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Alkali Metal Reactivity
In this dramatic demonstration, lithium, sodium, and potassium react with water to produce hydrogen gas and the hydroxides of the metals. Lithium reacts fairly slowly, fizzing. Sodium reacts more quickly, generating enough heat to melt itself and to occasionally ignite the hydrogen gas, producing a yellow-orange flame characteristic of sodium. The potassium reacts violently, immediately bursting into a flame which has the characteristic violet color of potassium. If desired, phenolphthalein or universal indicator may be added to the water to indicate the basic nature of the hydroxide product. The solution changes color as the metals react to form their hydroxides. These are all soft metals that can be cut with a lab spatula revealing a shiny surface characteristic of metals, but they all corrode quickly on exposure to air. They get softer and more reactive as you move down the period from lithium to sodium to potassium.
Curriculum Notes
This demonstration is good for illustrating the chemical characteristics of metals, reactivity of Group 1A metals trends, Ionization Energy Trend. This demo can be tagged to the "Indicator Cylinders" demo for the discussion of acid and base anhydrides. Allow about 5-6 minutes for the presentation of this demonstration. Allow about 8 minutes for the demonstration, clicker questions, and active learning group work.
A sample presentation is available to download on the menu.
One day of lead time is required for this project.
Discussion
The reaction of alkali metals with water is represented by the following equation:
$\ce{2 M(s ~or~ l) + 2 H2O(l) -> 2 MOH(aq) + H2(g)}$
Where M is the alkali metal.
The heat given off by this reaction immediately melts the sodium and potassium and is frequently sufficient to ignite the hydrogen gas produced:
$\ce{2 H2(g) + O2(g) -> 2 H2O(g)}$
The increasing vigor of the reaction as you move down the Group IA elements is caused by the decreasing ionization energy of the metals.
Element | First Ionization Energy (kJ/mol) |
Li | 520 |
Na | 496 |
K | 419 |
Going down the Group 1A elements from Li to K, the outermost electron is further away from the nucleus and the overall net force of attraction between the protons in the nucleus and the outermost electron decreases. (Effective Nuclear Charge, Zeff, and Coulomb's Force law). The ionization energy decreases making it is easier to remove electrons, making the element more reactive.
Materials
- three vials containing, respectively, small samples of lithium, sodium, and potassium immersed in mineral oil
- 3 ea. large glass cylinders about one-quarter filled with deionized water
- 3 ea. 100 mm Petri dish covers
- long forceps
- a square piece of wire gauze
- paper towels
- a spatula and Petri dish for dividing any samples that might seem to be too large
- dropper bottle of phenolphthalein solution or universal indicator (optional)
- waste bottle and funnel
Procedure
If desired, place a few dropperfuls of phenolphthalein or universal indicator solution in the water. Using forceps, remove a small piece of lithium from the vial in which it is immersed in mineral oil. Being careful not to touch the sample, wipe off the mineral oil with a paper towel. Again using forceps, drop the lithium into the cylinder containing a little water and immediately place the wire gauze screen over the mouth of the cylinder. When the reaction is complete, cover the glass cylinder with a Petri dish cover to prevent caustic alkali oxide vapors from escaping. Repeat this procedure in the other glass cylinders with sodium and then potassium using pea-sized samples.
Safety Precautions
All alkali metals are strong reducing agents that react strongly with water, including the water in your skin! Wear safety goggles. Avoid touching the metal samples. The reactions of sodium and potassium with water can be quite vigorous. Be sure to place the wire gauze over the mouth of the cylinders immediately to avoid spattering. Have an ABC fire extinguisher at hand to extinguish secondary fires, but do not attempt to extinguish an alkali metal fire using an ABC fire extinguisher. Instead, have a container of dry sand nearby to extinguish metal fires. When the hydrogen gas released by these reactions ignite, which it frequently does during these reactions, it produces a dense, white, caustic cloud of metal hydroxide. Avoid breathing these fumes. Cover each cylinder as soon as the reaction is complete to contain any caustic alkali hydroxide vapors.
Prep. Notes
Set up as in photo. When you break down this demo, be sure to pour the waste solution into the waste bottle before transporting it, because the cylinder can tip over very easily.