Determining the Empirical Formula of Zn and I Minneapolis Community and Technical College v.9.09

Transcription

1 Determining the Empirical Formula of Zn and I Minneapolis Community and Technical College v.9.09 I. Introduction The purpose of this experiment is for you to react zinc with iodine in order to determine the empirical formula of the compound that results. However, simply placing Zn metal in contact with solid I 2, won t produce a significant reaction since the materials are not in close enough contact with one another. For this reason, we will be reacting Zn metal with I 2 that is dissolved in hot methanol. The dissolved I 2 is now able to make much better contact with the Zn metal and the reaction proceeds at a much faster rate (suitable for a 2.5 hour lab period). II. MSDS: Material Safety Data Sheets Read through the following MSDS information and become familiar with the hazards associated with the chemicals used in this experiment before coming to lab. Zn: Hazards Identification WARNING! HARMFUL IF SWALLOWED OR INHALED. MAY CAUSE IRRITATION TO SKIN, EYES, AND RESPIRATORY TRACT. MAY FORM COMBUSTIBLE DUST CONCENTRATIONS IN AIR. WATER REACTIVE. MAY AFFECT THE GUM TISSUE, CENTRAL NERVOUS SYSTEM, KIDNEYS, BLOOD AND REPRODUCTIVE SYSTEM (lead component). Health Rating: 0 - None Flammability Rating: 1 - Slight Reactivity Rating: 2 - Moderate Contact Rating: 0 - None Lab Protective Equip: GOGGLES; LAB COAT Storage Color Code: Orange (General Storage) Potential Health Effects Inhalation: No adverse effects expected but dust may cause mechanical irritation. The effects may be expected to resemble those of inhaling an inert dust; possible difficulty in breathing, sneezing, coughing. When heated, the fumes are highly toxic and may cause fume fever. Ingestion: Extremely large oral dosages may produce gastrointestinal disturbances, due both to mechanical effects and the possibility of reaction with gastric juice to produce zinc chloride. Pain, stomach cramps and nausea could occur in aggravated cases. Skin Contact: May cause irritation. Eye Contact: May cause irritation. Chronic Exposure: No adverse health effects expected. Aggravation of Pre-existing Conditions: Persons with pre-existing skin disorders or impaired respiratory function may be more susceptible to the effects of the substance.

2 Methanol: Hazards Identification POISON! DANGER! VAPOR HARMFUL. MAY BE FATAL OR CAUSE BLINDNESS IF SWALLOWED. HARMFUL IF INHALED OR ABSORBED THROUGH SKIN. CANNOT BE MADE NONPOISONOUS. FLAMMABLE LIQUID AND VAPOR. CAUSES IRRITATION TO SKIN, EYES AND RESPIRATORY TRACT. AFFECTS THE CENTRAL NERVOUS SYSTEM, GASTROINTESTINAL TRACT AND LIVER. Health Rating: 3 - Severe (Poison) Flammability Rating: 3 - Severe (Flammable) Reactivity Rating: 1 - Slight Contact Rating: 1 - Slight Lab Protective Equip: GOGGLES & SHIELD; LAB COAT & APRON; VENT HOOD; PROPER GLOVES; CLASS B EXTINGUISHER Potential Health Effects Inhalation: A slight irritant to the mucous membranes. Toxic effects exerted upon nervous system, particularly the optic nerve. Once absorbed into the body, it is very slowly eliminated. Symptoms of overexposure may include headache, drowsiness, nausea, vomiting, blurred vision, blindness, coma, and death. A person may get better but then worse again up to 30 hours later. Ingestion:Toxic. Symptoms parallel inhalation. Can intoxicate and cause blindness. Usual fatal dose: milliliters. Skin Contact: Methyl alcohol is a defatting agent and may cause skin to become dry and cracked. Skin absorption can occur; symptoms may parallel inhalation exposure. Eye Contact: Irritant. Continued exposure may cause eye lesions. Chronic Exposure: Marked impairment of vision and enlargement of the liver has been reported. Repeated or prolonged exposure may cause skin irritation. Aggravation of Pre-existing Conditions: Persons with pre-existing skin disorders or eye problems or impaired liver or kidney function may be more susceptible to the effects of the substance. Iodine: Hazards Identification POISON! DANGER! CORROSIVE. CAUSES SEVERE IRRITATION OR BURNS TO EVERY AREA OF CONTACT. MAY BE FATAL IF SWALLOWED OR INHALED. VAPORS CAUSE SEVERE IRRITATION TO SKIN, EYES AND RESPIRATORY TRACT. STRONG OXIDIZER. CONTACT WITH OTHER MATERIAL MAY CAUSE FIRE. AFFECTS THE CARDIOVASCULAR AND CENTRAL NERVOUS SYSTEMS. MAY CAUSE ALLERGIC SKIN OR RESPIRATORY REACTION. Health Rating: 3 - Severe (Life) Flammability Rating: 0 - None Reactivity Rating: 2 - Moderate Contact Rating: 3 - Severe (Corrosive) Lab Protective Equip: GOGGLES; LAB COAT; VENT HOOD; PROPER GLOVES Potential Health Effects Inhalation: Corrosive. Vapors severely irritate and can burn the mucous membranes and respiratory tract. Excessive tears, rhinitis, tightness in the chest, sore throat, headache and delayed pulmonary edema can result. Inhalation of concentrated vapors may be fatal. Ingestion: Corrosive. Can cause severe burns of the mouth, throat and stomach. Causes abdominal pain, diarrhea, fever, vomiting, stupor and shock. Probable lethal dose is 2 to 4 gm of free iodine. Skin Contact: Corrosive. Liquid contact may cause blistering burns, irritation, and pain. Vapors may be severely irritating to the skin. Eye Contact: Corrosive! Vapors are severely irritating and may cause damage to the eyes. Contact may cause severe burns and permanent eye damage. Chronic Exposure: Chronic exposure to iodine may cause insomnia, conjunctivitis, inflammation of the nasal mucous, bronchitis, tremor, rapid heart beat, diarrhea and weight loss. Allergic sensitization may occur. Aggravation of Pre-existing Conditions:Persons with pre-existing skin disorders, eye problems, impaired respiratory function, or disease of the thyroid, lungs, or kidney may be more susceptible to the effects of the substance.

