Problem 14 - Chemistry An Atoms First Approach

Question

When hydrogen is burned in oxygen to form water, the composition of water formed does not depend on the amount of oxygen reacted. Interpret this in terms of the law of definite proportion.

Step-by-Step Solution

Answer

The composition of water formed from the reaction between hydrogen and oxygen follows the law of definite proportion, which states that a chemical compound always contains the same elements in the same proportions by mass. In the case of water, the atoms of hydrogen and oxygen combine in a specific, consistent ratio not affected by the amount of reactants involved. Water retains the same proportion of hydrogen to oxygen, which is a ratio of 2:1 in moles or 2g of hydrogen to 16g of oxygen by mass.

Step 1: Understanding the law of definite proportion

The law of definite proportions, also known as the law of constant composition, states that a chemical compound always contains the same elements in the same proportions by mass, regardless of the amount or source of the compound. In other words, the atoms of each element combine in a specific and consistent ratio to form a compound.

Step 2: Writing out the chemical reaction for hydrogen and oxygen forming water

The chemical reaction of hydrogen and oxygen combining to form water can be written as: \(2 H_2 (g) + O_2 (g) \rightarrow 2 H_2O(l)\) Here, two moles of gaseous hydrogen react with one mole of gaseous oxygen to produce two moles of liquid water.

Step 3: Analyzing the composition of water

To understand how the law of definite proportion applies to this reaction, let's first determine the mass proportion of hydrogen and oxygen in water. Water has a molecular formula of H2O, which means it consists of 2 hydrogen atoms (H) and 1 oxygen atom (O). The atomic mass of hydrogen is approximately 1 gram per mole, while the atomic mass of oxygen is approximately 16 grams per mole. Therefore, the mass proportion of hydrogen to oxygen in water is approximately: \(M_{H_2O} = 2 \times M_{H} + 1 \times M_{O}\)

Step 4: Applying the law of definite proportion to the reaction

No matter the amount of hydrogen and oxygen reacted, the resulting water will always have the same proportion of hydrogen to oxygen by mass, as dictated by the law of definite proportion. In this reaction, the definite proportion is a ratio of 2:1 in moles (or 2g of hydrogen to 16g of oxygen by mass). Even if there is an excess of one reactant, the product will always have a consistent composition, with the same proportions of hydrogen and oxygen. In conclusion, the composition of water formed from the reaction between hydrogen and oxygen does not depend on the amount of oxygen reacted, following the law of definite proportion. The atoms of hydrogen and oxygen in water always combine in a specific, consistent ratio that is not affected by the amount of reactants involved in the reaction.