Don’t Set That on Fire! You Won’t Get it Back!
How do you cook the perfect marshmallow?
Roasting marshmallows over an open fire is a favorite pastime for campers, outdoor
cookouts, and just gathering around a fire in the backyard. The trick is to get the
marshmallow a nice golden brown, without catching it on fire. Too often we are not
successful, and we see the marshmallow burning on the stick…and we have to start over.
Nonreversible reactions
Nonreversible reactions are exactly what they sound like: reactions that work one way,
but not the other. This is an easy way to tell a chemical change (a chemical reaction)
from a physical change (not a reaction). A good example is a combustion reaction, like
the burning marshmallow.
Combustion reactions
A combustion reaction is a reaction in which a substance reacts with oxygen gas,
releasing energy in the form of light and heat. Combustion reactions must involve O2 as
one reactant, and always produce water (H2O) as a product. Usually, the other product is
carbon dioxide.
Combustion reactions are exothermic, which means they release energy. Combustion of
hydrocarbons (like fossil fuels) are a very important way for us to get energy in things
like cars and propane grills. This is the reaction for combustion of propane:
C3H8 + 5O2  3CO2 + 4H2O
Think about it:
1. Give THREE examples of combustion reactions in everyday life.
2. What are two forms of energy that are released in combustion reactions?
3. Combustion of fossil fuels is an important source of energy in the world. Think about
what you already know about burning fossil fuels: what are some problems with this
source of energy? What are some alternatives?
4. Combustion reactions are NONREVERSIBLE. What does that mean, and why does
it matter?