Net ionic equations are a very important aspect of chemistry, as they only represent entities that are changed within a chemical reaction. Normally, this type of equation is used for the chemical redox reactions (in jargon simply called 'redox reactions'), double exchange and acid-base neutralization The main steps to obtain a net ionic equation are three: balance the molecular equation, transform it into a complete ionic equation (specifying for each chemical species how it exists in solution), obtain the net ionic equation.
Steps
Part 1 of 2: Understanding the Components of a Net Ion Equation
Step 1. Understand the difference between molecules and ionic compounds
The first step in obtaining a net ionic equation is to identify the ionic compounds involved in the chemical reaction. Ionic compounds are those that ionize in an aqueous solution and possess an electrical charge. Molecular compounds are chemical compounds that do not have an electrical charge. Binary molecular compounds are characterized by two non-metals and are sometimes also referred to as 'covalent compounds'.
- Ionic compounds can consist of: elements belonging to metals and non-metals, metals and polyatomic ions or multiple polyatomic ions.
- If you are unsure of the chemical nature of the compound, research the elements that compose it within the periodic table.
- The net ionic equations apply to reactions involving strong electrolytes in water.
Step 2. Identify the degree of solubility of the compound
Not all ionic compounds are soluble in an aqueous solution and therefore not dissociable in the single ions that compose it. Before proceeding further, you must therefore identify the solubility of each compound. Below, or find a brief summary of the main solubility rules of a chemical compound. For more details on this and to identify exceptions to these rules, refer to the graphs relating to the solubility curves.
- Follow the rules described in the order in which they are proposed below:
- All Na salts+, K+ and NH4+ they are soluble.
- All salts NO3-, C2H.3OR2-, ClO3- and ClO4- they are soluble.
- All Ag salts+, Pb2+ and Hg22+ they are not soluble.
- All salts Cl-, Br- and I.- they are soluble.
- All CO salts32-, OR2-, S2-, OH-, BIT43-, CrO42-, Cr2OR72- and SO32- they are not soluble (with some exceptions).
- All SO salts42- they are soluble (with some exceptions).
Step 3. Determine the cations and anions present in the compound
Cations represent the positive ions of the compound and are generally metals. Conversely, anions represent the negative ions of the compound and are normally non-metals. Some non-metals are capable of forming cations, while the elements belonging to metals always and only generate cations.
For example, in the NaCl compound, sodium (Na) is the positively charged cation because it is a metal, while chlorine (Cl) is a negatively charged anion because it is a non-metal
Step 4. Recognize the polyatomic ions present in the reaction
Polyatomic ions are electrically charged molecules tightly bound together that do not dissociate during chemical reactions. It is very important to recognize these elements since they have a specific charge and do not break down into the individual elements of which they are made up. Polyatomic ions can be both positively and negatively charged.
- If you are taking a standard chemistry course, you will most likely have to try to memorize some of the more common polyatomic ions.
- Some of the more well-known polyatomic ions include: CO32-, NO3-, NO2-, SO42-, SO32-, ClO4- and ClO3-.
- Obviously there are many others; you can find them in any chemistry book or by searching the web.
Part 2 of 2: Writing a Net Ion Equation
Step 1. Balance the molecular equation completely
Before you can write a net ion equation, you need to be sure you are starting with a fully balanced equation. To balance a chemical equation, you need to add the coefficients of the compounds until all the elements present in both members reach the same number of atoms.
- Note the number of atoms of each compound in both sides of the equation.
- Add a coefficient to each element, other than oxygen or hydrogen, to balance both sides of the equation.
- Balance the hydrogen atoms.
- Balance the oxygen atoms.
- Recount the numbers of atoms in each member of the equation again to make sure they are the same.
- For example, the Cr + NiCl equation2 CrCl3 + Ni becomes 2Cr + 3NiCl2 2CrCl3 + 3Ni.
Step 2. Identify the state of matter for each compound in the equation
Often, within the text of the problem, you will be able to identify keywords that will indicate the state of matter of each compound. However, there are some useful rules for determining the status of an element or compound.
- If no status is provided for a given element, use the status shown in the periodic table.
