Laboratory:  Chemical Formulas and Equations

CH104  Introductory Chemistry Laboratory

Higginbotham

This lab activity is adapted from a book titled Chemical Activities, by Borgford and Summerlin.  The American Chemical Society published the book in 1988.  The activity is titled “Precipitation:  When Ions Find New Partners.” 

Objectives:

• Identify and use chemical symbols to correctly describe ionic precipitation reactions.

• Determine ion charge for main group elements using the periodic table.

• Determine ion charge in ionic compounds by deductive logic, when given counterion charges.

• Recognize and become familiar with polyatomic ions.

• Apply an understanding of ion charge to determine correct formulas for ionic compounds.

Introduction:  During this activity you will gain practice looking at chemical reactions, writing formulas, naming inorganic compounds, and describing chemical changes with equations. 

Your first challenge today is to figure out which of the compounds you have in front of you react with one another when mixed.  In all of the reactions you will see today, the product of such reactions will be an insoluble solid.  The formation of this precipitate indicates a reaction has occurred.

Once you know which substances react, you then need to write chemical equations to describe those reactions.  The reactions we do today are all of one type, called double displacement reactions.  In each, the cations and anions of the reactants switch places, forming new pairings.  It's a bit like changing dance partners.  Cations remain cations but end up with a new, different anion partner.

Writing proper formulas for the products of these reactions can be challenging, especially when you are new at this.  The practice you get today is intended to help you get over the difficulties and get you to a place where it doesn't seem difficult. 

You will be combining solutions containing dissolved ionic substances below.  Chart out your chemical tests in an organized table that provides a space to record the result of the test (reacted or not, plus a brief description of the precipitate if it did react).  There are somewhere between 15 and 20 possible tests:  you figure out just how many you have to do to hit every possible combination.  Note that anytime you have two substances with the same cation (sodium, for instance) or anion, no reaction is possible because the ions will be unable to find a new partner.  Also, of course, a substance can not react with itself.

The table of reaction tests and results, and the written equations describing each reaction will constitute your lab report for this week.  I will scrutinize your equations to see that all formulas and equations are properly written, and that the equation syntax is correct.

Procedure:  To perform each test, you will need to place approximately 1 mL (20 drops) of the first solution to a test tube.  Then add about 10 drops of the second solution one drop at a time.  When/if you see the formation of a precipitate, you have witnessed a reaction.  You will be required to (a) find the correct formula for the ionic products, and (b) write and balance the entire equation for each reaction you witness. 

 Your tests should include every possible combination of the following substances, available to you in a dropper bottle as a solution in water: 

sodium sulfate     Na₂SO₄ 

sodium chloride     NaCl

silver nitrate         AgNO₃

sodium iodide         NaI

lead nitrate         Pb(NO₃)₂

ammonium carbonate  (NH₄)₂CO₃

calcium chloride     CaCl₂ 

The charges on polyatomic ions can be figured out by examining the cations they are paired with.  Ammonium (the polyatomic cation) has a charge of +1.  The silver cation also has a charge of +1.  Lead is +2.  You also can find polyatomic ions by name or formula, with charges, in any of several chemistry texts around the lab. 

If you run out of test tubes, clean them out with soap, rinse VERY thoroughly, and use them again.  Do not try to dry out the test tubes with paper towels—they often get stuck and will leave behind bits of paper.  Wet but clean is fine.

Precautions:

Label things carefully.  Test tubes containing lead or silver should have their contents dumped into the special waste jar in the lab.  Please do not pour these solutions down the drain.
 

Keep things clean.  You may want to use gloves when handling lead and silver compounds.  Silver nitrate stains skin.  The discoloration will not go away until it has worn off. 

Leave droppers with the bottles they belong to.  Hauling droppers around is a bad idea, since they may drip and they also could end up in the wrong bottle, thus contaminating the entire bottle of stuff.

Move dropper bottles rather than moving racks filled with test tubes.  Test tubes fall out of racks if the rack gets turned a bit sideways.

Data Sheet

Write BOTH word equations (with names of reactant and product compounds written out) and chemical formula equations for every reaction which produced a precipitate.  If you have questions about compound or formula names, ask before you leave the lab for the day.