Types of chemical reactions

Drop a copper wire into silver nitrate solution and silver crystals grow on it while the liquid turns blue: copper has swapped places with silver.

There are millions of known reactions, but most of them follow a handful of patterns. Learning the patterns lets you predict products before you see them. Some textbooks sort reactions by what happens to the formulas: things joining, splitting or swapping. Others sort them by what happens to particles: solids forming, protons moving, electrons moving. The two systems overlap, and it helps to know both.

Joining and splitting

In a synthesis, or combination, reaction two or more substances join to make one product: A + B → AB. Nitrogen and hydrogen combine to make ammonia, N₂ + 3H₂ → 2NH₃, the industrial route to ammonia. Iron and oxygen combine into iron(III) oxide. A decomposition reaction runs the other way, one substance breaking into two or more: AB → A + B. Hydrogen peroxide slowly decomposes into water and oxygen, 2H₂O₂ → 2H₂O + O₂, and limestone heated in a kiln decomposes into lime and carbon dioxide.

Swapping partners

In a single replacement reaction an element takes the place of another element in a compound: A + BC → AC + B. Copper metal in silver nitrate solution is the classic case, Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. Copper atoms become blue copper ions in the solution and silver ions come out as metal. Magnesium in hydrochloric acid replaces hydrogen and fizzes: Mg + 2HCl → MgCl₂ + H₂.

In a double replacement reaction two compounds trade partners: AB + CD → AD + CB. Mix clear solutions of silver nitrate and sodium chloride and a white solid appears at once, because silver chloride does not dissolve in water: AgNO₃ + NaCl → AgCl + NaNO₃. A reaction that makes a solid from solutions like this is called a precipitation reaction. Chemists predict them with solubility rules, short lists of which ionic compounds dissolve and which do not.

Burning, neutralizing and moving electrons

A combustion reaction is a fuel reacting quickly with oxygen and giving off heat and usually light. When the fuel contains only carbon and hydrogen, or carbon, hydrogen and oxygen, burning it completely gives carbon dioxide and water, as with methane, propane and ethanol. A neutralization reaction is an acid reacting with a base to give a salt and water: HCl + NaOH → NaCl + H₂O. Antacid tablets work this way on stomach acid, and the acids and bases course covers it in detail.

Many of these patterns share a deeper feature: electrons move from one substance to another. When sodium reacts with chlorine, each sodium atom loses an electron and each chlorine atom gains one. Loss of electrons is called oxidation and gain is called reduction, and since one cannot happen without the other, such reactions are called oxidation-reduction or redox reactions. Combustion, rusting and single replacement are all redox reactions. Precipitation and neutralization are not, because no element changes its oxidation number.