Problem 112 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Iodobenzene is prepared from aniline \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\right)\) in a two-step process as shown here: \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}+\mathrm{HNO}_{2}+\mathrm{HCl} \longrightarrow\) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2}^{+} \mathrm{Cl}^{-}+2 \mathrm{H}_{2} \mathrm{O}\) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2}^{+} \mathrm{Cl}^{-}+\mathrm{KI} \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I}+\mathrm{N}_{2}+\mathrm{KCl}\) In an actual preparation, \(9.30 \mathrm{~g}\) of aniline was converted to \(16.32 \mathrm{~g}\) of iodobenzene. The percentage yield of iodobenzene is \((\mathrm{I}=127)\) (a) 8\% (b) \(50 \%\) (c) \(75 \%\) (d) \(80 \%\)

Step-by-Step Solution

Answer

(b) 50%

Step 1: Calculate the molar mass of aniline

Aniline (C6H5NH2) has a molar mass calculated by adding the molar masses of all atoms in the molecule. There are 6 carbons (C, 12.01 g/mol each), 5 hydrogens (H, 1.008 g/mol each), and 1 nitrogen (N, 14.01 g/mol). Therefore, the molar mass of aniline (C6H5NH2) is: Molar mass = (6 * 12.01 g/mol) + (5 * 1.008 g/mol) + (1 * 14.01 g/mol) + (2 * 1.008 g/mol).

Step 2: Determine moles of aniline used

Using the molar mass of aniline from Step 1, convert the mass of aniline used in the reaction to moles by dividing the mass by the molar mass of aniline.

Step 3: Calculate the molar mass of iodobenzene

Iodobenzene (C6H5I) has a molar mass calculated by adding the molar masses of all atoms in the molecule. There are 6 carbons (C, 12.01 g/mol each), 5 hydrogens (H, 1.008 g/mol each), and 1 iodine (I, 127 g/mol). Molar mass = (6 * 12.01 g/mol) + (5 * 1.008 g/mol) + (127 g/mol).

Step 4: Determine the theoretical yield of iodobenzene

Using stoichiometry, for every mole of aniline used, one mole of iodobenzene should theoretically be produced. Calculate the theoretical yield in grams of iodobenzene by multiplying the moles of aniline by the molar mass of iodobenzene.

Step 5: Calculate the percentage yield of iodobenzene

The percentage yield is calculated using the formula: Percentage yield = (actual yield / theoretical yield) * 100%. The actual yield is given as 16.32 g of iodobenzene. Use the result from Step 4 as the theoretical yield to find the percentage yield.

Key Concepts

Stoichiometry

Stoichiometry is like a recipe for chemists. It tells you how much of each reactant you need to produce a certain amount of product. In our example, the stoichiometry of the reaction for creating iodobenzene from aniline shows a one-to-one ratio; each mole of aniline will theoretically produce one mole of iodobenzene.

Understanding stoichiometry allows you to predict the outcomes of chemical reactions. Just like you wouldn't want to run out of flour while baking a cake, chemists need to know the precise amounts of reactants to use to ensure a reaction goes to completion and not waste any materials or produce unwanted byproducts.

Molar Mass Calculation

Molar mass calculation is crucial for converting between grams and moles, which is a common task in chemistry. To get the molar mass, we simply add up the molar masses of each individual atom in the compound. For example, aniline's molar mass is found by totaling the molar masses of its carbon, hydrogen, and nitrogen atoms.

Breaking it Down

• Find each atom's molar mass using the periodic table.
• Multiply each atom's molar mass by the number of times it appears in the molecule.
• Add all the totals together to get the compound's molar mass.

Doing this step accurately is vital since it lays the foundation for the rest of the stoichiometry calculations.

Theoretical Yield

The theoretical yield is the amount of product expected if a chemical reaction runs perfectly and all the reactants convert to products without any losses. It's calculated using stoichiometry based on the limiting reactant – from which the product can form – and the balanced chemical reaction.

In the creation of iodobenzene, theoretical yield determines how much iodobenzene we can expect from a given amount of aniline. It's the chemist's 'best case scenario' but often, due to various factors such as side reactions or incomplete reactions, the actual yield will be lower than the theoretical yield.

Chemical Synthesis

Chemical synthesis is the process of creating new chemical compounds from simpler substances through chemical reactions. It's like cooking, but with atoms and molecules, creating exciting new concoctions. In our exercise, the synthesis of iodobenzene involves two steps starting with aniline.

Keys to Successful Synthesis

• Controlled reaction conditions.
• Precise measurements of reactants.
• Understanding the stoichiometry for proper ingredient ratios.

A successful synthesis should maximize the yield of the desired product while minimizing the side products, making understanding each step of the process critical.