1 mole of phenyl ethylene, the volume of hydrogen required is 22.4 liters. Comparing this with the given options, the correct choice is (2) 22.4 liters.

Key concepts

Hydrogenation reaction

In chemistry, a hydrogenation reaction involves adding hydrogen (H_2) to another compound. This process is often used to convert unsaturated molecules (those with double or triple bonds) into saturated molecules (those with single bonds). In the case of phenyl ethylene, which has a double bond between carbon atoms, hydrogenation removes this double bond. During the hydrogenation of phenyl ethylene (C_6H_5CH=CH_2), hydrogen gas is used to convert it into ethylbenzene (C_6H_5CH_2CH_3). This reaction simplifies the structure of the original compound by saturating the double bond.

Platinum catalyst

A platinum catalyst is often employed to speed up the hydrogenation reaction. Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. Platinum is especially effective in hydrogenation due to its high chemical reactivity and surface area. In practical terms, using a platinum catalyst means the reaction can occur more quickly and efficiently. This is why it's used in industrial applications where rapid and complete hydrogenation is desired.

Standard Temperature and Pressure (STP)

Standard Temperature and Pressure (STP) is a reference point in chemistry used to define the conditions under which gas volumes and other properties are compared. STP is defined as a temperature of 0°C (273.15 K) and a pressure of 1 atmosphere (atm or 101.3 kPa). Under these conditions, 1 mole of any ideal gas occupies 22.4 liters. When dealing with gas volumes in chemical reactions, STP provides a common standard for calculations. In the case of the hydrogenation reaction of phenyl ethylene, knowing that 1 mole of hydrogen gas equals 22.4 liters at STP allows us to determine the volume of hydrogen used.

Balanced chemical equation

A balanced chemical equation ensures that the number of atoms for each element is the same on the reactant and product sides of the reaction. This follows the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. For the hydrogenation of phenyl ethylene, the balanced equation is: C_6H_5CH=CH_2 + H_2 → C_6H_5CH_2CH_3. This indicates that 1 mole of phenyl ethylene reacts with precisely 1 mole of hydrogen gas to create 1 mole of ethylbenzene. Balancing chemical equations is crucial for accurately calculating the amounts of reactants and products in a given reaction.