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Periodic Trends: Ionization Energy

The minimum amount of energy required to remove an electron.

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How to Blow Up the Bathtub

How to Blow Up the Bathtub

Credit: Alan Light
Source: http://www.flickr.com/photos/42274165@N00/8439211800

Have you ever slipped in the tub and wanted to exact revenge?  Does one of your tub toys know too much and need to be “taken care of?”  Whatever your reasons, there are many possible ways to explode a bathtub.  One of the simplest ways (at least chemically) is to fill it up with water and drop in a small chunk of the appropriate metal.  However, not just any metal will do. Learn more about the sometimes violent reactions between alkali metals and water below.

Amazing But True!

• Alkali metals are highly reactive with oxygen. This is why pieces of the metal are often stored using mineral oils to prevent the oxidation of the metal.
• Credit: Tomihahndorf
Source: http://en.wikipedia.org/wiki/File:Lithium_paraffin.jpg

Lithium stored in paraffin [Figure2]

• The reactivity of various alkali metals with water is compared in the following video: http://www.youtube.com/watch?v=m55kgyApYrY
• For each metal, the first reaction that takes place is the generation of a metal hydroxide and hydrogen gas. Using sodium as an example, the balanced equation can be written as follows: 2Na(s)+2H2O(l)2NaOH(aq)+H2(g)\begin{align*}2 Na(s)+2H_2O(l) \rightarrow 2 NaOH(aq)+H_2(g)\end{align*}
• If enough heat is given off in a short period of time, the hydrogen gas can ignite, causing the following combustion reaction: 2H2(g)+O2(g)2H2O(g)\begin{align*}2 H_2(g) + O_2(g) \rightarrow 2 H_2 O(g)\end{align*}
• As you move down the periodic table, the reactions become progressively more violent.

Show What You Know

1. The standard heat of formation for aqueous LiOH is -508 kJ/mol, and the standard heat of formation for aqueous CsOH is -488 kJ/mol. Which would give off more energy after reacting completely with water, 1 gram of lithium or 1 gram of cesium?
2. Neutral alkali metals react with water (and most other substances) by giving up their single valence electron. Does it require more energy to remove an electron from lithium or cesium? Which ionization do you think occurs faster?
3. Based on your answers to the previous two questions, which seems to be more important for generating a big explosion: having a very exothermic reaction, or having a very fast reaction?
4. In the video above, the narrator says that they were “not allowed to have” any francium. Are there reasons other than its reactivity that francium might be difficult to obtain?

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