Ionic compounds can be between metals and nonmetals, metals and polyatomic ions, or multiple polyatomic ions. If you are unsure of a compound, look up the elements of the compound on the periodic table. [4] X Research source Net ionic equations apply to reactions involving strong electrolytes in water. [5] X Research source
Follow these rules in the order stated below: All Na+, K+, and NH4+ salts are soluble. All NO3-, C2H3O2-, ClO3-, and ClO4- salts are soluble. All Ag+, Pb2+, and Hg22+ salts are insoluble. All Cl-, Br-, and I- salts are soluble. All CO32-, O2-, S2-, OH-, PO43-, CrO42-, Cr2O72-, and SO32- salts are insoluble (with some exceptions). All SO42- salts are soluble (with some exceptions).
For example, in NaCl, Na is the positively charged cation because it is a metal while Cl is the negatively charged anion because it is a non-metal.
If you are in a standard chemistry course, you will likely be expected to memorize some of the most common polyatomic ions. Some common polyatomic ions include CO32-, NO3-, NO2-, SO42-, SO32-, ClO4-, and ClO3-. [8] X Research source There are many more and can be found in tables in your chemistry book or online. [9] X Research source
Write the number of atoms that comprise each compound on either side of the equation. Add a coefficient in front of elements that are not oxygen and hydrogen to balance each side. Balance the hydrogen atoms. Balance the oxygen atoms. Re-count the number of atoms on each side of the equation to make sure they are equal. For example, Cr + NiCl2 –> CrCl3 + Ni becomes 2Cr + 3NiCl2 –> 2CrCl3 + 3Ni.
If no state is provided for an element, use the state found on the periodic table. If a compound is said to be a solution, you can write it as aqueous, or (aq). If there is water in the equation, determine whether or not the ionic compound will dissolve using a solubility table. If it has high solubility, the compound will be aqueous (aq), if it has low solubility, it will be solid (s). If there is not water, the ionic compound is a solid (s). If the problem mentions an acid or a base, they will be aqueous (aq). For example, 2Cr + 3NiCl2 –> 2CrCl3 + 3Ni. Cr and Ni in their elemental forms are solids. NiCl2 and CrCl3 are soluble ionic compounds, therefore, they are aqueous. Rewritten, this equation becomes: 2Cr(s) + 3NiCl2(aq) –> 2CrCl3(aq) + 3Ni(s).
Solids, liquids, gases, molecular compounds, low solubility ionic compounds, polyatomic ions, and weak acids will not dissociate. The oxides and hydroxides with alkali or alkaline earth metals will dissociate completely. High solubility ionic compounds (use solubility table) and strong acids will ionize 100% (HCl(aq), HBr(aq), HI(aq), H2SO4(aq), HClO4(aq), and HNO3(aq)). [10] X Research source Keep in mind, although polyatomic ions do not dissociate further, if they are a component of an ionic compound they will dissociate from that compound.
In our example, NiCl2 dissociates into Ni2+ and Cl- while CrCl3 dissociates into Cr3+ and Cl-. Ni has 2+ charge because Cl has a minus charge, but there are 2 atoms of it. Therefore, it must balance the 2 negative Cl ions. Cr has a 3+ charge because it must balance the 3 negative Cl ions. Remember that polyatomic ions have their own specific charge. [11] X Research source
Solids, liquids, gasses, weak acids, and low solubility ionic compounds will not change state or separate into ions. Simply leave them as they are. Molecular substances will simply disperse in solution, so their state will change to (aq). Three exceptions that do not become (aq) are: CH4(g), C3H8(g), and C8H18(l). Continuing our example, the total ionic equation looks like this: 2Cr(s) + 3Ni2+(aq) + 6Cl-(aq) –> 2Cr3+(aq) + 6Cl-(aq) + 3Ni(s). When Cl is not in a compound, it is not diatomic; therefore, we multiplied the coefficient by the number of atoms in the compound to get 6 Cl ions on both sides of the equation.
Spectator ions do not participate in the reaction, but they are present. Finishing the example, there are 6Cl- spectator ions on each side that can be canceled out. The final net ionic equation is 2Cr(s) + 3Ni2+(aq) –> 2Cr3+(aq) + 3Ni(s). To do a check to see if your answer works, the total charge on the reactant side should equal the total charge on the product side in the net ionic equation.
