Calculating Reactant and Product Volumes in a Chemical Reaction

Understanding the Concept

When dealing with chemical reactions involving gases, it's important to understand that the volume of a gas does not directly translate to the number of molecules present under non-specific conditions. However, when gases are at the same temperature and pressure, equal volumes hold the same number of molecules, as per Avogadro's Law. This principle is crucial in calculating the volumes of reactants and products in a reaction where gases are involved.

Question and Context

The question at hand is: 40 cm3 of H2 reacts with 40 cm3 of O2. What will be the volume of reactants and products present in the vessel after the completion of the reaction?

Stoichiometry and Limiting Reactants

To solve this problem, we need to look at the balanced chemical equation for the reaction between hydrogen (H2) and oxygen (O2) to form water (H2O):
2H2 O2 → 2H2O

From the balanced equation, it is evident that the mole ratio of H2 to O2 is 2:1. This means that for every 2 moles of H2 needed, 1 mole of O2 is required. Conversely, the mole ratio of O2 to H2 is 1:2.

Given 40 cm3 of H2 and 40 cm3 of O2, let's find out which of the reactants is the limiting one.

Determining the Limiting Reactant

First, we calculate the volume of O2 required to react with the given volume of H2:
Volume of O2 required (1/2) × 40 cm3 20 cm3

Since the initial amount of O2 is 40 cm3 and we only need 20 cm3 to react with the H2, the excess amount of O2 will be 40 cm3 - 20 cm3 20 cm3, which will remain unreacted.

Next, let's calculate the volume of H2 required to react with the given volume of O2:
Volume of H2 required 2 × 40 cm3 80 cm3

However, we only have 40 cm3 of H2 initially, which is insufficient to react with the 40 cm3 of O2 (as we need 80 cm3 of H2 for full reaction with O2). Therefore, H2 is the limiting reactant.

As the limiting reactant (H2) dictates the amount of product (H2O) formed, we can calculate the volume of H2O produced as follows:

From the balanced equation, the mole ratio of H2 to H2O is 1:1. Hence, the volume of H2O produced is equal to the volume of H2 used up in the reaction.
Volume of H2O produced 40 cm3

Conclusion

After the completion of the reaction, the volume of reactants and products in the vessel will be as follows:

Volume of O2 20 cm3 (excess, unreacted) Volume of H2 0 cm3 (used up completely) Volume of H2O produced 40 cm3

Understanding the principles of stoichiometry and limiting reactants is essential for accurately predicting the outcome of chemical reactions in such scenarios. This example highlights the importance of these concepts in chemical calculations involving gases.