AP Chemistry
Equation Writing – Helpful Tips
Before we get into classifying, predicting, writing, and balancing equations, it will be helpful to
collect some information that will make our task easier. Below are collections of tips that will
make equation writing easier. Please keep them in mind as you encounter various types of
reactions.
1. General Ideas
a. Compounds that are listed as “SOLID”, “LIQUID”, or “GAS” must be written in
molecular form and will appear in the final equation.
b. Phrases like “in solution”, “a solution of”, etc., mean the compound is dissolved
and has a chance to ionize. This also means there is a chance of spectator ions.
c. Ions like Na+, K+, and NO3- are almost always spectator ions and will not be in the
final equation (A redox reaction would be an exception to this).
d. Any free element present indicates a redox reaction.
e. Strong acids and strong bases are ALWAYS written in the ionized form.
Weak acids and weak bases are ALWAYS written in the molecular form.
Strong Acids – HCl, HBr, HI, and any polyatomic acid whose ratio of oxygen
atoms to ionizable hydrogen atoms is 2 or more.
Weak Acids – The remaining acids (HF and the majority of organic acids)
Strong Bases – All column IA hydroxides plus Sr(OH)2 and Ba(OH)2
Weak Bases – Ammonia and the amine family (RNH2, R2NH, R3N)
f. Watch for common complexing agents (NH3, OH-, SCN-, CN-, Cl-) included in a
reaction description such as “excess” or “concentrated”. This will point to a
complexation reaction.
2. Aqueous Reactions
a.
b.
c.
d.
Are free elements present? If so, expect a single replacement redox reaction.
Does a precipitate form? Know your solubility table.
Does a gas form? Look for CO2, SO2, NH3, or H2S
Is a weak acid or base a reactant or product. If so it must be written molecularly.
Strong acids and bases are written ionized.
3. Oxidation/Reduction Reactions
Most of this will be covered in the electrochemistry section. Occasionally it is needed to
predict products of nontrivial redox reactions by knowing the oxidation/reduction pairs.
Common Oxidizing Agents
Nitrate (nitric acid), NO3
Permanganate,
Reduced To
-
NO (dilute acid), NO2 (concentrated)
MnO4-
Mn+2 (acid)
Hydrogen peroxide, H2O2
Dichromate/chromate,
Cr2O72-,
H2O
2-
CrO4
-
+3
Cr (acid)
I-
Iodate, IO3
Standard state halogens
Halide ions
2-
Sulfate (sulfuric acid), SO4
SO2
Common Reducing Agents
Oxidized To
Hydrogen peroxide, H2O2-
O2
Sulfite, SO3-
SO42-
Hydrogen gas, H2
H+
Free Metals, Mo
M+
Iodide, I-
I2