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 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 drop in
the temperature of the reaction mixture. Both physical and chemical changes can be endothermic or exothermic.
Changes of state involve a solid melting, a liquid freezing, a liquid boiling, or a gas condensing. When steam condenses,
heat is released. Likewise when liquid water freezes, heat is given off. Our experience makes it easy for us to realize that
to boil water or any liquid and thus convert into a gas, heat is required and the process is endothermic. It is less easy to
see that when a gas condenses to a liquid, heat is given off and the process is exothermic.
Like phase changes, chemical reactions can occur with the addition or release of heat. Those that require heat to occur
are described as endothermic, and those that release heat as exothermic. We are probably very familiar with exothermic
chemical reactions: Almost everyone has experienced the warmth of a fireplace or campfire. Burning wood provides
heat through the exothermic chemical reaction of oxygen (O) with wood (C6H10O5) to produce carbon dioxide (CO),
steam (H2O) and heat. The chemical reaction describing the process is C6H10O5 + 6O2 → 6CO2 + 5H2O + heat. All
combustion reactions (such as burning fuel) are exothermic. The reaction between iron and moist air that creates rust is
a very exothermic process and produces lots of heat, but this reaction takes place so slowly that the release of heat is
undetectable. Fireworks, explosives and fuels, on the other hand, all involve very fast and extremely exothermic
chemical reactions.
An easy way to tell if a reaction is
endothermic is if it feels cold. This
doesn’t always happen, though; anytime
you cook something, you are performing
an endothermic reaction. This is because
you have to input heat to make the
chemical reaction occur.
Exothermic Processes
making ice cubes
formation of snow in clouds
condensation of rain from water vapor
a candle flame
rusting iron
burning sugar
forming a chemical bond
Endothermic Processes
melting ice cubes
conversion of frost to water vapor
evaporation of water
baking bread
cooking an egg
producing sugar by photosynthesis
breaking a chemical bond
Guidelines for Graphic Notes
The notes are “graphic”…meaning they MUST contain both WRITTEN information AND DRAWINGS.
They will go on pg _____ of your notebook.
1. Keep your drawings simple! They need to be easy to understand, neat (not sloppy) and in color.
2. Make sure to include:
 Definitions for endothermic and exothermic reactions
 At least 3 examples of each type of reaction (great place for pictures “hint, hint”)
 The obvious effects of each type of reaction (in other words, how can you tell if it was an exothermic or
endothermic reaction?)
3. Use color appropriately: Use red for your vocab and definitions, and black or blue ink for all other content/key
facts.
Sources:
http://www.pbs.org/weta/roughscience/series3/ice/handwarmers.html
http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermic-examples.shtml
http://www.scientificamerican.com/article/what-is-an-exothermic-rea/