Classifying chemical reactions
Chemical reactions are changes in which new substances are formed. Magnesium
burning in oxygen to form magnesium oxide is a chemical change. On the
other hand, ice melting to form liquid water is a physical change because
ice and water are the same chemical substances.
What happens during chemical reactions?
chemical reactions the bonds between atoms or ions are broken, the atoms
rearranged and new substances formed. A chemical reaction always forms
one or more new substances.
Chemical changes can be identified by looking for changes and the release
or absorption of heat, i.e. the reaction vessel gets hot or cold. For
example, when magnesium, a silvery metal, burns, a brilliant white light
is seen, heat is released and a white powder is formed.
Evidence of chemical reactions
You cannot see the atoms and molecules breaking bonds and forming new
bonds, but you can see the results of these changes. A cake rising in
the oven, gas bubbles forming in an antacid drink or sausages burning
on a BBQ are all evidence of chemical change.
There are four key indicators to watch for in deciding if a chemical
reaction has occurred:
Although there are millions of different chemical
reactions, many occurring as complex sequences of events, there is some
pattern to the chemistry involved. Most simple chemical reactions can
be divided into four types based on the number of reactants and how the
reactant molecules and atoms behave.
The four types of chemicals reactions are:
- Decomposition reactions
-This reaction occurs when a single substance breaks down, or decomposes,
into two or more simpler substances. One example is electrolysis of
water when an electric current is being passed through water forming
hydrogen and oxygen gas.
- Synthesis reactions
- These are also known as combination or addition reactions. Synthesis
occurs when two or more simple substances combine to form one more complex
substance. Synthesis is the opposite to addition reactions. Rockets
are propelled by combining hydrogen and oxygen to form water!
- Single replacement reactions
- In these reactions, an atom or group of atoms replaces another atom
or group of atoms in a compound to form a new compound. Copper plating
a coin or knife blade is an example of single replacement.
- Double replacement reactions
- More complex than single replacement reactions, these reactions involve
two pairs of ions in solution swapping partners to form two new compounds.
Acid/base reactions are double replacement reactions.