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Air and Combustion: Oxygen

4.0 Air and Combustion

4.4 Oxygen

We have learnt that oxygen can be obtained from fractional distillation of liquefied air (Section 4.2). How else can oxygen be obtained?

How can we prepare oxygen gas in the laboratory?

Materials and substances required
  1. Hydrogen peroxide
  2. Manganese (IV) oxide
  3. Flask, retort stand dropping funnel, delivery tube, gas jar, wooden splint

Set-up preparation of oxygen in the laboratory high school chemistry

Alternative A: Direct collection in a gas jar

preparation of oxygen in the laboratory high school chemistry

Alternative B: Collection over water

The tap is partially opened to add hydrogen peroxide (dilute solution, about 20%) dropwise to manganese (IV) oxide. Reaction occurs, producing oxygen gas which flows into the gas jar. The gas can be tested for smell, effect on litmus paper, and glowing splint.

Open the video below, preparation and properties of oxygen gas,

(Courtesy Youtube - To Prepare Oxygen & Examine its Properties by Simple Science & Maths) and take observations.

Questions 4.4(a)

  1. State the observations made when hydrogen peroxide is added to manganese (IV) oxide
  2. What is the colour of oxygen gas?
  3. What is the effect of oxygen gas on a glowing (not burning) wooden splint?
  4. What is the effect of oxygen gas on burning magnesium and glowing charcoal?
  5. What is the effect of oxygen gas, if any, on wet blue and red litmus paper?
  6. Does oxygen burn? Give a reason for your answer.
  7. In the Alternative A arrangement, the gas jar was initially full of air. Based on what has been observed, is oxygen gas denser or less dense than air? Explain your answer.
  8. Oxygen gas can be collected over water as in Alternative B. From this observation, does oxygen dissolve much in water?
  9. With the collection over water, the reaction is normally allowed to proceed for a few seconds before placing the gas jar to collect oxygen. Suggest a reason for this.

Answers to Questions 4.4(a)

In this reaction, manganese (IV) does not take part in the reaction; that is why it retains its black colour. But its presence is required to speed up the reaction. It is a catalyst. A catalyst is a substance which speeds up a chemical reaction but remains chemically unchanged.

NB: A close examination of hydrogen peroxide in the dropping funnel shows that it is producing small burbles, meaning it is already reacting. Manganese (IV) oxide only speeds up the reaction; it does not cause the reaction.

A catalyst is placed over the arrow, because it is neither a reactant nor a product.

In this case, hydrogen peroxide does not react with any substance. Rather, it decomposes (breaks up) into simpler substances: water and oxygen. Such a reaction is called a decomposition reaction.

At home

Hydrogen peroxide is commonly used as an antiseptic (to disinfect fresh wounds) and as a mouth wash to kill germs in the mouth.

Properties of oxygen gas

From our answers to Questions 4.4, the properties of oxygen are as follows

  1. It is colourless and odourless (has no smell).
  2. It is denser than air.
  3. It is neutral (has no effect on blue or red litmus).
  4. It is slightly soluble in water.
  5. It rekindles (relights) a glowing splint.

NB: Rekindling of a glowing splint is the test for oxygen. It is only oxygen gas that is both odourless and rekindles a glowing splint. Oxygen does not burn. But it supports burning.

Other than from hydrogen peroxide, how else can we obtain oxygen in the laboratory?

There are many other reactions which produce oxygen, including the following.

  1. Decomposition by heating (thermal decomposition) of potassium manganate (VII) oxide solid
    thermal decomposition of potassium manganate (VII) high school chemistry
  2. All the substances represented in this equation are solids except oxygen.

  3. Thermal decomposition of metal nitrates (e.g. sodium nitrate)
  4. Sodium nitrate and sodium nitrite are solids.

  5. Reaction between sodium peroxide (solid) with water
  6. Sodium peroxide is a solid. Sodium hydroxide forms a solution with water.

Use the table of Common Chemistry Laboratory Chemicals to identify the colours of manganese (IV) oxide, sodium nitrite, and sodium hydroxide.

Questions 4.4(b)

  1. From the information given in 1-3 above, suggest the arrangement you would use to prepare and collect oxygen from
    1. sodium peroxide and water
    2. potassium manganate (VII) or sodium nitrate
  2. How would you know when the gas jar is full? (Hint: Oxygen is colourless but rekindles a glowing splint.)

Answers to Questions 4.4(b)