You must learn that acids react with bases to form salts and water, a chemical reaction called neutralization. You must be able to predict names and formulas of salts that incorporate polyatomic ions using the same methods introduced for binary ionic compounds. You must be able to write balanced equations for neutralization reactions using inspection and your knowledge of the neutralization process..
Acids are compounds that dissolve in water to produce solutions containing hydronium ions, H3O+ (aq), and bases dissolve in water to produce hydroxide ions, HO- (aq). When an acidic solution and a basic solution are mixed, hydroxide ions react with hydronium ions to produce water molecules:
Abbreviating the hydronium ion as H+ (aq) makes it easier to count the water molecules produced.
This chemical process is called neutralization. What happens to the other ions that were present in the original solutions? Nothing! They are still separately solvated and moving independently, unaffected by the reaction going on around them. Let's look at the reaction between hydrochloric acid and sodium hydroxide:
Equations 1 and 2 represent the formation of the solutions of hydrochloric acid and sodium hydroxide, and equation 3 shows what happens when they are mixed. Notice that sodium ions and chloride ions appear on both sides of equation 3, called the total ionic equation. If we cancel out these spectator ions we are left with equation 4, the net ionic equation, which represents the chemistry of the neutralization reaction. If we represent the sodium ions and chloride ions as NaCl (aq), and write formulas for hydrochloric acid and aqueous sodium hydroxide, equation 3 becomes equation 5, which summarizes the overall reaction.
If we were to evaporate the water from the solution after this neutralization reaction, we would be left with solid sodium chloride, commonly known as table salt. In general, the term salt refers to any ionic compound that could be produced by a neutralization reaction. Salts contain metal cations, and anions that may be monatomic (like chloride) or polyatomic anions derived from oxyacids. The formula of a salt can be predicted from the known charges of the cation and anion as we have seen previously for binary ionic compounds. Sodium sulfate, for example, has the formula Na2SO4. When a formula includes more than one polyatomic ion, it is enclosed in parentheses with a subscript to indicate the relative number of ions. Magnesium nitrate is Mg(NO3)2. There are two nitrate ions (charge 1- each) for every Mg2+ ion. When necessary, we include the charge on the cation as part of the name. Mercury(II) acetate is Hg(C2H3O2)2.
Some salts form crystalline solids which include some water molecules as part of the structure. Such salts are called hydrates, and we include the water molecules in the formula using a dot and a coefficient. For example, copper(II) sulfate forms blue crystals with five water molecules per formula unit, so the formula is CuSO4.5H2O. Hydrates can be dried by strong heating, which drives the water molecules out of the crystal.
Equations for neutralization reactions are usually written in the form of equation 5 above, a summary using the formulas for the reactants and products. To be useful, a chemical equation must be balanced, i.e. for each element there must be the same number of atoms on both sides of the equation. This is because the law of conservation of mass requires that atoms are not created or destroyed during a chemical reaction.
One way to balance the equation for a neutralization reaction is to remember that the underlying chemical process is the reaction of hydrogen ions and hydroxide ions in a one to one ratio. We balance the overall equation by adding coefficients so that the numbers of hydrogen ions and hydroxide ions are equal.
Consider as an example the reaction of hydrogen bromide with barium hydroxide. Hydrogen ions from the acid will react with hydroxide ions from the base, and barium and bromide ions will be left in solution. The ionic compound made up of barium ions and bromide ions is barium bromide, which has the formula BaBr2 (remember that barium is a group IIA metal and forms Ba2+ cations). We first write an overall equation for the process using the correct formulas for all of the compounds involved:
We can then quickly balance the equation by recognizing that we have 2 moles of hydroxide ions from 1 mole of barium hydroxide, so we need 2 moles of hydrogen bromide and will get 2 moles of water:
Inspection shows us that the other elements, barium and bromine are also balanced. When balancing an equation, start with the correct formula for each compound and change the coefficients not the formulas.
Most equations that we will meet can be balanced by direct inspection, but for more complicated situations an understanding of the chemical processes going on in the reaction can help.