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Preparing and Collecting Oxygen

Middle schoolChemistryHydrogen peroxide decomposition with a prescribed first-order catalyst model, mixed delivery-tube air and saturated water-displacement collection at 25 °C

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About this experiment

Assemble an oxygen collection bench, start decomposition and compare the quantity of gas with its purity. Initial air in the delivery tube makes the first sample less pure. Discarding that gas improves the later sample, while water vapor remains part of gas collected over water.

Learning goals

  • Relate peroxide consumption to the two-to-one peroxide-to-oxygen mole ratio.
  • Distinguish gas volume, conversion and oxygen purity.
  • Investigate how purging and atmospheric pressure change the collected sample.

How to use it

  1. 1Connect the collector, keep the initial tube air at 10 mL and collect without discarding the first gas.
  2. 2Start a fresh trial, discard 20 mL and compare purity after the same reaction time.
  3. 3Increase catalyst activity and compare the time to the same conversion rather than the final stoichiometric yield.
  4. 4Change pressure, collect again and record wet volume, remaining peroxide and sample purity.

The model

First-order oxygen generation
Water-vapor correction to ideal-gas collection

Assumptions

  • The reaction is 2H₂O₂ → 2H₂O + O₂. Its first-order constant is 0.035 times the selected catalyst activity per second, a prescribed teaching rate rather than an experimental calibration.
  • All gas is ideal at 298.15 K. Collected gas is saturated with water vapor at a fixed partial pressure of 3.17 kPa. Hydrostatic head and dissolved oxygen are excluded.
  • The initial delivery-tube gas contains 21 percent oxygen and is perfectly mixed during flushing. The collector begins full of water. A disconnected collector cannot retain generated gas.

Try this in class

  1. Q1Why can a larger collected volume still have a lower oxygen fraction?
  2. Q2Why does a catalyst change the time curve without changing the peroxide-to-oxygen stoichiometry?

Frequently asked questions

Why is purity below 100 percent even after a long purge?

The wet sample still contains water vapor, and a finite purge may leave a small amount of initial tube air.

Does the activity setting specify a real catalyst recipe?

No. It is a normalized rate multiplier for comparing curves, not a measured catalyst dose or preparation protocol.