Heating Group 1 metals in air and in chlorine

Heating Group 1 metals in air and in chlorine

Demonstration

This is a demonstration that shows the reactions of Group 1 metals in air and in chlorine. It does not clearly show the trends in reactivity of Group 1 metals, which is better demonstrated by the reactions in water, which follow on well from this demonstration.

To view a video of this demonstration experiments.
 

Lesson organisation

This experiment must be done as a demonstration. If you have not attempted this experiment before, it is strongly advised that you try it before performing the demonstration in front of students.

The first step is to generate chlorine, which can be done in advance, and requires a fume cupboard. The rest of the demonstration can be done in a well-ventilated laboratory. Goggles should be worn during the chlorine generation and the demonstration. The class should also wear eye protection during the demonstration.

How long the demonstration takes depends largely on how much talking you do between each part of the experiment. To speed things up a bit, the metals can be pre-cut into appropriately sized pieces, but they should be returned to the oil until just before they are used. For some classes it may be appropriate to do just one part of the experiment and heat the metals in either air or chlorine.

Chemicals

Lithium (HIGHLY FLAMMABLE, CORROSIVE) (Note 3 and 4)

Sodium (HIGHLY FLAMMABLE, CORROSIVE) (Note 3)

Potassium (HIGHLY FLAMMABLE, CORROSIVE) (Note 3)

Sodium chlorate(I) solution, 10-14% (w/v) (CORROSIVE), fresh (Note 2)

Hydrochloric acid, 5M (IRRITANT AT THIS DILUTION) (Note 2)

Apparatus

Goggles for the demonstrator, eye protection for the audience

Fume cupboard (only for generating the chlorine)

Clean, dry bricks with at least one flat surface, 3 (Note 1)

Gas jars with lids, 3 (Note 1)

Bunsen burner

Heat resistant mat

Scalpel

Forceps or tweezers

Tile

Filter paper

Universal Indicator paper 

Chlorine generator (TOXIC, DANGEROUS FOR THE ENVIRONMENT) See Standard Techniques: Generating collecting and testing gases (Note 2) 

Health and Safety and Technical notes

Demonstrator to wear goggles or face shield, class to wear eye protection. 

Chlorine - see CLEAPSS Hazcard. 

Lithium - see CLEAPSS Hazcard.

Sodium - see CLEAPSS Hazcard.

Potassium - see CLEAPSS Hazcard.

Sodium chlorate(I) - see CLEAPSS Hazcard. 

Hydrochloric acid - see CLEAPSS Hazcard and Recipe book.

1 The mouth of the gas jar must be narrower than the brick, to reduce the amount of gas escaping during the demionstration.

2 There are two methods given in the standard techniques for generating chlorine. The method that uses sodium chlorate(I) is safer than the method that uses potassium manganate(VII), but will not work well if the sodium chlorate(I) is an old sample as the concentration will be too low. Note that sodium chlorate(I), NaOCl, is NOT the same as chlorate(V), NaClO₃.

3 It is very helpful to have the 3 mm cubes of lithium, sodium and potassium cut ready to use. These should still be kept under oil until they are required.

4 Do not heat lithium in crucibles or other porcelain material – explosions have occurred.

Procedure

Heating the metals in air
a Starting with lithium, use the tweezers to pick up a small piece of metal and place it on a tile. Use a scalpel to cut a small cube with an edge of about 3 mm. Show the students the freshly cut surface which soon tarnishes, showing that the metal reacts quickly with oxygen. Blot off the oil using the filter paper, and place it onto the flat surface of the brick.

b Heat the metal from above using the hottest part of a roaring Bunsen flame just beyond the blue cone. Once the metal is on fire, remove the Bunsen flame. You should be able to observe the classic red of a lithium flame. (You may initially see a yellow flame, but this is the burning of any oil which was not removed.)

c Once the metal has stopped burning, test the residue with damp indicator paper and show that it is alkaline.

d Repeat for sodium and potassium.
 

Heating the metals in chlorine
a Prepare gas jars on chlorine in advance using a chlorine generator. See Standard Techniques: Generating collecting and testing gases.

b Check that the mouths of the gas jars of chlorine are narrower than the brick to reduce the amount of escaping gas, and that the colour of the gas in the jar is green. If it is not then there is not enough chlorine present for the demonstration to be successful.

c Starting with lithium, cut a small cube with an edge of about 3 mm. Blot off any excess oil. Place it on the clean, dry brick.

d Heat the piece of metal from above using the Bunsen burner (see diagram below). When the metal is burning, take away the Bunsen, invert the gas jar, remove the lid and immediately place over the burning metal. It helps to have a second pair of hands to do this. The metal continues to burn, producing fumes of white chloride. This method avoids producing FeCl₃ or CuCl₂, which can occur when a combustion spoon, or deflagration spoon made of iron or brass, is used.

e Repeat for sodium and potassium.

·         Heating sodium in chlorine Reproduced from CLEAPSS Guide: L195 'Safer Chemicals, Safer Reactions', section 9.3, by permission of CLEAPSS®

·         Heating sodium in chlorine Reproduced from CLEAPSS Guide: L195 'Safer Chemicals, Safer Reactions', section 9.3, by permission of CLEAPSS®

 Teaching notes

When heating in both air and chlorine, the expected pattern of lithium being the least reactive through to potassium being the most reactive may not be observed as it is hard to see potassium burning without the Bunsen flame. This may well be due to the potassium reacting faster than the others and an oxide coating being formed almost as soon as you begin to heat it.

In air:

4Li (s) + O₂ (g) → 2Li₂O (s)

Sodium and potassium produce a mixture of oxides, peroxides and superoxides.

In chlorine:

2Na (s) + Cl₂ (g) → 2NaCl (s) and similarly for the others.

The typical flame colours for lithium (red) and sodium (yellow) can usually be seen and sometimes the lilac of the potassium flame.