Classic Chemistry II – ZimmerName

Identification of Unknown Ionic Compounds

Due: Period 5 – Tuesday, Dec 9th

Period 4 & 6 - Wednesday , Dec 10th

Introduction:

There are two main kinds of questions that chemists often address using the tools of analytical chemistry: (1) How much do I have? and (2) What is this?

The first question requires quantitative analysis, which involves stoichiometry and careful measurements of mass and/or volume. The titration lab we did before Thanksgiving is one example of a quantitative analysis lab. The second question requires qualitative analysis, where chemical tests are used to identify the components of a sample. Qualitative analysis is an important technique in “real-world” science, and its applications include testing for pollutants in water samples, identifying material from a crime scene, etc.

The capstone lab is of the qualitative analysis – you will be determining the chemical formulas of unknown compounds. This lab will act as a great review of many of the reaction typeswe'vestudied this semester. You’ll hopefully also find it fun and challenging to solve the puzzle by identifying your mystery substances.

Objective:

Your group will be given a set of vials of unknown (but pure) solid ionic substances, and a list of the cations and anions that could make up the compound in each vial. Your goal is to design and carry out a procedure to positively identify each of the unknowns. In addition to the unknowns themselves, you will have access to the following reagents and materials:

  • Distilled water
  • Dilute nitric acid
  • Dilute sulfuric acid
  • Dilute hydrochloric acid (3M)
  • Concentrated hydrochloric acid
  • Dilute sodium hydroxide
  • Concentrated sodium hydroxide (6M)
  • 6M ammonia
  • Limewater
  • Potassium thiocyanate solution
  • Silver nitrate solution
  • Iron(II) sulfate solution
  • 0.1 M sodium sulfide
  • 0.1 M strontium nitrate
  • Solid iron(II) sulfate
  • Chlorine water
  • Hexanes
  • pH test strips
  • 12 well drop plate
  • Small test tubes
  • Centrifuge types and a centrifuge
  • Glass stir rods
  • Beral plastic pipets

Prelab:

You need to familiarize yourself with the various chemical tests that can be used to identify the cations and anions that may be present in your unknowns. Please read over the qualitative analysis info on the course site. Then, use the information you find to answer the following questions.

  1. How would you distinguish between sodium nitrate and sodium chloride?
  2. How would you distinguish between calcium hydroxide and aluminum hydroxide?
  3. How would you distinguish between ammonium nitrate and potassium nitrate?
  4. How would you distinguish between magnesium bromide and magnesium iodide?

List of possible cations

  • Sodium
  • Potassium
  • Calcium
  • Magnesium
  • Iron(III)
  • Copper(II)
  • Zinc
  • Aluminum
  • Silver

List of possible anions

  • Hydroxide
  • Chloride
  • Bromide
  • Iodide
  • Nitrate
  • Sulfate
  • Carbonate
  • Sulfide

Experimental Plan: start working on it with prelab but to be finished by end of long block period!

Use the website from the pre-lab and what you know about chemical reactions to design a procedure that will be applied to each of your unknowns, so that at the end of the procedure you will have determined which compound it is by identifying both the cation and the anion present. A flow chart is a great way to visualize your procedure – below is a simple example of what a flow chart might look like.

Important notes regarding Lab Procedure:

Goggles and gloves should be on during all tests, these are unknowns so you don’t know how safe or unsafe the chemical is!

Use very small amounts of the unknown when testing – smaller than a pea-size amount.

All waste (except hexane – organic waste) should go in the waste beaker on table. Rinse small test tubes out with water 3x to ensure they are clean. If you have a hexane containing test tube set in beaker off to the side for separate disposal. Nothing should go down the drain!

Data and Results: (In lab notebook, due at end of class Monday)

Carry out your procedure on as many unknowns as you have time for. Make sure to record all tests and observations as you do them. A table, something like the one below, might be a good way to organize your results. Make it BIG, so you have room to write your observations.

Unknown / Test / Observations / Inference

Identify all the unknowns that you can (at least 1, hopefully two or more). For any that you aren’t sure about, list the possible cations and anions it could contain. For each identification or partial identification, briefly summarize the evidence you have to support your conclusion.

Post-lab:(see due dates at top of document)

From your data and results, find at least one example of each of the following types of reactions. For each reaction you identify, indicate (with a symbol or letter in your data table) where in your experiments it occurred, then write and balance the net ionic equation for the reaction.

  • Precipitation
  • Acid-base reaction
  • Redox reaction (any type)
  • Double-displacement with gas formation
  • Complex ion formation