- If the compound is described as a solution, you can refer to it as an aqueous solution (aq).
- When water is present in the equation, determine whether or not the ionic compound is soluble using a solubility table. When the compound has a high degree of solubility, it means that it is aqueous (aq), on the contrary if it has a low degree of solubility, it means that it is a solid compound (s).
- If there is no water in the equation, the ionic compound in question is solid (s).
- If the problem text refers to an acid or a base, these elements will be aqueous (aq).
- Take for example the following equation: 2Cr + 3NiCl2 2CrCl3 + 3Ni. Chromium (Cr) and nickel (Ni), in their elemental form, are solid. The ionic compounds NiCl2 and CrCl3 they are soluble, so they are aqueous elements. By rewriting the example equation, we will get the following: 2Cr(s) + 3NiCl2 (aq) 2CrCl3 (aq) + 3Ni(s).
Step 3. Determine which chemical species will dissociate (ie separate into cations and anions)
When a species or compound dissociates, it means that they split into their positive (cations) and negative (anions) components. These are the components we will need to balance to get our net ionic equation.
- Solids, liquids, gases, molecular compounds, ionic compounds with a low degree of solubility, polyatomic ions and weak acids do not dissociate.
- Oxides and hydroxides with alkaline earth metals completely dissociate.
- Ionic compounds with a high degree of solubility (use the solubility tables to identify them) and strong acids ionize at 100% (HCl(aq), HBr(aq), HI(aq), H2SO4 (aq), HclO4 (aq) well no3 (aq)).
- Remember that although polyatomic ions do not dissociate, if they are a component of an ionic compound, they will dissociate from it.
Step 4. Calculate the electric charge of each of the dissociated ions
Remember that metals represent positive ions (cations), while non-metals represent negative ones (anions). Using the periodic table of elements, you can determine the electric charge of each element. You will also need to balance the charge of each ion present within the compound.
- In our example equation, the element NiCl2 dissociates into Ni2+ and Cl-, while the component CrCl3 dissociates into Cr3+ and Cl-.
- Nickel (Ni) has a 2+ electric charge because it has to balance chlorine (Cl) which, despite having a negative charge, is present with two atoms. Chromium (Cr) has a 3+ charge because it has to balance the three negative chlorine ions (Cl).
- Remember that polyatomic ions have their own specific charge.
Step 5. Rewrite your equation so that the soluble ionic compounds present are broken down into the individual constituent ions
Any element that dissociates or ionizes (strong acids) will simply separate into two distinct ions. The state of matter will remain aqueous (aq) and you will need to be sure that the equation obtained is still balanced.
- Solids, liquids, gases, weak acids and ionic compounds with a low degree of solubility do not change state and do not separate into the single ions that constitute them; then simply leave them as they appear in their original form.
- Molecular substances in solution simply disperse, so in this case their state will become aqueous (aq). There are 3 exceptions to this last rule, in which the state of matter does not become aqueous in solution: CH4 (g), C3H.8 (g) and C8H.18 (l).
- Continuing with our example, the complete ionic equation should look like this: 2Cr(s) + 3Ni2+(aq) + 6Cl-(aq) 2Cr3+(aq) + 6Cl-(aq) + 3Ni(s). When chlorine (Cl) does not appear in a compound, the latter is not diatomic, so we can multiply the coefficient by the number of atoms that appear in the compound itself. In this way, we get 6 chlorine ions in both sides of the equation.
Step 6. Remove the ions called "spectators"
To do this, delete all identical ions present in both sides of the equation. You can only cancel if the ions are 100% identical in both sides (electric charge, subscript, etc.). When the deletion is complete, rewrite the equation omitting all the removed species.
- The spectator ions do not participate in the reaction, however they are present.
- In our example, we have 6 spectator ions of Cl- in both sides of the equation which can then be eliminated. At this point, the final net ion equation is as follows: 2Cr(s) + 3Ni2+(aq) 2Cr3+(aq) + 3Ni(s).
- To verify the work done and be sure of its correctness, the total charge on the reactive side of the net ionic equation should be equal to the total charge on the product side.
