Net Ionic Equations

Net Ionic Equations Let’s think back to the video that we watched that showed sodium chloride (salt) being dissolved – the sodium ions became separated from the chlorine ions and they dispersed in the water. This is called dissociation - separation into ions that occurs when a soluble ionic substance dissolves.

So if we dissolve salt in water we now have separate cations and anions floating around in the solution. This solution can now conduct electricity because the ions make a path for the electricity to flow through. Pure water is covalent and does not conduct electricity. An electrolyte is the name for a substance that conducts electricity in solution. A non-electrolyte does not. Electrolytes can be strong or weak.

Let’s look at a double replacement reaction:

AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)

If a compound is aqueous, that means that it is dissolved in water – if it is dissolved in water then it has dissociated. Let’s re-write this equation showing how the ions have separated (The AgCl is an insoluble solid, so it did not dissociate):

+ - + - + - Ag + NO3 + Na + Cl  AgCl + Na + NO3

This is called an ionic equation. Looking at it, we can see that some of the ions on the reactant side are the same as the ions on the product side. These are called spectator ions – they did not actually participate in the formation of a new compound. They just floated around and stayed dissolved. Let’s cross these spectator ions out to simplify the reaction: Ag+ + Cl-  AgCl

The final equation that we are left with here is called a net ionic equation. It shows only the ions that have actually reacted. This final equation should also be balanced by adding coefficients.

Let’s write the net ionic equation for this chemical reaction:

KI (aq) + Pb(NO3)2 (aq) ----> KNO3 (aq) + PbI2 (s)

First, let’s show the dissociation of all aqueous compounds:

+ - +2 - + - K + I + Pb + NO3  K + NO3 + PbI2

Now, let’s get rid of the spectator ions:

- +2 I + Pb  PbI2

- +2 Lastly, let’s balance the net ionic equation: 2I + Pb  PbI2

Let’s practice writing some more net ionic equations. Remember – only aqueous compounds dissociate! Solids, liquids, and gases do not. If water is a product it is a liquid, not aqueous.

1. CuCl2 (aq) + Al (s)  Cu (s) + AlCl3 (aq)

2. ZnI2 (aq) + NaOH (aq)  NaI (aq) + Zn(OH)2 (s)

3. KOH (aq) + HCl (aq)  KCl (aq) + H2O (l)

4. Pb (s) + AgNO3 (aq)  Ag (s) + Pb(NO3)2 (aq)