Expt 030 -- Gases: Preparation and Properties

Several gases are produced in a modified Petri dish apparatus, and their properties are studied on the basis of reactions with test papers.

Background

• Procedures for the generation and study of gases contained in closed, Petri-dish like containers, was developed some years ago. In this experiment, plastic Petri dishes are modified by constructing a barrier from a stripe of caulking material incorporated into the bottom of the dish. Moistened test papers are placed on the top of the dish. When chemicals are mixed inside the dish, gases produced are trapped and react with the test papers.
• Litmus and pH paper test the acidity of the gas. Starch iodide paper tests for oxidizing agents. The copper sulfate paper special reagent for identifying specific substances. These are spot tests for gases.

Safety

Many chemicals are used in the various procedures given here. Most are toxic, and many are corrosive. The gases produced often are toxic and noxious. Cover the dishes before mixing the chemicals, and keep them covered until they are washed. Wear goggles and apron. Wash spills immediately with large amounts of water. The room must be well ventilated. Uncover dishes under the hood. Wash hands after the experiment.

Procedure

1. Follow the same procedure for each gas.
2. Obtain a partitioned Petri dish. Elevate one side of the bottom plate so that the partition effectively separates chemicals on one side from the other. Place 5 pieces of test paper, each about 8-mm x 8-mm on the inside top of the Petri dish. Arrange these so that they are on one side of the dish. The five test strips should include:
   • red litmus moistened with water
   • blue litmus moistened with water
   • pH paper moistened with water
   • paper moistened with KI, starch
   • paper moistened with 1 M CuSO₄
3. There are 8 possible gases to produce. Place one set of chemicals on the left, and the other on the right. Prepare the cover with the test strips indicated.

   !!!Click here to See Picture.

4. Cover the dish. Tilt the dish so as to mix the chemicals, and let them run onto the side of the bottom dish away from the test papers. (Gas bubble production can spray chemicals onto the test strips and give misleading results.) After initial bubbling subsides, allow the solution to move under the papers to minimize the distance the gas must diffuse.

   !!!Click here to See Movie.

5. Do not open the dish in the lab. However, sometimes an odor characteristic of the gas is strong enough that sniffing at the edge of the gas indicates the odor.

   Gas	Left	Right
   ammonia	NH4Cl solid	10 dps 1 M NaOH

   !!!Click here to See Picture.

   nitrogen dioxide

   10 dps 6 M HNO3

   small piece Cu

   !!!Click here to See Picture.

   bromine	10 dps dilute bleach	5 dps 1 M CH3COOH 10 dps 1 M KBr
   iodine	10 dps 3% H2O2	5 dps 1 M HCl 10 dps 1 M KI
   chlorine	10 dps dilute bleach	10 dps 1 M HCl
   sulfur dioxide	10 dps 1 M NaHSO3	10 dps 1 M HCl
   carbon dioxide	10 dps 1 M NaHCO3	10 dps 1 M HCl
   hydrogen sulfide	10 dps 0.1 M Na2S	10 dps 1 M HCl

   • Note hydrogen sulfide is very toxic. Do not sniff or inhale.
6. Go to the hood. Remove the top of the dish under the hood. Transfer the contents of the bottom dish to a large disposal jar filled with water. Scrape the test papers into a different jar.

Questions

1. Of the 8 procedures for preparing samples of gases, 4 involve acid-base reactions. write equations to describe the 4 acid base reactions.
2. Two of the gases react with Cu²⁺. Write balanced equations for these reactions.
3. Four gases react with the KI/starch solution. Write balanced equations for these reactions.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 030 Gases: Preparation and Properties

Watch the movies and carefully record the changes observed.

	ammonia	nitric oxide
pink litmus
blue litmus
pH
Starch I-
Cu2+

H1. Which of the these gases are oxidizing agents?

H2. Which of these gases are acidic?

H3. Which of these gases are basic?

Answer the questions.

Curriculum-

This is a late experiment, one used to summarize large amounts of chemical knowledge. It fits well into an applied chemistry course.

Safety-

Many chemicals are used in the various procedures given here. Most are toxic, and many are corrosive. The gases produced often are toxic and noxious. Cover the dishes before mixing the chemicals, and keep them covered until they are washed. Wear goggles and apron. Wash spills immediately with large amounts of water. The room must be well ventilated. Uncover dishes under the hood. Wash hands after the experiment.

Time-

Teacher Preparation: 30 minutes (Additional 5 minutes per dish to modify the Petri dishes the first time. This process could be done by students. See Lab Hints.)

Class Time: 20-40 minutes depending on the number of gases studied.

