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Ionic Equations

Ions are formed when atoms or molecules lose or gain electrons; therefore, ions have an overall positive or negative charge.

If an atom or a molecule loses an electron 🡪 positively charged ion

If an atom or a molecule gains an electron 🡪 negatively charged ion

As we already know, solution results from a solute that is dissolved in a solvent. In other terms, we can state that when a compound that is composed of ions is dissolved in liquid, we get a solution. After dissolving in liquid, these ions will separate out. 

Ionic equations can be written for any reaction involving aqueous substances (ions in a solution). 

Ionic equations are similar to ordinary chemical equations, but in this case, electrolytes (aqueous substances) are written in the form of dissociated ions. 

Considering the properties to conduct the electricity, substances are divided into two main groups:

  1. Electrolytes – substances that conduct electricity and completely ionize in an aqueous solution;
    - Common examples are – salts, acids, bases, etc. 
  2. Nonelectrolytes – substances that DO NOT conduct electricity and, therefore, do not dissociate into ions in an aqueous solution.
    - Typical examples are – DI water, ethanol, sugar (C6H12O6 – glucose), some other organic compounds, etc. 

Electrolytic dissociation, also referred to as ionic dissociation, occurs when an electrolyte is dissolved in aqueous solution (meaning that it is dissociated into ions). Since water is a polar molecule, it is considered as a dipole. 

Even though we denote ions as X+ or Y-, we have hydrated ions in the solution, meaning that water molecules surround the ions. 

Let’s consider the simple dissociation of an acid:

HCl (aq) + H2O (aq) 🡪 H3O+ (aq) + Cl- (aq)

In reality, we have H3O+ in a solution as a result of ionic dissociation of hydrochloric acid, but for simplicity, we just write H+ instead of H3O+.

Let’s consider dissociation of different substances:

  • Dissociation of acids 

Acids consisting of 1 hydrogen atom dissociate completely in one step:
HCl (aq)  🡪 H+ (aq) + Cl- (aq)

Acids consisting of 2 or more hydrogens dissociate in steps
Step I: H2SO4 (aq)  🡪 H+ (aq)  + HSO4- (aq)
Step II: HSO4- (aq)  🡪 H+ (aq)  + SO42- (aq)
Overall dissociation: H2SO4 (aq) 🡪 2H+ (aq)  + SO42- (aq)

  • Dissociation of bases

Soluble bases (alkali bases) dissociate releasing metal cations and hydroxyl anions
NaOH (aq)  🡪 Na+ (aq)  + OH- (aq)
Ca(OH)2 (aq) 🡪 Ca2+ (aq)  + 2OH- (aq)

Insoluble bases DO NOT dissociate

  • Dissociation of salts

Only soluble salts dissociate into ions
KNO3 (aq) 🡪 K+ (aq) + NO3- (aq)
NaHCO3 (aq) 🡪 Na+ (aq)  + H+ (aq) + CO3- (aq)
NaH2PO4 (aq) 🡪 Na+ (aq) + 2H+ (aq) + PO42- (aq)
CuOHCl (aq) 🡪 Cu2+ (aq) + OH- (aq) + Cl- (aq)
Al(OH)2NO3 (aq) 🡪 Al3+ (aq) + 2OH- (aq) + NO3- (aq)

Ionic Equations

As we already discussed in the article, we can write an ionic equation for every reaction which involves aqueous reactants.

We should define several terms before we continue with specific ionic equations.

The molecular equation is a balanced chemical equation that involves ionic compounds but in forms of molecules. 

The complete ionic equation is a balanced chemical equation that involves component ions of the ionic compounds. 

The net ionic equation is a shortened version of the complete ionic equation since we omit the ions that are unaltered on both sides of the equation. 

Spectator ions are ions that have excluded from the net ionic equation. 

As we discussed the major definitions, let’s consider several specific examples of ionic equations.

Sample Problem

Write a molecular, complete ionic, and net ionic equation for the reaction between hydrochloric acid and sodium hydroxide. Determine the spectator ions. 

Step 1: Determine the molecular formulas for the given substances and expected products to write the molecular equation.

HCl (aq) + NaOH (aq) 🡪 NaCl (aq) + H2O (l) 🡨 balanced molecular equation

Step2: Write the complete ionic equation

H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) 🡪 Na+ (aq) + Cl- (aq) + H2O (l)      🡨 complete ionic equation

Step 3: Exclude the ions that remain unaltered on both sides of the equation

H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) 🡪 Na+ (aq) + Cl- (aq) + H2O (l)      

H+ (aq) + OH- (aq) 🡪 H2O (l) 🡨 net ionic equation

Step 4: determine the spectator ions by stating the ones that have been omitted from the net ionic equation

Na+ and Cl- 🡨 spectator ions

Sample Problem

Write a molecular, complete ionic, and net ionic equations for the reaction between calcium chloride and sodium carbonate. Determine the spectator ions. 

Following the same instructions as in the previous problem we get:

Molecular equation:

CaCl2 (aq) + Na2CO3 (aq) 🡪 CaCO3 (s) + 2NaCl (aq)

Complete ionic equation

Ca+ (aq) + 2Cl- (aq) + 2Na+ (aq) + CO32- (aq) 🡪 CaCO3 (s) + 2Na+ (aq) + 2Cl- (aq)

Net ionic equation

Ca+ (aq) + CO32- (aq) 🡪 CaCO3 (s)

Spectator ions

Na+ and Cl-

NOTE

We can only dissociate the aqueous compounds. If any of the products is in liquid, solid, or gaseous form, we leave the compound as it is. 

For example, as we did in the last two sample problems.

We have not dissociated liquid water into ions; rather, we wrote H2O (l).

We have not dissociated solid CaCO3 into ions since it was precipitated after the reaction. So, we simply wrote CaCO3 (s).

Summary

All of the key terms and concepts discussed in the article are summarized in the table below:

Electrolytessubstances that conduct electricity and completely ionize in an aqueous solution
Nonelectrolytessubstances that DO NOT conduct electricity and, therefore, do not dissociate into ions in an aqueous solution
Electrolytic dissociationoccurs when an electrolyte is dissolved in aqueous solution.
Ionic dissociationsame as electrolytic dissociation
The complete ionic equationbalanced chemical equation that involves component ions of the ionic compounds
The net ionic equationshortened version of the complete ionic equation since we omit the ions that are unaltered on both sides of the equation.
Spectator ionsions that have excluded from the net ionic equation.