Ions (1 day)

Purpose: To study the properties of ions in solution.

Introduction: Ionic solids dissolve in water to form ions, so their solutions conduct electricity. A solution contains two different kinds of ions: positive ions called cations (pronounced cat-ions) and negative ions called anions pronounced an-ions). The entire solution is electrically neutral.

When mixing solutions of ionic solids, certain combinations of ions form precipitates (insoluble solids which settle to the bottom of the container). The solution remaining above the precipitate (ppt) is called the supernatant. The ions which do not combine, but just remain in the supernatant are called “spectator ions.”

Water is nearly always the dissolving agent (solvent) for ionic solids, but it is not included as a term in the chemical equation. The substances only dissolve in it, not react chemically with it. What actually happens is that a water molecule attaches itself to each ion in solution. This is possible because water is a polar molecule; that is, there is a slight separation of charge within the neutral molecule. The oxygen end of the bent-shaped molecule has excess electrons, so that end is slightly negative, while the hydrogen end is slightly positive. When an ionic solid dissolves in water, the ions are separated by the water molecules.

The accurate way of writing an ion formula, then, is to write “(aq)” after it – (aq) means aqueous (pronounced a-kwi-us). In this course we will usually omit the (aq) for convenience, but always remember that each ion actually has a water molecule attached to it.

Example: NaCl dissolved in water forms Na⁺ (aq) + Cl^- (aq).

There are three different types of equations which may be used to represent reactions of ionic solids in solution:

Molecular equation (shows the molecular formula of each substance).

Pb(No₃)₂ + 2NaCl → PbCl₂ + 2NaNO₃

Ionic equation (shows each solid split into its ions, with a molecular formula only for insoluble solids – notice what happens to the coefficients)

Pb⁺⁺ + 2NO₃^- + 2Na⁺ + 2Cl^- → PbCl₂^↓ + 2Na⁺ + 2NO₃^-

Net ionic equation (shows only the ions involved in change – spectator

ions are omitted)

Pb⁺⁺ + 2Cl^- → PbCl₂^↓

IMPORTANT: In either kind of ionic equation, charges must balance on each side of the equation.

Of the three types of equations, the net ionic is the most accurate representation. It focuses on the action that is taking place, and leaves out noninvolved ions. Learning to write net ionic equations is an important skill in chemistry.

Materials: 0. 1-M solutions of ionic solids on Report Sheet

Small test tubes & rock.

Procedure: Part I: Ion Colors

Examine and record the color of each solution listed on the Report Sheet. From this information, determine the colors of the individual ions.

Part II: Ion Reactions

In a small test tube, mix about 1ml (1/2”) of each solution in the combinations shown on the Report Sheet, and shake the tube. Record the information requested.

IONS

Part I: Ion Colors

Solution Color Cation Color Anion Color

KOH ________ ________ ________ ________ ________

NaCl ________ ________ ________ ________ ________

CuCl₂ ________ ________ ________ ________ ________

AgNO₃ ________ ________ ________ ________ ________

KI ________ ________ ________ ________ ________

Fe(NO₃)₃ ________ ________ ________ ________ ________

Na₂CrO₄ ________ ________ ________ ________ ________

Cu(NO₃)₂ ________ ________ ________ ________ ________

KMnO₄ ________ ________ ________ ________ ________

Pb(NO₃)₂ ________ ________ ________ ________ ________

KNO₃ ________ ________ ________ ________ ________

K₂Cr₂O₇ ________ ________ ________ ________ ________

CoCl₂ ________ ________ ________ ________ ________

Ni(NO₃)₂ ________ ________ ________ ________ ________

Part II: Ion Reactions

(1) AgNO₃ + CuCl₂ (2) AgNO₃ + CoCl₂

Color of precipitate: _______________ _______________

Color of supernatant: _______________ _______________

Ionic equation: (1) →

(2) →

Net ionic equation: (1) →

(2) →