Experiment Four

Qualitative Analysis – Identification of Anions and Cations

Chemical Nomenclature

Procedural Tips

FAQs

Introduction/Goals

Background

Pre-lab

Additional Problems

Supplies and Chemicals

Safety Precautions

Waste Disposal Instructions

You will practice writing formulas and names of ionic and covalent compounds.                       You will observe the chemical reactions that various cations and anions undergo by testing solutions of known composition. You will then identify the cations and anions in an unknown solution based on its reactivity.

Print a copy of the Grading Sheet for this lab.

Procedural Tips:

Evidence of Chemical Reactions

The Flame Test

Using the Bunsen Burner

How to Note Odors

Preparation of a Water Bath

Watch the reaction for the carbonate ion carefully, bubbles of carbon dioxide may be hard to see.

FAQ:

Why do I have to wear safety goggles?

Why don't I see a lilac color with the potassium solution?

The potassium color is very short-lived and often difficult to see. Observe the color within a few seconds of heating.  You may find the color of the potassium flame easier to see when viewed through a cobalt blue glass.

Introduction/Goals:

Introduction:

Cations (positive ions) and anions (negative ions) in solution have unique chemical properties. You will use the chemical reactions that the ions undergo to identify the chemical composition of samples.

This area of chemistry is known as qualitative analysis and is an important part of the chemistry field. Qualitative analysis tells you what is present, but not the quantity of substance present. Qualitative analysis is important in many areas of study and is an example of the importance of chemistry in our world. Some "real world" examples of qualitative analysis are:

Goals:

Background:

Neutral atoms may gain or lose electrons to become charged ions. If an atom loses an electron, it becomes a positively charged cation. An atom which has gained an electron is a negatively charged anion. A cation and an anion will be attracted to each other because of the attractive force between oppositely charged particles. This attraction results in an ionic bond and an ionic compound is formed. Formulas used to represent ionic compounds show the combining ratio of the positive and negative ions in the compound. This ratio is determined by the charges on the respective ions. For example, magnesium (Mg) will lose 2 electrons per atom to become Mg²⁺. Chlorine (Cl) will gain one electron per atom to become Cl^-. Since no atom can lose electrons unless another atom is available to accept them, it will require two chlorine atoms to accept the electrons from magnesium. Thus, the ionic formula for magnesium chloride will be MgCl₂.

By observation of chemical reactions the identification of the cations and anions that compose an ionic compound can be made. The number of each ion cannot be determined using these techniques. Only the identity of the components of the compound is determined.

Determination of Cations:

Na⁺, K⁺, Cu ²⁺ Ca²⁺ and Li⁺ will be determined by the characteristic colors the ions give in flame tests.  Ca²⁺ can be further identified by the reaction of Ca²⁺ with ammonium oxalate to give a white precipitate.

Ammonium ion, NH₄⁺, can be converted to ammonia gas, NH₃ by the addition of base to an ammonium salt. The ammonia gas produced is detected by odor and by its reaction to red litmus paper.

Iron(III), Fe³⁺, will react with potassium thiocyanide to produce a red product. The presence of a red color when thiocyanide is added to a solution indicates the presence of ferric (Fe³⁺) ions.

Determination of Anions:

The chloride ion (Cl^-) is determined by its reaction with Ag⁺ to form the insoluble ionic compound, AgCl. This product is identified by the formation of a white precipitate that does not dissolve in HNO₃.

The sulfate ion (SO₄^2-) reacts with Ba²⁺ to form the insoluble compound, BaSO₄. BaSO₄ is a white precipitate that is insoluble in HNO₃. Phosphate and carbonate ions will also form a white precipitate with Ba²⁺. However, these barium compounds are soluble in HNO₃.

Phosphate ions (PO₄^3-) react with ammonium molybdate to form the yellow precipitate, ammonium phosphomolybdate.

The test for the carbonate ion, CO₃^2-, is characterized by the formation of carbon dioxide, CO₂, gas when HCl is added. The presence of carbon dioxide gas is indicated by the formation of bubbles.

Pre-lab:

Read the introductory material on the webpage.

Read Experiment Four in the laboratory manual.

Answer the pre-lab study questions on for Experiment Four.

You are encouraged to do part C and D of Experiment Four before coming to lab.

Additional Problems:

1. What is wrong with the following names?

   1. magnesium(II) bromide

   2. Sulfur chloride

   3. Iron chloride

2. Indicate whether each of the following ions are likely to form?

   1. K²⁺

   2. Li^-

   3. Zn^2-

   4. Ar⁺

3. Tell whether the following elements would most likely form mainly ionic bonds, mainly covalent bonds, or both ionic and covalent bonds?

   1. nitrogen

   2. sodium

   3. sulfur

   4. chlorine

   5. magnesium

4. Do questions 5.87 and 5.89 on page 138 in your McMurray textbook.

Supplies and Chemicals:

Safety Precautions:

SAFETY GOGGLES MUST BE WORN AT ALL TIME IN THE LABORATORY

Hydrochloric acid (HCl) is very corrosive. If you get any on your skin, wash it off immediately with cold water and tell your instructor.

Nitric acid (HNO₃) is very corrosive. If you get any on your skin, wash it off immediately with cold water and tell your instructor. NITRIC ACID MAY CAUSE YOUR SKIN TO DISCOLOR TO A YELLOW COLOR.

Sodium hydroxide (NaOH) is very caustic and may damage your skin. If you get any on your skin, wash it off immediately and tell your instructor.

Silver nitrate may stain your skin black.

Carefully waft ammonia fumes toward your nose.

Waste Disposal Instructions:

Excess CuCl₂ should be placed in the container marked "Copper Waste".

Wastes containing silver salts should be placed in the container marked "Silver Waste".

Wastes containing barium salts should be placed in the container marked "Barium Waste".

Pour other liquid wastes down the sink followed by large amounts of water.