Net Ionic Equations

20 May, 2019

Objective: In this lesson, the student will learn how to identify and write net ionic equations.

Net ionic equations result from double displacement reactions in which free ions in aqueous solution react to form either a precipitate, a gas, a weak electrolyte, or water.

Strategy for Writing Net Ionic Equations

If one reactant is an acid, check for gaseous products (i.e., CO2, H2S, SO2, or NH3 gases).
If one reactant is a base, check for gaseous products (i.e., NH3 or CO2 gases).
An acid + a base produces a salt + water.
Check for precipitates using the activity series of metals and a solubility table.

Activity Series of Metals & Solubility Table

Remember!

All chemical equations must be balanced.
In double displacement reactions, the sum of ion charges on both sides of the equation must be balanced.

Example

Write a balanced net ionic equation for the reaction of aqueous solutions of sodium sulfide and hydrochloric acid.

Na2S (aq) + HCl (aq) → ?

If one reactant is an acid (HCl), check for gaseous products.

Na2S (aq) + HCl (aq) → H2S (g) + NaCl (aq)

Balance.

Na2S (aq) + 2 HCl (aq) → H2S (g) + 2 NaCl (aq)

The above is called a molecular equation: all reactants and products are expressed as compounds.

From the molecular equation, identify the ions in solution. Note that the H2S is a gas and is not in aqueous solution.

2 Na+ (aq) + S2- (aq) + 2 H+ (aq) + 2 Cl- (aq) → H2S (g) + 2 Na+ (aq) + 2 Cl- (aq)

The aqueous 2 H+ and S2- ions react to form H2S (g). The remaining aqueous 2 Na+ and 2 Cl- ions appear on both sides of the equation and are spectators. That is, they do not participate in the reaction and can be eliminated.

2 Na+ (aq) + S2- (aq) + 2 H+ (aq) + 2 Cl- (aq) → H2S (g) + 2 Na+ (aq) + 2 Cl- (aq)

After eliminating the spectator ions from the reaction, the net ionic equation is

S2- (aq) + 2 H+ (aq) → H2S (g)

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My tutoring experience started when I was an ‘A’ biology and chemistry student at County College of Morris and was hired to work at the College’s Science Resource Center as a peer tutor. Subsequently I won the CRC Press Chemistry Achievement Award in recognition of outstanding scholastic achievement in chemistry, graduated from the College with an A.S. in chemistry, and continued my education at Montclair State University, where I earned a B.S. in biochemistry. For those high school students planning on taking the AP Chemistry Exam and/or the SAT II Subject Test in Chemistry administered by the College Board, I offer extensive preparation on a very wide breadth of content. In addition, for those of you who are interested in becoming a chemistry teacher in New Jersey, I also offer tutoring in the Chemistry: Content Knowledge Praxis Test (5245) required by the State of New Jersey Department of Education. Chemistry and biology need not be esoteric, exotic hot-house plants!