_{adapted by M. J Simpson from a lab originally by S. F. Sontum, K. Jewett and R. Sandwick}

Learning goals: work collaboratively with a lab partner, follow instructions to complete a chemistry experiment, collect experimental data, formulate logical conclusions based on experimental results

Introduction

Much as a cook uses recipes to guide her/him in preparing dishes, chemists use chemical equations as ways of expressing the results of chemical reactions.  Sometimes it is more descriptive to use the ions involved in a reaction rather than the ionic compounds. For example, to express that hydrochloric acid reacts with sodium hydroxide to give water and sodium chloride, we could write:

Molecular equation: HCl_(aq) + NaOH_(aq) → H₂O_(l) + NaCl_(aq)

The above equation is fine in some ways, but in actuality NaOH and HCl solutions do not contain NaOH and HCl molecules, and there are no NaCl molecules in the solution after the reaction occurs. The better description for this reaction is:

Total ionic equation: H⁺_(aq) + Cl^–_(aq) + Na⁺_(aq) + OH^–_(aq)  →  Cl^–_(aq) + Na⁺_(aq) + H₂O_(l)

Net ionic equation: H+_(aq) + OH-_(aq) → H₂O_(l)

The first equation is called the total ionic equation, while the second is termed the net ionic equation.  A net ionic equation summarizes the changes that have taken place as a result of a chemical reaction.

Background

The solubility of a substance in a solvent is the maximum amount of the substance that can be dissolved in a given amount of solvent. While there is no exact definition for the boundary between soluble and not soluble, a general guide might be:

SOLUBILITY                                      TERM

> 0.1 M                                                 Soluble

0.01 to 0.1 M                                       Moderately soluble

< 0.01 M                                              Slightly soluble

Some guidelines have been prepared to help estimate a compound’s solubility. These are written in a manner where statements above other statements have priority. For example, an alkali metal carbonate is soluble since “all alkali metals (#2) are soluble” have priority over “All carbonates are insoluble.” [There are exceptions that are not included here. (See textbook)]    

Solubility Rules:

1. All nitrates (NO₃-) and acetates (C₂H₃O₂-) are soluble.
2. All salts of alkali metals and NH₄+ are soluble.
3. All common halides (Cl-, Br^–, I^–) are soluble, except those of Ag⁺, Pb²⁺, Cu⁺, Hg₂²⁺
4. Hydroxides (OH-) are insoluble except for Na⁺, Ca²⁺, and Ba²⁺.
5. All sulfates (SO₄^2-) are soluble, except BaSO₄ which is slightly soluble, and CaSO₄ and Ag₂SO₄, which are moderately soluble.
6. All carbonates (CO₃^2-) and phosphates (PO₄^3-) are insoluble.

Procedure

Note: perform this experiment with a partner, but be sure to observe the tests together.

Make up the following standard solutions:

50.0 mL of 0.10 M KI_(aq) from KI_(s)

25.0 mL of 0.10 M CuCl_{2 (aq)} from CuCl₂ ● 2H₂O _(s)

50.0 mL of 0.10  M H₂SO_{4 (aq)} from 1.00 M H₂SO_4(aq)

25.0 mL of 0.10 M NaOH _(aq) from 1.00 M  NaOH_(aq)

50.0 mL of 0.10 M Na₂CO_{3 (aq)} from Na₂CO_3(s)

Three test solutions will be prepared for you:  0.10 M AgNO₃, 0.10 M BaCl₂, and 0.10 M Co(C₂H₃O₂) ₂.

To determine how to prepare a solution from a solution of higher concentration, use the formula: C_i V_i = C_f V_f

where:

C_i = initial concentration of higher concentration solution

V_i = initial volume of the higher concentration solution

C_f = final concentration (of the diluted solution)

V_f = final volume of the diluted solution (volume of the initial solution plus water)

Tests for chemical reactions:

For each of the solutions you prepared, test each solution with the following test solutions:

0.10 M AgNO₃

0.10 M BaCl₂

0.10 M Co(C₂H₃O₂) ₂

0.10 M Na₂CO₃ (the solution you prepared)

A test involves taking 5 mL of your solution and adding 3 – 5 drops of the test solution. Make observations in the table on the worksheet.

Things to note:

(a)  a change of color;

(b)  a separate phase is formed;

(c)  any other change.

Report

Complete the worksheet for your lab report. Optional survey.

The following procedure may help you when writing a net reaction for any chemical reaction.

1. Determine the principal forms present of each reactant in solution.
2. Determine the principal forms present after the reaction has occurred.
3. Write a balanced equation for the reaction.
4. Cross out reactants and products that do not change (the spectator ions).

Example:  Mix HNO₃ solution + KOH solution

Principal substances before reaction:  H+, NO₃-, K+, OH-

Principal substances after reaction: NO₃-, K+

Product formed:  H₂O

Net reaction:  H+_(aq) + OH-_(aq) → H₂O _(l)