Several types of reactions occur in water. When water is the solvent for a reaction, the reaction is said to occur in aqueous solution, which is denoted by the abbreviation (aq) following the name of a chemical species in a reaction. Three important types of reactions in water are precipitation, acid-base, and oxidation-reduction reactions.
In a precipitation reaction, an anion and a cation contact each other and an insoluble ionic compound precipitate out of solution. For example, when aqueous solutions of silver nitrate, AgNO3, and salt, NaCl, are mixed, the Ag+ and Cl- combine to yield a white precipitate of silver chloride, AgCl:
Ag+(aq) + Cl-(aq) → AgCl(s)
For example, when hydrochloric acid, HCl, and sodium hydroxide, NaOH, are mixed, the H+ reacts with the OH- to form water:
H+(aq) + OH-(aq) → H2O
HCl acts as an acid by donating H+ ions or protons and NaOH acts as a base, furnishing OH- ions.
In an oxidation-reduction or redox reaction, there is an exchange of electrons between two reactants. The species that loses electrons is said to be oxidized. The species that gains electrons are said to be reduced. An example of a redox reaction occurs between hydrochloric acid and zinc metal, where the Zn atoms lose electrons and are oxidized to form Zn2+ ions:
Zn(s) → Zn2+(aq) + 2e-
The H+ ions of the HCl gain electrons and are reduced to H atoms, which combine to form H2 molecules:
2H+(aq) + 2e- → H2(g)
The overall equation for the reaction becomes:
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
Two important principles apply when writing balanced equations for reactions between species in a solution:
- The balanced equation only includes the species that participate in forming products. For example, in the reaction between AgNO3 and NaCl, the NO3- and Na+ ions were not involved in the precipitation reaction and were not included in the balanced equation.
- The total charge must be the same on both sides of a balanced equation. Note that the total charge can be zero or non-zero, as long as it is the same on both the reactants and products sides of the equation.