Net ionic equations are an important topic in chemistry as they serve to represent only entities that undergo a change in a chemical reaction. They are typically used in redox reactions, double displacement reactions, and acid-base neutralizations. To write a net ionic equation, you must follow three basic steps: balance the molecular equation, transform it into a complete ionic equation (how each species exists within the solution), and then write the net ionic equation.
Steps
Part 1 of 2: Understanding the Components of an Ionic Equation
Step 1. Learn the difference between ionic and molecular compounds
The first step in writing a net ionic equation is to identify the ionic compounds in the reaction. Ionic compounds are those that ionize in an aqueous solution and have a charge. Molecular compounds are compounds that are never charged. They are made of two nonmetals and are often referred to as covalent compounds.
- Ionic compounds can be formed between metals and non-metals, metals and polyatomic ions, or between various polyatomic ions.
- If you are in doubt about a compound, look up the elements of that compound on the periodic table.
Step 2. Identify the solubility of a compound
Not all ionic compounds are soluble in an aqueous solution and therefore will not dissociate into individual ions. Before continuing with the rest of the equation, you need to identify the solubility of each compound. Below you will see a brief summary of the solubility rules. To find more information and to see exceptions to the rule, look at a solubility table.
- Follow the rules that appear below, respecting their order:
- All Na salts+, K+, and NH4+ salts are soluble.
- All salts NO3-, C2H3OR2-, ClO3-, and ClO4- they are soluble.
- All Ag salts+, Pb2+, and Hg22+ they are insoluble.
- All salts Cl-, Br-, and I- they are soluble.
- All CO salts32-, OR2-, S2-, OH-, PO43-, CrO42-, Cr2OR72-, And so32- they are insoluble (with some exceptions).
- All salts SO42- they are soluble (with some exceptions).
Step 3. Determine the cation and anion in a compound
Cations are the positive ions of a compound and are generally metals. Anions are the negative ions of a compound and are nonmetals. Some non-metals are capable of forming cations, but metals always form cations.
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 nonmetal
Step 4. Recognize the polyatomic ions in a reaction
Polyatomic ions are charged molecules that bond so tightly that they cannot disassociate during chemical reactions. It is important to recognize polyatomic ions as they have a specific charge and do not separate into individual components. Polyatomic ions can be both positively charged and negatively charged.
- If you are going to study these topics for a standard chemistry class, you will be asked to memorize some of the most common polyatomic ions.
- Some of the most common polyatomic ions are CO32-, NO3-, NO2-, SO42-, SO32-, ClO4-, and ClO3-.
- There are many more and you can find them in tables in your chemistry book or online.
Part 2 of 2: Writing a Net Ionic Equation
Step 1. Balance the entire molecular equation
Before writing a net ionic equation, you must make sure that your initial equation is completely balanced. To balance an equation, you add coefficients in front of the compounds to equal the number of atoms of each element on both sides of the equation.
- Write the number of atoms that each compound comprises on both sides of the equation.
- Add a coefficient in front of the elements other than oxygen and hydrogen to balance both sides.
- Balance the hydrogen atoms.
- Balances the oxygen atoms.
- Count the number of atoms on both sides of the equation again to make sure it is exactly the same.
- For example, Cr + NiCl2 CrCl3 + Ni would become 2Cr + 3NiCl2 2CrCl3 + 3Ni.
Step 2. Identify the states of matter for each compound in the equation
It is often possible to identify keywords in the problem that will tell you what the state of matter is for each compound. There are some rules that can help you determine the state of an element or compound.
- If they don't give you the state of an element, use the state found on the periodic table.
- If they tell you that a compound is a solution, you can assume that its state is 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 no water, the ionic compound is a solid (s).
- If the problem mentions something about acids or bases, then it is aqueous (aq).
- For example, 2Cr + 3NiCl2 2CrCl3 + 3Ni. Cr and Ni in their elemental form are solid. NiCl2 and CrCl3 they are soluble ionic compounds, and therefore they are aqueous. The rewritten equation will be as follows: 2Cr(s) + 3NiCl2 (aq) 2CrCl3 (aq) + 3Ni(s).
Step 3. Determine which species will dissociate (separate into cations and anions) in the solution
When a species or compound dissociates, it separates into its positive (cations) and negative (anions) components. These will be the components that balance out at the end for the net ionic equation.
- Solids, liquids, gases, molecular compounds, low solubility ionic compounds, polyatomic ions, and weak acids will not dissociate.
- Highly soluble ionic compounds (see solubility chart) and strong acids will ionize 100% (HCl(aq), HBr(aq), HI(aq), H2SW4 (aq), HClO4 (aq), and HNO3 (aq)).
- Keep in mind that, although polyatomic ions do not continue to dissociate, when they are components of an ionic compound, they do dissociate from that compound.
Step 4. Find the charge of each dissociated ion
Remember that metals will be the cation (positive), while nonmetals will be the anion (negative). Using the periodic table you can determine which element will have charge. You must also balance the charges of each ion with the compound.
- In the example above, NiCl2 dissociates into Ni2+ and Cl- while CrCl3 dissociates into Cr3+ and Cl-.
- Ni has a +2 charge because Cl has a negative charge, but they are two atoms of that element. Therefore, you must balance the two negative ions of Cl. Cr has a 3+ charge because it must balance the 3 negative ions of Cl.
- Remember that polyatomic ions have their own specific charge.
Step 5. Rewrite the equation with the soluble ionic compounds separated into their individual ions
Anything that dissociates or ionizes (strong acids) will simply separate into two different ions. The state of matter will still be (aq) but you have to make sure the equation is still balanced.
- Solids, liquids, gases, weak acids, and low solubility ionic compounds will not change state or separate into ions, so just leave them as is.
- Molecular substances will only disperse in solution, so their state will change by (aq). There are only three exceptions that no will become (aq): CH4 (g), C3H8 (g), and C8H18 (l).
- Continuing with the example, the total ionic equation will be as follows: 2Cr(s) + 3Ni2+(aq) + 6Cl-(aq) 2Cr3+(aq) + 6Cl-(aq) + 3Ni(s). When Cl is not a compound, it is not diatomic, and therefore the coefficient is multiplied by the number of atoms in the compound to obtain 6 Cl ions on both sides of the equation.
Step 6. Eliminate the spectator ions by canceling the identical ions on both sides of the equation
You can only cancel if they are 100% equal on both sides (also the loads, subscripts, etc.). Rewrite the action excluding the canceled species.
- To end the example, there are 6Cl- Spectator ions on each side that you can cancel. The final net ionic equation is 2Cr(s) + 3Ni2+(aq) 2Cr3+(aq) + 3Ni(s).
- To check and see if the answer is correct, the total charge on the reactant side must equal the total charge on the product side in the net ionic equation.