# Identifying Reaction Types

I need help on identifying precipitation, acid-base, and redox reactions. I know what each one is in theory:

• Precipitation: Creating a solid
• Acid-Base: neutralization
• Redox: exchange of electrons

But given a chemical reaction like: HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq)

I need to understand how to classify them. Are there any tell-tale signs to look out for when determining. For example, does a precipitation reaction ALWAYS produce a (s) as a product and therefore any reaction with a (s) on the product side is a precipitation reaction?

• They are fairly simple concepts, check your textbook again, or Wikipedia. – Alex Jun 12 '13 at 0:33

It's true that a precipitation reaction will always yield at least one solid product, but not every reaction that yields a solid is, strictly speaking, properly categorized as a precipitation reaction. Precipitation reactions are ones in which at least one of the reactants is in the aqueous phase (i.e., dissolved in water), and a solid forms on the product side which was not present on the reactant side. If all the reactants are solid, then a solid product forming should not be called a precipitate. To be called a precipitate, an insoluble product must form from within solution, either from a solid and solute interacting, or from an interaction strictly between solutes.

Examples of precipitation reactions:

$\ce{AgNO3_{(aq)} + NaCl_{(aq)} -> AgCl_{(s)} + NaNO3_{(aq)}}$

$\ce{CuCl2_{(aq)} + Zn_{(s)} -> ZnCl2_{(aq)} + Cu_{(s)}}$

Notice that at least one of the reactants was in the aqueous phase, and a new solid chemical species is produced on the product side. $\ce{AgCl_{(s)}}$ and $\ce{Cu_{(s)}}$, respectively, are the precipitates. Incidentally, the second reaction is also an example of a redox reaction, since it involved a change in the oxidation states of the reactants (here, zinc is oxidized while copper is reduced).

Identifying acid-base and redox reactions is a lengthier topic, so I suggest you consult a textbook or any of the numerous online resources.

• so a reaction can be more than 1 type? In the 2nd example, it is both a precipitation and redox. – Richard Jun 12 '13 at 0:40
• @Richard, yes, categories of chemical reactions are not necessarily mutually exclusive, and there are countless reactions that can be reasonably classified in more than one way. Combustion reactions, for example, are also redox reactions. Combination and decomposition reactions may or may not be redox reactions. Which term is used depends on context. – Greg E. Jun 12 '13 at 0:44