Expt:Alkali metals

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Introduction

These demonstrations show the similarity of the physical and chemical properties of the alkali metals and the trend in reactivity down Group 1 of the Periodic Table.

Apparatus and chemicals

• Goggles or a face shield for the demonstrator, eye protection for the audience

All experiments

• Tweezers or forceps
• Filter paper
• Beaker (150 cm³)
• Ceramic tile
• Scalpel or sharp knife to cut the metals
• Small bottles of oil containing small pieces of:

• Lithium (Highly flammable, Corrosive), 5 mm cubes (see note 1)
• Sodium (Highly flammable, Corrosive), 4 mm cubes (see note 1)
• Potassium (Highly flammable, Corrosive), 3 mm cubes (see note 1)
• Ethanol (Highly flammable) or Industrial denatured alcohol (IDA) (Highly flammable, Harmful), 150 cm³ (see note 2)
• 2-methylpropan-2-ol (Highly flammable, Harmful), 150 cm³ (see note 2)

Experiment 1

• Protective gloves (preferably nitrile)
• Small iron nail
• Power supply (~6 V)
• Light bulb in holder (for testing conductivity)
• Electrical leads
• Petri dishes, with lids
• Adhesive tape (to seal petri dishes)

Experiment 2

• One or more large glass troughs (5 dm³ capacity)
• Safety screens (2, at least)
• Glass or Perspex sheets to cover troughs (optional)
• Detergent, 1 drop
• Universal indicator solution (Highly flammable)

Technical notes

Lithium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 58

Sodium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 88

Potassium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 76

Ethanol (Highly flammable) Refer to CLEAPSS® Hazcard 40A

Industrial denatured alcohol (Highly flammable, Harmful) Refer to CLEAPSS® Hazcard 40A

2-methylpropan-2-ol (Highly flammable, Harmful) Refer to CLEAPSS® Hazcard 84B

Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and Recipe card 36

1. A technician should prepare the pieces of metal and store them under oil. Using the tweezers, remove a large piece of the alkali metal from the oil. Ensure that conditions are dry. Place the metal on a tile and, using a scalpel or sharp knife, cut pieces of lithium (5 mm cubes), sodium (4 mm cubes) and potassium (3 mm cubes). Place the small pieces in separate bottles of oil, labelled with the metal name and the hazard symbol. Cut each alkali metal separately and return the larger piece to its bottle before starting the next one.
2. Place any apparatus used to cut (and later handle) the metal (filter paper, scalpels etc) in a trough of water after use. Small pieces of alkali metal for disposal should be allowed to react fully with ethanol (for lithium and sodium) or 2-methylpropan-2-ol (for potassium) until fizzing stops, before washing away with water.

Procedure

Health & Safety

Demonstrator to wear goggles or a face shield. Pupils to be 2 – 3 m away and wearing eye protection.

Procedure

Experiment 1: Physical properties

Do the following with a small piece of each metal, lithium, sodium and potassium

A. Remove the metal from the oil bottle with tweezers.

B. Cut the metal with a scalpel lithium is the hardest to cut). Place any pieces that will not be used back into the oil.

C. Wearing protective gloves, squeeze the remaining metal in your gloved hand to show its softness. Lithium is the hardest to mould.

D. Drop each metal from a height of a few centimetres on to a piece of filter paper on the bench. Compare with a piece of iron of about the same size. Note the gentle impacts that reveal that the metals have a low density.

E. Use the circuit to show that the metals conduct electricity well.

F. Place the piece of metal in a petri dish and pass around the class. They must not touch the piece of metal – tape the lid shut if necessary.

G. Dispose of the lithium and sodium in ethanol, and dispose of the potassium in 2-methylpropan-2-ol.

Experiment 2: Reaction with water

A. Fill the trough(s) about half-full of water. Add a drop of detergent (to stop the metals sticking to the side). Place enough Universal indicator solution into each, with stirring, until the colour is clearly visible.

B. At least two safety screens should be used, as close to the troughs as possible. Alternatively use glass plates to cover the trough(s).

C. Ensure that stock bottles are closed (and no metal comes into contact with splashed water) before the reaction begins.

D. Ensure that the students cannot pick up pieces of the alkali metals.

E. Using dry tweezers, remove a small piece of lithium from the stock bottle, place the lithium on a filter paper and close the bottle. Use the filter paper to wipe off the oil.

F. Using tweezers drop the piece of metal on to the water surface in a trough (and place the cover over the trough).

G. Repeat with a piece of filter paper floated on the surface of the water, placing another piece of lithium on the paper.

H. In both cases, the lithium should float and fizz – giving off hydrogen. The water in the trough turns alkaline. The lithium does not catch fire.

I. Use a fresh trough (or fresh water) for the next metal. Note that the reaction heats up the water.

J. Repeat with sodium. This time the piece on the water floats, moves around more vigorously and fizzes. It may catch fire if it sticks to the side of the trough. On the filter paper it will only catch fire. It burns with a yellow flame and gives off a white smoke. CARE – when sodium and potassium catch fire, they give off corrosive oxides as smokes. The students must be far enough away not to breathe these (or the troughs should be covered).

K. Replace the water and repeat with potassium. Do not use the filter paper. Be careful as potassium is very reactive. The potassium moves around vigorously, melts, fizzes and catches fire, burning with a lilac flame.

L. Be careful to ensure that no traces of the metals remain on the tweezers, filter paper etc. The simplest way is to place them in one of the troughs of water to react. Small pieces of metals can be disposed of in ethanol (for lithium and sodium) or 2-methylpropan-2-ol (for potassium) and the residue flushed down the sink with when all reaction has ceased.

Reference

This experiment has been reproduced from Practical Chemistry: http://practicalchemistry.org/experiments/alkali-metals,155,EX.html

Useful resource

www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/alkali-reac.html, explains simply order of reactivity