Chemistry 5070/12 — October/November 2011

Walkthrough

The question describes the preparation of oxygen from hydrogen peroxide using manganese(IV) oxide as a catalyst. The oxygen is collected by downward displacement of water (over water).

When any gas collection apparatus is set up, the flask, delivery tube, and the space above the water in the trough initially contain air. Air is a mixture of gases, and its most abundant component is nitrogen (approximately 78%), with oxygen making up about 21%.

As soon as the hydrogen peroxide is added and the reaction starts, the oxygen gas produced will expand and push out whatever gas is currently in the apparatus. The first gas to travel down the delivery tube and into the inverted test-tube is therefore the air that was originally in the flask and tube. This air is contaminated with a large amount of nitrogen.

To collect a pure sample of oxygen, the first few tubes (or the first few minutes of gas collection) are discarded. This ensures that the air (and thus the nitrogen) is completely expelled before the pure oxygen is collected.

Let's evaluate the other options:

• A (hydrogen): The reaction is $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$. No hydrogen gas is produced.
• B (hydrogen peroxide): Hydrogen peroxide is an aqueous solution (a liquid). It does not vaporise under these conditions to contaminate the collected gas as a vapour in the same way air does.
• D (water vapour): Because the gas is collected over water, the collected oxygen will indeed be saturated with water vapour. However, this is true for all the gas collected, not just the first few tubes. The specific reason for rejecting the first tubes is to remove the air (nitrogen) from the apparatus.

Key Takeaways

• When collecting a gas over water, the initial gas displaced from the reaction vessel is air, not the product gas.
• Air is primarily composed of nitrogen (~78%).
• The first portion of gas collected must be discarded to ensure the final sample is pure and not contaminated with air.

Common Mistakes

• Choosing water vapour (D): Students often remember that gas collected over water is wet and choose water vapour. However, water vapour is present throughout the entire collection, so it is not the reason for rejecting only the first few tubes.
• Choosing hydrogen (A): Students may confuse this reaction with the reaction of a metal with an acid, which produces hydrogen gas. Here, only oxygen is produced.

Things to Be Careful About

• Read the question carefully: it asks why the first few tubes are rejected, not why the gas is generally impure. Gas collected over water is always wet, but that is an accepted characteristic of the method, not a reason to discard the initial fraction.
• Remember that the apparatus is not evacuated before the experiment; it is filled with ordinary laboratory air.