Understanding Stoichiometry and Gas Volume Relationships in Reactions

Introduction

When studying chemical reactions, it's crucial to understand the relationship between the volume of gases and the number of moles involved. At standard temperature and pressure (STP), volume and number of moles are directly proportional. This relationship is key to understanding how much of a reactant is needed to complete a reaction. In this article, we will explore the specific example of the reaction between hydrogen gas (H2) and oxygen gas (O2), and how to determine the volume of oxygen required to react completely with a given volume of hydrogen gas.

Stoichiometric Equation and Reaction Analysis

The balanced chemical equation for the reaction between hydrogen gas and oxygen gas to form water (H2O) is:

H2(g) 1/2 O2(g) → H2O(l)

This equation tells us that one volume of hydrogen gas reacts with half a volume of oxygen gas to produce water. Since water is a liquid under normal conditions, its volume is not directly considered in the gas volume analysis at STP. Instead, we focus on the volume of gases involved.

Calculating Oxygen Volume Required for Complete Reaction

Given that we have 3.6 liters of hydrogen gas, we need to calculate the volume of oxygen gas required for the reaction to proceed completely. Using stoichiometry, we know that:

2 volumes of hydrogen gas reacts with 1 volume of oxygen gas.

Therefore, to find the volume of oxygen required, we use the proportional relationship:

3.6 L (H2) × (1/2) 1.8 L (O2)

This calculation shows that for 3.6 liters of hydrogen gas, you need 1.8 liters of oxygen gas to react completely. This is derived from the stoichiometric equation and the proportional relationship between the volumes of gases at STP.

Alternative Method for Volume Calculation

Another way to approach this problem is to use the balanced equation and the mole concept. Using the balanced equation:

2H2 O2 → 2H2O

We can set up a proportion to find the volume of oxygen needed:

2 mol H2 react with 1 mol O2

Given that we have 3.6 liters of H2, we need to find the volume of O2 required:

2 L H2 / 1 L O2 3.6 L H2 / X L O2

Solving for X, we find:

X (3.6 L H2) × (1/2) 1.8 L O2

Thus, the volume of oxygen required is 1.8 liters.

Example of Another Gas Volume Calculation

To further illustrate the concept, let's consider another example. If 2.5 liters of hydrogen gas react with stoichiometric dichlorine gas (Cl2), what volume of hydrogen chloride (HCl) gas will result?

H2(g) Cl2(g) → 2HCl(g)

Using the stoichiometric equation, we see that 1 volume of hydrogen gas reacts with 1 volume of chlorine gas to produce 2 volumes of hydrogen chloride gas. Therefore:

2.5 L (H2) × 2 5 L (HCl)

Hence, the volume of hydrogen chloride gas produced is 5 liters.

Conclusion

Understanding the relationship between the volume of gases and the number of moles in a chemical reaction is fundamental to solving problems involving gases at STP. By using stoichiometric equations and proportional relationships, we can accurately determine the volumes of reactants and products in gas reactions.