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Episode 4 - Ice

Hand Warmers
by Mike Bullivant

Exothermic and endothermic chemical reactions
Heat from cold wood ashes
Slaking Lime
The Thermite reaction
Making your own hand warmer
Further reading and websites

preparing limeChallenge
To make a safe, portable form of heating to take to the rest of the team working up on the glacier.

Exothermic and endothermic chemical reactions
When a chemical reaction takes place, chemical bonds in the reactants are broken and new bonds are formed. Depending on the nature of the bonds involved, a reaction will either release energy to, or absorb energy from, the surroundings as heat. When a process releases energy into the surroundings it's called exothermic (from the Greek thermo meaning heat, and exo meaning outside). The most obvious effect of an exothermic chemical reaction is a rise in the temperature of the reaction mixture. However, some processes absorb heat energy from the surroundings, in which case they're called endothermic reactions (endo in Greek means within). The effect of an endothermic chemical reaction is a fall in the temperature of the reaction mixture.

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Mike igniting aluminiumAll combustion reactions (such as the burning of coal) are exothermic. Incredibly, the reaction between iron and moist air that produces rust is a very exothermic process and generates lots of heat. Unfortunately, this particular reaction takes place so slowly that the liberation of heat is undetectable. Fireworks, explosives and fuels, on the other hand, all involve very fast and extremely exothermic chemical reactions.

For our hand warmer, do we need to use an exothermic process or an endothermic one? To generate heat and release it to the surroundings, we need an exothermic process, and it has to be suitable for use in a hot pack for warming the hands.

What properties do you think are necessary for a suitable chemical reaction for use in a hand warmer?

Well, the reaction must be portable and easily reproducible. It must generate and maintain a temperature that's neither too hot nor too cold. It must also be safe, and not involve the use of hazardous chemicals.

preparation of lime, rust and aluminiumWe decided to try three different exothermic processes to see if any met the above criteria:

Heat from cold wood ashes
Slaking lime
The thermite reaction

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Heat from cold wood ashes
The first 'reaction' that we tried for our hand warmer involved dissolving potassium carbonate (K2CO3) in water. Potassium carbonate is one of the chemicals left behind in the ashes of burnt wood. You can extract it from the ashes by boiling them in water and filtering off any undissolved solids. The potassium carbonate dissolves in the water, along with some other salts. Reducing the aqueous solution down by evaporating much of the water, then filtering it, will leave a clear solution that contains mostly potassium carbonate. If you evaporate nearly all the water, solid potassium carbonate will eventually fall out of solution as it becomes increasingly concentrated. It can then be filtered off and dried.

When we dissolved our dry, solid potassium carbonate in water, it only raised the temperature by a few degrees. Therefore the process wasn't suited to our purposes. We considered our alternatives, which would involve a chemical reaction of some sort.

Mike checking temperature of limeAlthough dissolving potassium carbonate (K2CO3) in water is an exothermic process, it isn’t a chemical reaction, because no chemical bonds are broken or formed as it takes place. But it generates heat, so can be described as exothermic.

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Slaking lime
The second exothermic process we tried for our hand warmer involved the 'slaking' of quicklime, which involves a chemical reaction or two. We took ordinary chalk (calcium carbonate, CaCO3) and heated it to between 2000 and 2500°F (1200 - 1400°C). At these temperatures, the calcium carbonate releases carbon dioxide gas (CO2) and is converted into quicklime (calcium oxide, CaO). When you add water to the quicklime, a process called 'slaking,' a vigorous reaction takes place and lots of heat is generated. The product of that reaction is a compound called 'slaked' lime (calcium hydroxide, Ca(OH)2).

Although aqueous solutions of 'slaked' lime are alkaline, its solubility in water is so low that the solution would not be a caustic hazard (causing chemical burns) used in a hand warmer. Given the right amount of lime, the amount of heat generated would be suitable for our purposes. The reaction is also portable and easily reproducible. But first, let's try one more exothermic reaction.

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igniting of materialsThe thermite reaction
The third process we tried was a chemical reaction called the thermite reaction (spelled thermit in the U.K.). This is a vigorous and highly exothermic reaction, which was used in the past to weld railway lines together. We managed to generate enough heat in our thermite reaction to weld two pieces of steel plate. That means the temperature generated by our reaction was higher than 2900°F (1600°C)!

The thermite reaction involves the reduction of certain metal oxides by aluminum (Al) powder. (Originally, reduction involved the chemical removal of oxygen from, or the addition of hydrogen to, a compound. Reduction is more accurately defined as a process in which atoms, molecules, or ions gain electrons.) We obtained aluminum powder by filing down old soda cans. We chose to use iron(III) oxide, or Fe2O3, known more commonly as rust. We got rust off an old corrugated iron shed at the sawmill. When we mixed our aluminum powder and rust in the right proportions, and kick-started the reaction, a glowing mass of molten iron was formed — along with an amazing amount of heat and light.

Well, this was a little too vigorous a reaction to be used in making hand warmers, so we decided to stick to slaking lime.

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rust and aluminiumMaking your own hand warmer
The best reaction for use in our hand warmers was the 'slaking' of lime. It generated enough heat for our purposes, and involved chemicals (water, calcium oxide and calcium hydroxide) that are comparatively safe and readily available. The reaction is also portable and easily reproducible.

TO MAKE YOUR OWN HAND WARMER YOU WILL NEED:

2 sealable plastic bags (approximately 4 inches square)
1/2 ounce (14 grams) powdered lime (calcium oxide)
2 tsp. (10 cc's) of tap water
a teaspoon
a plastic pipette or eye dropper
a bucket of cold water

Before you try the following experiment, make sure that you are wearing protective glasses and latex or rubber gloves. Do NOT handle these chemicals with your bare hands. Don't use a nylon apron or gloves.

NOTE: Be careful! When mixing the contents you may find they become so hot that they can cause serious burns.

Take a small, self-sealing plastic bag and put it inside another bag of the same type and size. Using the teaspoon, carefully add the half ounce (14 grams) of powdered lime to the inner bag, and use the pipette or eye-dropper to add about 2 teaspoons (10 cc's) of tap water. Seal both bags securely, and mix the contents together by carefully manipulating the powder and the water with your fingers. After a few seconds, you will notice that the temperature of the hand warmer starts to rise dramatically.

team with handwarmersWhen we took our slaked-lime hand warmers to the rest of the team on the Franz Josef Glacier, we found that so much heat was generated that some of the plastic bags actually melted. If this happens when you try the experiment, drop the bags into a bucket of cold water and dispose of the resulting solution by flushing it down the toilet. Be sure to wash your hands immediately and thoroughly in lots of cold running water.

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Further reading and web sites:
Metals and Chemical Change, edited by David Johnson, published by The Open University and the Royal Society of Chemistry (2002), ISBN 0-85404-665-8

Hampden - Sydney College
An educational site looking at the various uses of lime.

Wikipedia: Chemical Reactions
This article from Wikipedia, the Internet's free encyclopedia, explains a variety of chemical reactions, including exothermic and endothermic reations.

Washington University in St Louis
This site provides an experiement for an exothermic reaction.

University of Siegen - Germany
Selection of experiments of Thermite reactions

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