Aim and chemicals
Aim: to make oxygen in the lab, collect it and prove what it is.
- Method A (room temperature): hydrogen peroxide solution + a spoon of manganese dioxide (MnO₂). Equation: 2H₂O₂ → 2H₂O + O₂ (MnO₂ is the catalyst).
- Method B (heating): potassium permanganate, heated. 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂.
A catalyst changes the speed of a reaction and is still there, unchanged, at the end. We can filter and dry the black powder to show this.
Assembling the apparatus
- Pick a clean flask (a conical flask or a test tube).
- Put the hydrogen peroxide in the flask (Method A). For Method B put the KMnO₄ in a hard glass test tube, slightly tilted with its mouth a little downward, and push a loose cotton plug near the mouth.
- Fit a one-hole cork with a bent delivery tube. Check that it is airtight: put the tube end in water, warm the flask with your hands and look for bubbles.
- Fill a gas jar with water, cover with a glass plate, turn it upside down in the trough and slide the plate off under the water.
- Put the end of the delivery tube under the mouth of the jar.
Parts to know: flask, cork, delivery tube, trough, gas jar, stand, burner (Method B).
Collecting oxygen
Oxygen is only slightly soluble in water, so it can be collected over water (downward displacement of water). The gas pushes the water out of the jar.
- Wait for steady bubbles. The first bubbles are just the air that was in the flask.
- When the jar is full of gas, slide a glass plate under water over its mouth, lift it out and stand it mouth up.
- Oxygen is a little heavier than air, so it can also be collected by upward displacement of air with the jar mouth up. This gives drier gas but it is harder to know when the jar is full.
Collect two or three jars. Throw away the first one, since it has some air in it.
Testing oxygen and staying safe
The test
Light a wooden splint, blow it out so it just glows red, and put it into the jar. In oxygen it relights and burns brightly. Other gases do not do this.
Other properties to show
Oxygen has no colour, no smell and no taste. It helps things burn but does not burn itself. It is a little soluble in water, which keeps fish alive.
Safety
- Wear safety glasses. Hydrogen peroxide can bleach skin; KMnO₄ stains.
- Take the delivery tube out of the water before you stop heating, otherwise cold water sucks back into the hot tube.
- No flames near the jar of oxygen except for the test.
- Clean up: wash the black powder and flask, then wash your hands.
Try it: record and explain
In your notebook draw the set-up with labels, write the chemicals, the equation, what you saw at each step, the test result and one safety rule. Then explain in two lines why you throw away the first jar.
Key formulas and definitions
- 2H₂O₂ → 2H₂O + O₂ (catalyst: MnO₂)
- 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂ (heat)
- Test: glowing splint relights = oxygen
- Molar volume at room temperature = about 24 L per mole of gas
- Collect: over water or by upward displacement of air
Worked examples
1. Why do we wait for steady bubbles before collecting the first jar?
The first bubbles are air pushed out of the flask and tube. Waiting makes sure the jar gets mostly oxygen.
2. In Method B, why do we put a cotton plug near the mouth of the test tube?
The heat can throw out purple powder. The cotton plug stops it entering the delivery tube and the water.
3. 17 g of hydrogen peroxide (34 g per mole) fully breaks down. How much oxygen gas is made at room temperature? Use 24 L per mole.
17 ÷ 34 = 0.5 mol H₂O₂. 2 mol H₂O₂ give 1 mol O₂, so 0.25 mol O₂. Volume = 0.25 × 24 = 6 L.
Common mistakes
- Collecting the first bubbles. They are air, not oxygen.
- Stopping the burner before taking the delivery tube out of the water.
- Using a flaming splint for the test. It must be glowing only.
- Thinking MnO₂ is used up. It is a catalyst and is left unchanged.