http://www.ck12.org Chapter 11. Chemical Reactions
FIGURE 11.6
Mass changes for steel wool burning in
airIntroduction
MEDIA
Click image to the left for more content.The video above at http://www.youtube.com/watch?v=_Y1alDuXm6A (1:12) shows the decomposition of mer-
cury(II) oxide into liquid mercury and oxygen gas. This reaction was an important one in the history of chemistry,
because it helped early chemists to understand the relationship between reactants and products.
In the last lesson, we began investigating how a chemical equation can represent a given chemical reaction. In this
lesson, we are going to study the ways in which chemical reactions are classified. There are literally thousands
of chemical reactions that take place every day in our lives. Some reactions take place in the atmosphere, such
as the combustion of fossil fuels. Others occur in solution, like the reactions responsible for photosynthesis or the
reactions that break down our food to give us energy. Chemical reactions can take place in a variety of environments.
Reactions happen on the sea floor, in our cells, and in the upper atmosphere. As we look at chemical reactions, we
notice some commonalities and trends. When we studied the elements, we saw characteristics that allowed us to
categorize them by family. There are also various ways to categorize chemical reactions. Some reactions produce
heat, while others consume it. Some reactions are spontaneous, while others are not. Some reactions happen in
nanoseconds, while others happen over longer spans of time. Some produce electricity, some emit light, and some
release gaseous products. The products of chemical reactions tell us a lot about the chemistry of the process. In the
above video, we see mercury(II) oxide decomposing into elemental mercury and oxygen gas. Decomposition was
one of the first reaction types to be identified by chemists. Decomposition is one type of reaction you’ll learn about
in this lesson.
Combination Reactions
The first type of reaction that we will investigate is thecombination reaction, which is sometimes also referred to
as a synthesis reaction. In combination reactions, two or more chemical species combine to produce a single new
compound. A generic combination reaction might have the following form:
A+B→C