Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, \(4 \mathrm{NH}_{3}+5 \mathrm{O}_{2} \longrightarrow 4 \mathrm{NO}+6 \mathrm{H}_{2} \mathrm{O}\), when one mole of ammonia and one mole of oxygen are made to react to completion, then (a) \(1.0 \mathrm{~mol}\) of \(\mathrm{H}_{2} \mathrm{O}\) is produced (b) all the oxygen is consumed (c) \(1.5 \mathrm{~mol}\) of \(\mathrm{NO}\) is formed (d) all the ammonia is consumed

Step-by-Step Solution

Answer

The correct answer is (b) all the oxygen is consumed.

Step 1: Identify Limiting Reactant

To determine the limiting reactant, we compare the mole ratio from the balanced equation to the moles available. The balanced equation tells us that 4 moles of NH₃ reacts with 5 moles of O₂. Here, we have 1 mole of NH₃ and 1 mole of O₂. The mole ratio needed is \(\frac{4}{5}\), but we have \(\frac{1}{1}\). Thus, O₂ is the limiting reactant since 1 mole of O₂ will not be enough to react with 1 mole of NH₃.

Step 2: Calculate Product Formation based on Limiting Reactant

Since O₂ is the limiting reactant, we will use it to calculate the products formed. According to the equation, 5 moles of O₂ produce 6 moles of H₂O. Therefore, if 1 mole of O₂ is used, the moles of H₂O produced should be \(\frac{6}{5} = 1.2\) moles.

Step 3: Calculate NO Production

From the balanced equation, 5 moles of O₂ also produce 4 moles of NO. With 1 mole of O₂, the NO production would be \(\frac{4}{5} = 0.8\) moles.

Step 4: Assess Predictions

(a) Predicted H₂O production is 1.2 moles, not 1.0 mol. False. (b) Limiting reactant implies all O₂ is consumed. True. (c) Predicted NO production is 0.8 moles, not 1.5 mol. False. (d) Since only 0.8 moles of NH₃ would react with 1 mole of O₂ (enough to match the 1:1 mole actual ratio), all NH₃ is not consumed. False.

Key Concepts

Stoichiometry

Understanding stoichiometry is crucial in solving chemical problems like identifying the limiting reactant. It is the part of chemistry that deals with the calculation of reactants and products in chemical reactions. Stoichiometry is based on the law of conservation of mass, where the mass of reactants equals the mass of products.
When faced with a balanced chemical equation, stoichiometry allows us to predict the amounts of products formed. For example, given the balanced equation:

• \(4 \mathrm{NH}_{3} + 5 \mathrm{O}_{2} \rightarrow 4 \mathrm{NO} + 6 \mathrm{H}_{2} \mathrm{O} \)

This tells us the exact number of moles of ammonia (NH₃) that react with a specific number of moles of oxygen (O₂) to produce nitric oxide (NO) and water (H₂O). Here, stoichiometry involves converting between moles of different substances using their ratios, as provided by the balanced equation.

Chemical Reactions

In chemistry, a chemical reaction occurs when substances, known as reactants, are transformed into different substances called products. Reactions are governed by specific rules and involve a rearrangement of atoms.
The reaction we're analyzing can be expressed as:

• \(4 \mathrm{NH}_{3} + 5 \mathrm{O}_{2} \rightarrow 4 \mathrm{NO} + 6 \mathrm{H}_{2} \mathrm{O} \)

This is an example of a combustion reaction where ammonia is oxidized by oxygen to produce nitrogen monoxide and water. It's important to read the chemical equation carefully as it reveals how reactants interact and how products form. In practice, knowing which reactants are in excess and which are limiting can affect the quantity of products synthesized.

Mole Ratio

The concept of mole ratio is key in stoichiometry. Derived from the coefficients in the balanced chemical equation, the mole ratio indicates the proportion of each reactant and product involved in a reaction. In our example:

• The mole ratio of NH₃ to O₂ is 4:5
• The mole ratio of O₂ to NO is 5:4

This means that for every 4 moles of NH₃, 5 moles of O₂ are required. Similarly, 5 moles of O₂ produce 4 moles of NO.

The concept of mole ratio helps in determining the limiting reactant. Comparing actual moles of reactants with the mole ratios needed gives insight into which reactant will be exhausted first, dictating the extent of the reaction.

Product Formation

Product formation in a chemical reaction is directly related to the limiting reactant. Once the limiting reactant is completely used up, the reaction stops, and no further products can be formed.
In our scenario with 1 mole of NH₃ and 1 mole of O₂, the limiting reactant is O₂. This means product formation calculations should be performed using the starting amount of O₂.

• For every 1 mole of O₂, 1.2 moles of H₂O are formed (from the 6:5 ratio).
• Similarly, 0.8 moles of NO is produced per mole of O₂ consumed.

Understanding this concept helps in accurately predicting how much product results from given reactants and can be applied to various chemical reactions for efficient use of materials.