Net ionic equations are an important aspect of chemistry because they only represent the state of matter that changes in a chemical reaction. This equation is commonly used in redox reactions, double replacement reactions, and acid-base neutralization. There are three basic steps to writing a clean ionic equation: balancing the molecular equation, converting it to a fully ionic equation (how each type of substance exists in solution), and writing a clean ionic equation.
Step
Part 1 of 2: Understanding the Elements of Ionic Equations
Step 1. Know the difference between a molecular compound and an ionic compound
The first step to writing a net ionic equation is to identify the ionic compounds of the reaction. Ionic compounds are compounds that will ionize in aqueous solution and have a charge. Molecular compounds are compounds that never have a charge. These compounds are formed from two nonmetals and are often referred to as covalent compounds.
- Ionic compounds can be formed from metals and nonmetals, metals and polyatomic ions, or several polyatomic ions.
- If you are unsure of 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 aqueous solution. Thus, the compound will not dissolve into individual ions. You must identify the solubility of each compound before continuing with the rest of the equation. The following is a brief summary of the rules for solubility. Look up the solubility tables for more details and exceptions to these rules.
- Follow these rules in the order listed below:
- All salt Na+, K+, and NH4+ can dissolve.
- All salt NO3-, C2H3O2-, ClO3-, and ClO4- can dissolve.
- All Ag. salt+, Pb2+, and Hg22+ cannot dissolve.
- All Cl. salt-, Br-, and I- can dissolve.
- All CO. salts32-, O2-, S2-, OH-, PO43-, CrO42-, Cr2O72-, and SO32- insoluble (with a few exceptions).
- All salt SO42- soluble (with a few exceptions).
Step 3. Determine the cations and anions in a compound
A cation is a positive ion in a compound and is usually a metal. Anions are non-metallic negative ions in a compound. Some nonmetals can form cations, but metals will always form cations.
For example, in NaCl, Na is a positively charged cation because Na is a metal, while Cl is a negatively charged anion because Cl is a nonmetal
Step 4. Identify the polyatomic ions in the reaction
Polyatomic ions are charged molecules that are held together so tightly that they do not dissolve in chemical reactions. It is important to recognize polyatomic ions because they have a definite charge and do not split into their individual elements. Polyatomic ions can be positively or negatively charged.
- If you are taking a regular chemistry class, you will likely be asked to remember some of the most commonly used polyatomic ions.
- Some polyatomic ions include CO32-, NO3-, NO2-, SO42-, SO32-, ClO4-, and ClO3-.
- There are many other polyatomic ions and can be found in the tables in your chemistry book or online.
Part 2 of 2: Writing a Net Ionic Equation
Step 1. Balance the complete molecular equation
Before writing a clean ionic equation, you must first make sure that your original equation is actually equivalent. To balance an equation, you add coefficients in front of the compounds until the number of atoms for each element on both sides of the equation is the same.
- Write down the number of atoms that make up each compound on both sides of the equation.
- Add the coefficients in front of the non-oxygen and hydrogen elements to balance each side.
- Balance the hydrogen atoms.
- Balance the oxygen atoms.
- Count the number of atoms on each side of the equation to make sure that the number is the same.
- For example, Cr + NiCl2 CrCl3 + Ni to 2Cr + 3NiCl2 2CrCl3 + 3Ni.
Step 2. Identify the state of matter of each compound in the equation
Often, you can identify keywords in a problem that describe the substance of each compound. There are several rules to help you determine the substance of an element or compound.
- If the form of the substance of an element is not listed, use the form of the substance on the periodic table.
- If a compound is a solution, you can write it as aqueous or (aq).
- If there is water in the equation, determine if the ionic compound will dissolve or not using the solubility table. If the compound has a high solubility, the compound is aqueous (aq). If the compound has a low solubility, the compound is a solid (s).
- In the absence of water, the ionic compound is a solid (s).
- If the question mentions an acid or a base, this compound is aqueous (aq).
- For example, 2Cr + 3NiCl2 2CrCl3 + 3Ni. Cr and Ni in elemental form are solids. NiCl2 and CrCl3 It is a soluble ionic compound. Thus, both compounds are aqueous. If rewritten, this equation becomes: 2Cr(s) + 3NiCl2(aq) 2CrCl3(aq) + 3Ni(s).
Step 3. Determine what kind of compound will dissolve (separate into cations and anions) in solution
When a type or compound dissolves, it separates into positive (cations) and negative (anions) elements. These are the compounds that are balanced in the end for a net ionic equation.
- Solids, liquids, gases, molecular elements, ionic compounds with low solubility, polyatomic ions, and weak acids will not dissolve.
- Ionic compounds with high solubility (use solubility table) and strong acids will ionize 100% (HCl(I), HBr(I), HI(I), H2SO4(aq), HClO4(aq), and HNO3(aq)).
- Remember that although polyatomic ions are insoluble, if they were elements of an ionic compound, they would have dissolved from that compound.
Step 4. Calculate the charge of each dissolved ion
Remember that the metal will be the positive cation, while the non-metal will be the negative anion. Using the periodic table, you can determine which element will have how much charge. You must also balance the charges of each ion in the compound.
- In our example, NiCl2 dissolve into Ni2+ and Cl- while CrCl3 dissolves into Cr3+ and Cl-.
- Ni has a 2+ charge because Cl has a negative charge, but there are 2 Cl atoms. Thus, we must balance the 2 negative Cl ions. Cr has a charge of 3+ because we have to balance the 3 negative Cl ions.
- Remember that polyatomic ions have a certain charge of their own.
Step 5. Rewrite the equation with the soluble ionic compounds, broken down into their individual ions
Anything that is soluble or ionized (a strong acid) will separate into two different ions. The state of the substance will remain the same (aq), but you have to make sure that the equation remains equal.
- Solids, liquids, gases, weak acids, and ionic compounds with low solubility, will not change shape or separate into ions. Leave these substances alone.
- Molecules will dissolve in solution. So, the form of the substance will change to (aq). The three exceptions that do not become (aq) are: CH4(g), C3H8(g), and C8H18(l).
- Finishing our example, the total ionic equation would look like this: 2Cr(s) + 3Ni2+(I) + 6Cl-(I) 2Cr3+(I) + 6Cl-(I) + 3Ni(s). Although Cl is not a compound, it is not diatomic. Thus, we multiply the coefficient by the number of atoms in the compound to get 6 Cl ions on both sides of the equation.
Step 6. Eliminate the spectator ions by removing identical ions on each side of the equation
You can remove ions only if they are 100% identical on both sides (charge, small number at the bottom, etc.). Rewrite the reaction without the substance being removed.
- Completing the example, there are 6 Cl. spectral ions- on each side that can be removed. The net ionic equation finally is 2Cr(s) + 3Ni2+(I) 2Cr3+(I) + 3Ni(s).
- To check if your answer is correct, the total charge on the reactant side should equal the total charge on the product side in the net ionic equation.
