With millions of chemicals to choose from, the number of chemical reactions that can be carried out is without limit. Faced with such possibilities, dealing with chemical equations can seem daunting. Matters are greatly simplified once you realize that there are a few basic types of reactions that represent most any reaction you are likely to encounter. The chemicals change, but the reactions follow similar patterns. We will discuss six, the same that I teach to my first year chemistry students. (This is not intended for organic chemists.)
Synthesis comes to us from Greek, by way of Latin, and literally means to “put together”. A synthesis reaction is one in which elements or small compounds are combined to make a more complex compound. The simplest synthesis reactions are easy to recognize because they have only one product. (More complex synthesis reactions may have one large product along with a smaller product.)
Sample synthesis reactions could include:
2 Na + Cl2 -> 2 NaCl
SO2 + H2O -> H2SO3
CH2CH2 + HBr -> CH3CH2Br
(this is an organic reaction, but still easy to recognize as synthesis)
Decomposition is roughly the opposite of synthesis it is a breaking down of one molecule into two (or more) smaller molecules or elements. These reactions are easy to recognize because they start with one relative large reactant and finish with multiple smaller products. Sometimes a catalyst, electricity, light or heat may be required to cause the decomposition, and they may be indicated in the reaction, either as a reactant or placed over the reaction arrow.
Sample decomposition reactions could include:
2 H2O + electricity -> 2 H2 + O2
CaCO3 -> CaO + CO2
3. Single Replacement
In a single replacement reaction, one element replaces a similar element in a compound. These reactions are easy to recognize because both sides of the equation will have a single element. In most single replacement reactions either a metal or a halogen is being replaced.
Sample single replacement reactions could include:
2 NaI + Br2 -> 2 NaBr + I2
AgCl + K -> KCl + Ag
ZnF2 + 2 Li -> 2 LiF + Zn
4. Double Replacement
Double replacement reactions are sometimes hard to recognize at first. A simpler name for them would be a “trade” or “swap”. Most often we are concerned with ionic compounds when discussing double replacement reactions, and the reaction takes place in aqueous solution. Under these conditions, all the ions are dissociated from one another and just floating around, bumping into one another in solution. No reaction actually takes place unless the new combination of anion and cation creates an insoluble compound. If that happens, a precipitate forms, falling out of solution as a solid, and leaving behind the remaining ions. In a chemical equation, all these details are usually left out, so you lack the most obvious clues [a down arrow indicates a precipitate, a subscripted (s) indicates a solid, and (aq) means aqueous dissolved in water]. The actual chemicals in the reaction should be clue enough, however. Look for ionic compounds where the two cations (the positive, usually metal ions) have traded anions (the negative ions) with one another. (It helps to ignore the number of ions in the formula.)
Sample double replacement reactions could include:
2 NaOH + CuSO4 -> Na2SO4 + Cu(OH)2
K3PO4 + Al(NO3)3 -> 3 KNO3 + AlPO4
There is one case to watch out for an acid-base reaction. When an acid (which has an H+ ion) reacts with a base (which has an OH- ion), one product is water. If chemists and teachers were kind, we would write water’s formula as HOH, depicting its nature as the product of an H+ and an OH- ion. Normally, we don’t. Instead, we write H2O, and expect you to remember that it can also be HOH. To illustrate, look at the reaction below. In the first case, it is obviously double replacement, but in the second, you may easily overlook the swap if you aren’t careful.
NaOH + HCl -> NaCl + HOH
NaOH + HCl -> NaCl + H2O
Being aware of this, you may want to get in the habit of rewriting water as HOH when asked to identify reaction types.
5. Oxidation-Reduction (Redox)
In an oxidation-reduction reaction, at least one element must be oxidized (lose electrons) and at least one element must be reduced (gain electrons). This net change in oxidation number (a method of counting electrons, basically) is all that is necessary for a reaction to be considered a redox reaction. Many synthesis, decomposition, and single replacement reactions are also redox reactions. Unless you are asked for multiple classifications, save the oxidation-reduction label for those reactions which do not fit the other categories. When you are looking at a reaction and it doesn’t fit one of the other categories, it is a good bet that it is redox. (Redox reactions can get ugly.) To be absolutely sure though, you’ll have to go through and assign oxidation numbers to the elements in the reaction and see if any oxidation numbers are different between the reactants and products. If they are, you’ve got yourself a redox reaction. If you aren’t sure how to assign oxidation numbers, you can refer to the Helium article “Assigning Oxidation Numbers”.
Sample oxidation-reduction reactions could include:
3 P4 + 10 KClO3 -> 6 P2O5 + 10 KCl
CH4 + 4 Cl2 -> CCl4 + 4 HCl
H2SO4 + 8 HI -> H2S + 4 I2 + 4 H2O
Combustion reactions are a special case of the oxidation-reduction reaction. The extra requirements are that the reaction produces a lot of heat (often as flame) and that oxygen is one of the reactants (and is reduced during the reaction). Much of the time, combustion reactions deal with the burning of hydrocarbons compounds made exclusively from hydrogen and carbon. When this is the case, the products are always carbon dioxide and water, unless otherwise indicated. (When there isn’t enough oxygen, carbon monoxide may be produced instead.) This makes a combustion reaction very easy to recognize, because it is usually “Something + O2 -> CO2 + H2O”.
Sample combustion reactions could include:
CH4 + 2 O2 -> CO2 + 2 H2O
(combustion of methane / natural gas)
2 C4H10 + 13 O2 -> 8 CO2 + 10 H2O
(combustion of butane)
2 C3H8O + 9 O2 -> 6 CO2 + 8 H2O
(combustion of isopropanol / rubbing alcohol)