Materials-

• modified Petri dishes
• plastic transfer pipets for solutions
• Centrifuge tubes for solids
• pH paper
• red and blue litmus paper
• Prepare these solutions to make indicating papers
  • 1 M CuSO₄ -- (Dissolve 12.5 g copper(II) sulfate pentahydrate (CuSO₄¥5H₂O) in enough water to make 50 mL solution.)
  • 1.5 % KI / 0.5% starch (Boil 100 mL of distilled water. Cautiously and carefully add spray starch from a can (sold with laundry products at a grocery store), with stirring, until a bluish gray tinge appears in the liquid. Cool to room temperature. Add 1.5 g KI.)
• Select several gases to make and prepare reagent for those.
• 0.1 g Cu -- use small piece of strands or granular or foil
• 0.1 g NH₄Cl
• dilute bleach -- (Dilute commercially available household laundry bleach with an equal volume of water.)
• 1 M CH₃COOH -- (Add 33.3 mL 3 M acetic acid (CH₃COOH) to enough water to make 100 mL solution.)
• 1 M KBr -- (Dissolve 5.95 g potassium bromide (KBr) in enough water to make 50 mL solution.)
• 3% H₂O₂ -- (Use commercially available solutions as purchased.)
• 3 M HCl -- (Use the stock solution as prepared.)
• 1 M HCl -- (Add 33.3 mL 3 M hydrogen chloride (HCl) to enough water to make 100 mL solution.)
• 1 M KI -- (Dissolve 8.300 g potassium iodide (KI) in enough water to make 50 mL solution.)
• 1 M NaOH -- (Add 33.3 mL 3 M sodium hydroxide (NaOH) to enough water to make 100 mL solution.)
• 1 M NaHSO₃ -- (Dissolve 5.203 g sodium hydrogen sulfite (NaHSO₃) in enough water to make 50 mL solution.)
• 1 M NaHCO₃ -- (Dissolve 5.299 g sodium hydrogen carbonate (NaHCO₃) in enough water to make 50 mL solution.)
• 0.1 M Na₂S -- (Purchase the sodium sulfide as a 0.1 M solution.)
• 6 M HNO₃ -- (Add 19.0 mL 15.8 M nitric acid (concentrated HNO₃) to enough water to make 50 mL solution.)

Disposal-

Dispose of used test papers with ordinary solid trash. Neutralize the liquid in the disposal jar using vinegar or NaHCO₃. Discard the neutralized solution at the sink with a very large volume of running water.

Lab Hints-

• Bisect the petri dish with a line of silicone bathtub caulk. Leave one third to one half of the line open for mixing the chemicals.

  !!!Click here to See Picture.

• Let the dishes stand overnight to cure. The dishes may be reused for several years.

  !!!Click here to See Picture.

• You may wish to try other combinations and test papers. Oxygen and hydrogen may be made easily. You might want to have student suggest and try different test papers with these oxidizing and reducing agents.
• Many damp intensely colored flower petals will undergo both oxidation-reduction reactions and acid-base reactions. Red roses and purple clematis change distinctly.

Answers-

Q1. Of the 8 procedures for preparing samples of gases, 4 involve acid-base reactions. write equations to describe the 4 acid base reactions.

A1. S^2- + 2 H⁺ --> H₂S

CO₃^2- + 2 H⁺ --> CO₂ + H₂O

HSO₃^- + H⁺ --> SO₂ + H₂O

NH₄⁺ + OH^- --> NH₃ + H₂O

Q2. Two of the gases react with Cu²⁺. Write balanced equations for these reactions.

A2. Cu²⁺ + H₂S --> CuS + 2 H⁺

Cu²⁺ + 4 NH₃ --> Cu(NH₃)₄²⁺

Q3. Four gases react with the KI/starch solution. Write balanced equations for these reactions.

A3. I₂ + I^- --> I₃^-

Cl₂ + 3 I^- --> 2 Cl^- + I₃^-

Br₂ + 3 I^- --> 2 Br^- + I₃^-

2 NO₂ + 3 I^- + 2 H⁺ --> 2 HNO₂ + I₃^-

Handout Ans.-

	Initial	ammonia	nitric oxide
pink litmus	pink	blue	pink
blue litmus	blue	blue	pink
pH	orange	dk.blue	red
Starch I-	white	white	blueblack
Cu2+	very pale blue	medium blue	no change

HQ1. Which of the these gases are oxidizing agents?

HA1. nitric oxide

HQ2.Which of these gases are acidic?

HA2. nitric oxide

HQ3.Which of these gases are basic?

HA3. ammonia

CoopLearn-

It makes sense to divide up the effort and have many groups studying 3-5 gases and sharing results.

Key Words 1-

gases, preparation, reduction, oxidation, redox, acid, base

Elements-

H O N S Cl Br I Cu C