To write ionic equations properly, these facts must be known because a reaction occurs only if a weak molecule or ion, a slightly soluble substance, or a volatile compound is formed.
| 1. | To determine if the substance (acid, base or salt) is strong or weak, use this summary. | |||||
| Acid | Base | Salt | ||||
| Soluble | ||||||
| a) | Write ionic form if strong | Most salts are strong | ||||
| H+ + Cl– | Na+ + OH– | Na+ + Cl– | ||||
| H+ + NO3– | K+ + OH– | Ag+ + NO3– | ||||
| 2H+ + SO2–4 | Ca+2 + 2OH– | Al+3 + 3NO3– | ||||
| b) | Write molecular form (undissociated) if weak | |||||
| Most acids are weak | ||||||
| HAc | H2S NH3 • H2O (weak base) | PbAc2 | ||||
| Soluble | ||||||
| c) | Write molecular form, underlined. | |||||
| H2Bo3 | Fe(OH)3 | AgCl | ||||
| d) | Water - Water is always in the molecular form (H2O). It is weak. | |||||
| 2. | To determine if a substance is soluble, or insoluble, use these steps: | ||
| a) | Soluble | All Na+, K+, NH4+, salts, all NO3– and Ac– salts. | |
| All Chlorides except those of Ag+, Hg2+2, Pb+2 | |||
| All Sulfates except Pb+2, Ba+2, (Ca+2, Hg2+2, Ag+ sparingly) | |||
| b) | Insoluble | All Carbonates and Phosphates except Na+, K+, and NH4+ | |
| All Hydroxides except Na+, K+, NH4+, (Ca+2 sparingly) | |||
| All Sulfides except Na+, K+, NH4+, Ba+2, Ca+2, Mg+2, Al+3 | |||
| 3. | Gases should always be written in molecular form: NO2, N2O, NO3, AsH3 | ||
| 4. | Complex ions are not dissociated: Cu(NH3)4+2, Co(NO2)63, Ag(NH3)2+ | ||
| Compounds formed from atoms with vastly different electron needs (expressed as electronegativities) are ionic in that the electron is no longer shared by both atoms but one atom has actually gained an electron and the other atom has lost an electron. Thus, in the solid state, such compounds are stabilized by electrostatic attractive forces (+ . . .–). Purely ionic compounds, as these are called, are always found as solids at normal temperatures and pressures. For such compounds to be soluble in water, the water must interact in some way to break the ion lattice. This energy needed to disrupt the ion lattice is called Energy of Solvation. Since energy can be translated to mean work, the Energy of Solvation may be considered to be the work needed to break the lattice. However, entropy effects must be taken into consideration. However, entropy is a measure of disorder and the measurement of it is not absolute. Only entropy differences can be measured. Thus all ionic solids, if they dissolve in water, do so to form solvated solutions of their ions. That is, they ionize completely. |
| Solvated ↓ |
| CuSO4 • 5H2Og • H2O = Cu • (H2O)6 + SO4-2 • (H2O)6 |
