It is important that you know the basic types of chemical reactions for the SAT II Chemistry test since the test often refers to reactions as being of one type or another. Here’s a list of the different types of reactions, with examples of each type included. Synthesis reaction: This is a reaction in which two or more elements or compounds combine to form a single product. This type of reaction follows the general equation where A and B may be either elements or compounds. Here are some examples:
2Na(s) + Cl2(g)2NaCl(s)
MgO(s) + H2O(l)Mg(OH)2(aq)
SO2(g) + H2O(l)H2SO3(aq)
Decomposition reaction: In this type of reaction, a single reactant, a compound, breaks into two or more parts. Often these are the most difficult to predict. Here is the general equation:
ABA + B
where A and B may be either elements or compounds. Here are some examples of decomposition reactions:
2H2O(l)2H2(g) + O2(g)
H2CO3(aq)H2O(l) + CO2(g)
CaCO3(s)CaO(s) + CO2(g)
2KClO3(s)2KCl(s) + 3O2(g)
Single replacement or displacement reaction: In this type of reaction, a more active element replaces a less active element in a compound. Among the halogens, F2 is the most active halogen, and the activity of the halogens decreases as you go down the group. For the metals, you will need to be given an activity series. General equation:
A + BCAC + B
where A is a metal. Here is an example of a displacement reaction in which a metal is involved:
Cu(s) + 2AgNO3(aq)2Ag(s) + Cu(NO3)2(aq)
General equation:
A + BCBA + C
where A is a nonmetal. Here is an example of a displacement reaction where a nonmetal is involved:
Cl2(g) + 2NaI(aq)2NaCl(aq) + I2(s)
Double replacement or displacement reaction: In this type of reaction, two compounds react to form two new compounds. The formation of a molecular compound such as water, the formation of a gas, or the formation of a precipitate usually drives these reactions. Here’s the general equation:
AB + CDAD + CB
And here are a couple of examples:
Pb(NO3)2(aq) + 2KI(aq)2KNO3(aq) + PbI2(s)
HCl(aq) + NaOH(aq)H2O(l) + NaCl(aq)
Combustion reaction: In this type of reaction, often a hydrocarbon is burned in the presence of oxygen gas to form carbon dioxide (in a complete combustion) or carbon monoxide (in an incomplete combustion, due to a limited amount of oxygen). Here is the general equation in the presence of plenty of oxygen:
CxHy + O2(g)CO2(g) + H2O(l) or (g)
An example of this is seen when methane gas is burned in the presence of excess oxygen (Bunsen burner reaction):
CH4(g) + 2O2(g)CO2(g) + 2H2O(g)
Here is the general equation for when a hydrocarbon is burned in an incomplete combustion (oxygen is in limited supply):