ice2.txt
ice2.txt - - - - heat sources, Exothermic and endothermic chemical reactions
Heat from cold wood ashes
Slaking Lime
The Thermit(e) reaction
Making your own hand warmer
Further reading and websites
Challenge
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 to 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 generally 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
generally a fall in the temperature of the reaction mixture.
Back to top
All combustion reactions (e.g burning of coal) are exothermic. Incredibly, the
reaction between iron and moist air to produce rust is a very exothermic process
that generates lots of heat. Unfortunately, this particular reaction takes place
so slowly that the liberation of heat is undetectable. Pyrotechnics, explosives
and fuels, on the other hand, all involve very fast and hugely 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 one that?s 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 be able to maintain a temperature that?s neither too hot nor too cold. It
must also be safe, and not involve the use of hazardous chemicals.
We decided to try three different exothermic processes to see if any met the
above criteria:
Heat from cold wood ashes
Slaking lime
The thermit(e) reaction.
Back to top
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 off much of the water,
followed by filtering, will leave a clear solution containing predominantly
potassium carbonate. If you evaporate off 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, a temperature
rise of only a few degrees Celsius occured . The process wasn?t therefore
suitable for our purposes. We considered alternatives involving a chemical
reaction of some sort.
Although 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.
Back to top
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 1 200 and 1 400 ºC. 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 the reaction is a compound called ?slaked?
lime (calcium hydroxide, Ca(OH)2).
Although aqueous solucitons of 'slaked' lime are alkaline, its solubility in
water is so low that the solution would not represent a caustic hazard in a hand
warmer. Given the right amount of lime, the amount of heat generated would be
suitable for our purposes. The reaction?s also portable and easily reproducible.
But let?s try one more exothermic reaction.
Back to top
The thermit(e) reaction
The third process we tried was a chemical reaction called the thermit reaction (sometimes
spelt thermite). 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 thermit reaction to weld two pieces of steel plate. The temperature
generated by the reaction must therefore have been higher than 1 600 ºC.
The thermit reaction involves the reduction (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) of certain metal oxides by aluminium (Al) powder. We got
our aluminium powder by filing down old drinks cans. We chose to use iron(III)
oxide, or Fe2O3, known more commonly as rust. We got ours off an old corrugated
iron shed at the sawmill. When we mixed our aluminium 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.
This was a little too vigorous a reaction to be used in making hand warmers so
instead we decided to stick to the slaking of lime.
Back to top
Making 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?s also portable and easily reproducible.
TO MAKE YOUR OWN HAND WARMER YOU WILL NEED
2 sealable polythene bags (approx. 10cm × 10cm)
14g powdered lime (calcium oxide)
10cm3 of tap water
teaspoon
plastic pipette or eye dropper
bucket of cold water
Before you try the following experiment, make sure that you are wearing
protective glasses nd latex or rubber gloves. Do NOT handle these chemicals with
your bare hands. Don?t use a Nylon garment.
NOTE: Be careful: when mixing the contents you may find they become so hot, they
may cause serious burns.
Take a small, self-sealing polythene bag and place it inside another bag of the
same type and size. Using the teaspoon, carefully add 14g of powdered lime to
the inner bag, and use the pipette or eye-dropper to add about 10cm3 of tap
water to it. Seal up 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.
When 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 by them 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 plenty of cold running water.
Back to top
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
Materials and Energy, Book 2 of the Open University Course ST240 Our Chemical
Environment, The Open University (2000), ISBN 0 7492 51425
Hampden - Sydney College
An educational site looking at the various uses of lime.
Wikipedia Encyclopedia
A site looking at 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 Thermit reactions
Back to top
Heat from cold wood ashes
Slaking Lime
The Thermit(e) reaction
Making your own hand warmer
Further reading and websites
Challenge
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 to 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 generally 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
generally a fall in the temperature of the reaction mixture.
Back to top
All combustion reactions (e.g burning of coal) are exothermic. Incredibly, the
reaction between iron and moist air to produce rust is a very exothermic process
that generates lots of heat. Unfortunately, this particular reaction takes place
so slowly that the liberation of heat is undetectable. Pyrotechnics, explosives
and fuels, on the other hand, all involve very fast and hugely 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 one that?s 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 be able to maintain a temperature that?s neither too hot nor too cold. It
must also be safe, and not involve the use of hazardous chemicals.
We decided to try three different exothermic processes to see if any met the
above criteria:
Heat from cold wood ashes
Slaking lime
The thermit(e) reaction.
Back to top
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 off much of the water,
followed by filtering, will leave a clear solution containing predominantly
potassium carbonate. If you evaporate off 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, a temperature
rise of only a few degrees Celsius occured . The process wasn?t therefore
suitable for our purposes. We considered alternatives involving a chemical
reaction of some sort.
Although 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.
Back to top
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 1 200 and 1 400 ºC. 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 the reaction is a compound called ?slaked?
lime (calcium hydroxide, Ca(OH)2).
Although aqueous solucitons of 'slaked' lime are alkaline, its solubility in
water is so low that the solution would not represent a caustic hazard in a hand
warmer. Given the right amount of lime, the amount of heat generated would be
suitable for our purposes. The reaction?s also portable and easily reproducible.
But let?s try one more exothermic reaction.
Back to top
The thermit(e) reaction
The third process we tried was a chemical reaction called the thermit reaction (sometimes
spelt thermite). 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 thermit reaction to weld two pieces of steel plate. The temperature
generated by the reaction must therefore have been higher than 1 600 ºC.
The thermit reaction involves the reduction (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) of certain metal oxides by aluminium (Al) powder. We got
our aluminium powder by filing down old drinks cans. We chose to use iron(III)
oxide, or Fe2O3, known more commonly as rust. We got ours off an old corrugated
iron shed at the sawmill. When we mixed our aluminium 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.
This was a little too vigorous a reaction to be used in making hand warmers so
instead we decided to stick to the slaking of lime.
Back to top
Making 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?s also portable and easily reproducible.
TO MAKE YOUR OWN HAND WARMER YOU WILL NEED
2 sealable polythene bags (approx. 10cm × 10cm)
14g powdered lime (calcium oxide)
10cm3 of tap water
teaspoon
plastic pipette or eye dropper
bucket of cold water
Before you try the following experiment, make sure that you are wearing
protective glasses nd latex or rubber gloves. Do NOT handle these chemicals with
your bare hands. Don?t use a Nylon garment.
NOTE: Be careful: when mixing the contents you may find they become so hot, they
may cause serious burns.
Take a small, self-sealing polythene bag and place it inside another bag of the
same type and size. Using the teaspoon, carefully add 14g of powdered lime to
the inner bag, and use the pipette or eye-dropper to add about 10cm3 of tap
water to it. Seal up 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.
When 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 by them 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 plenty of cold running water.
Back to top
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
Materials and Energy, Book 2 of the Open University Course ST240 Our Chemical
Environment, The Open University (2000), ISBN 0 7492 51425
Hampden - Sydney College
An educational site looking at the various uses of lime.
Wikipedia Encyclopedia
A site looking at 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 Thermit reactions
Back to top
posted by Arise! at 4:59 PM
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