Air and Combustion: Burning of substances in air

4.0 Air and Combustion

4.5 Burning of substances in air

We have learnt that the active part of air is oxygen. That is, it is oxygen that takes part in chemical reactions. Oxygen supports combustion (burning), including rusting which is a relatively slow combustion. Therefore burning in air is chemically the same as burning in oxygen. They lead to the same products, oxides.

How do elements burn in air and in pure oxygen?

Materials and substances required

1. Sodium metal, magnesium metal, sulphur
2. Deflagrating spoon, tongs, knife, floor tile
3. Bunsen burner
4. Gas jars, water, red and blue litmus paper
5. Apparatus and chemicals for preparing oxygen gas

1. Prepare four gas jars of oxygen A, B, C and D.
2. Cut a small piece of sodium metal, hold it with a deflagrating spoon and ignite it in a Bunsen flame. Observe how it burns in air.
3. Lower the burning sodium into one jar of pure oxygen and take observations.
4. Shake the product with a little water and test the pH of the solution.
5. Repeat the experiment using magnesium metal, sulphur, and carbon (charcoal).

Caution: The burning of sulphur should be done in a fume chamber or open space.

Open the videos, burning of sodium,

(Courtesy YouTube - Burning sodium metal in oxygen by Ale )
magnesium,
(Courtesy YouTube - Burning magnesium in oxygen by Nigel Baldwin )
sulphur,
(Courtesy YouTube - Combustion of sulphur in oxygen by Chemistry Channel )
and carbon in oxygen,
(Courtesy YouTube - Burning coal in pure oxygen by Felix Letkemann ) and take observations.

Questions 4.5(a)

1. Fill in the table that follows to represent your observations
   


2. Describe the observations made for each of the three elements.
3. What are the equations for the reactions of the elements with oxygen?
4. Make conclusions about the burning of
   1. metals in air (oxygen)
   2. non-metals in air (oxygen)

Answers to Questions 4.5(a)

From the above reactions, it can be concluded that metals burn in air to form solid basic oxides. Non-metals burn to form acidic oxides, most of them being gaseous.

The burning of phosphorus in air can be safely demonstrated using red phosphorus, but in a fume chamber or open space. White phosphorus burns with a fierce fire and must not be used.

Questions 4.5(b)

Complete the following word equations to represent the burning of elements in air.

Potassium + Oxygen
Calcium + Oxygen
Aluminium + Oxygen

Answers to Questions 4.5(b)

Some of the highly reactive metals such as sodium, magnesium, calcium and aluminium combine with nitrogen in addition to oxygen of the air. They form both oxides and nitrides. The example for magnesium is as follows.

At home

The match head and lining on a match box is red phosphorus, owing to its readiness to burst into flame when exposed to the air. The heat produced by friction when striking the matchbox is enough to ignite the phosphorus match head.

Competition for combined oxygen

Think of a rugby football with a player, R, holding a ball, O. In competition for the ball, a stronger and more aggressive player, Q, snatches the ball. In chemistry, we would say Q is more reactive than R. An equation for this might be.

Similarly, elements compete for combined oxygen. And through this we can find out which element is more reactive than the other.

Which element is more reactive in the following pairs: (a) copper and zinc (b) copper and iron?

Materials and substances required

1. Copper (II) oxide, zinc powder, iron dust
2. Ignition tube, Bunsen burner, test tube holder, white tile

Arrangement for copper (II) oxide and any metal powder

Open the link, reaction of copper (II) oxide with zinc.

(Courtesy Youtube - Reaction between zinc and copper (II) oxide by Chemistry edb) Observe the demonstrations keenly, including of the products when poured onto the white tile.

Questions 4.5(c)

1. 1. State the observations made when a mixture of copper (II) oxide and zinc powder is strongly heated.
   2. Name the substance corresponding to each colour.
2. Which of the two elements, copper and zinc, is more reactive?
3. Write a word equation for the reaction.

Answers to Questions 4.5(c)

Open the link, reaction of copper (II) oxide with iron.

(courtesy YouTube - Redoxreaktion CuO + Fe by Bruno Gunterswiler). Observe the demonstrations keenly, including of the products when poured onto the white tile.

Questions 4.5(d)

1. What are the observations made when a mixture of copper (II) oxide and iron powder is strongly heated?
2. 1. What are the colours of the substances formed when they are allowed to cool?
   2. Name the substance corresponding to each colour.
3. Which of the two elements, copper and iron, is more reactive?
4. Write a word equation for the reaction.

Answers to Questions 4.5(d)

In the competition for combined oxygen, the element that loses oxygen is reduced. That which gains oxygen is oxidized. Zinc and iron are oxidized while copper is reduced. Such reduction-oxidation reactions are used to extract metals such as copper, zinc and iron from their ores and oxides.

Reactivity series

From the two demonstrations above, zinc is more reactive than copper, because it takes combined oxygen from copper. Likewise, iron is more reactive than copper.

To find out whether zinc is more reactive than iron or the other way round, a further experiment involving iron (II) oxide and zinc is necessary. Through such experiments, the order of reactivity of the 9 most reactive ordinary metals has been found to be as follows.

Questions 4.5(e)

The table that follows shows different metal and oxide mixtures labelled a to z. In which of the mixtures a, d, g, i, j, l, m, o, p, t, w, and z will chemical reactions occur between the components when heated strongly? Optional: For further practice, feel free to answer all the parts a to z.

Answers to Questions 4.5(e)

Uses of oxygen

1. It is mixed with acetylene to form oxy-acetylene gas used in welding.
2. Used in high performance engine to ensure complete combustion fuel and production of more heat energy
3. It is mixed with helium and carried along by mountain climbers and deep sea divers.
4. In hospitals, a rich mixture of oxygen is administered to patients with breathing difficulty.
5. It is used in refining or purifying metals during their extraction, by burning away the impurities.