3 III. Procedure Your objective in this lab is to determine the mass of zinc that reacts with a specific mass of iodine and then to convert this information to moles. This means that you will have to weigh many times throughout the procedure outlined below. Suggestions: Failure to weigh at the correct time will mean having to start the experiment all over again!!!!!!! When in doubt, weigh it!!! Good lab technique and low sample loss is crucial to obtaining the correct empirical formula in today s experiment. Work carefully and don t spill anything. You will be working individually on this experiment. Equipment : 125 ml Erlenmeyer flask (a.k.a. reaction flask) 150 ml beaker (for your product) 50 ml beaker (preweigh Zn) Glass stirring rod Top loading balance Analytical balance Fume hood Chemicals: Solid Zn (s), I 2(s),, CH 3 OH (l) (methanol) Label and pre-weigh a 125 ml Erlenmeyer flask and a clean/dry 150 ml beaker on an analytical balance. Record their respective masses (ALL DISPLAYED DIGITS) in the data table. Don t forget to record the balance number (or model) on your data sheet. Use the same analytical balance for all weighings. Use a spatula and the top loading balance to weigh out between 0.9 and 1 grams of metallic Zn granules in a clean/dry 50 ml beaker. This method, known as pre-weighing, reduces traffic at the analytical balances. Try not to spill on the balance pan and please clean up your own messes! You will obtain a more accurate Zn weighing when you re-weigh your reaction flask on an analytical balance (next step). Deposit the pre-weighed Zn granules in the first pre-weighed 125 ml Erlenmeyer flask (a.k.a. the reaction flask) and reweigh the flask using an analytical balance. Calculate the mass of the zinc in the flask to 4 decimal digits and record this value on the data table. Obtain a vial containing pre-weighed I 2 crystals (~1 g) from your lab instructor. Deposit the entire amount of solid I 2 in the reaction flask. Keep the reaction flask covered with aluminum foil when not weighing to reduce gaseous I 2 released into the atmosphere. Reweigh the reaction flask using an analytical balance and record the new mass. Calculate the mass of the solid I 2 to 4 decimal digits and record this value on the data table. Use the graduated cylinder located near the methanol stock bottle (in the hood) to measure out approximately 15 ml of methanol. While working in the hood, add the methanol to the reaction flask. Leave the graduated cylinder in the hood for the next person to use. Note the color of the reaction mixture in the observations section of your data sheet. What is responsible for the solution s color? (Question #2) Use a small piece of aluminum foil to cover the top of the reaction flask. Obtain a hotplate and while working in the fume hood, gently heat the reaction flask (heat setting 2-3 on dial) for approximately minutes. It should gently boil. Record any color changes in the observations section of your data sheet. When the reaction is complete, the mixture will be colorless. (Done? You be the judge. If in doubt, cook longer.) Don t leave your solution unattended!!! If you must leave your station, have someone else watch things until you return. Don t allow the solution to boil dry!!! i.e. use the Al foil to reduce methanol loss. If the solution level drops significantly (compare to your neighbors), add additional methanol.

4 Now decant (pour off) the liquid into the pre-weighed 150 ml beaker. Leave the un-reacted Zn in the reaction flask. Rinse the un-reacted Zn by adding approximately 5 ml of methanol to the reaction flask and re-heating for 15 seconds. Continue working in the hood and again decant the rinse solution into the pre-weighed beaker that contains the first, original methanol solution. Repeat this procedure two more times. Place the reaction flask (containing the un-reacted Zn) on the hot plate and allow the Zn granules to dry. Agitate or gently shake the flask to accelerate the drying process. When the Zn in the reaction flask is dry, cover it with aluminum foil and let it cool before reweighing on the analytical balance. Be sure to remove the aluminum foil before weighing and record your result on the data sheet. Place the 150 ml beaker containing the methanol solution on the hot plate and boil to dryness This MUST be done gradually at low heat and in the fume hood to protect us all from methanol vapors. A glass stirring rod should be left in the beaker as it will improve the boiling behavior of the solution and reduce bumping. Remove the glass stirring rod before the liquid has completely been boiled off. Continue heating the beaker for several minutes after all liquid has visibly disappeared. Let the beaker cool and re-weigh it on an analytical balance. Keep in mind that the solid you collect is hygroscopic Prelab question: What does it mean to be hygroscopic? (Know this before coming to lab) From your measurements, determine the masses of zinc and iodine that have reacted. Convert these quantities into moles and determine the empirical formula of the product with the correct number of significant figures (we weren t using an analytical balance for nothing you know!)

5 C1151 Data Sheet Name Empirical Formula Date of Exp. Instructor Initials Lab Section Your individual experimental report will be due at the beginning of class next week. Data Table: (All entries must be in written in ink before you leave the lab). Balance # Mass (g) Before Heating After Heating a. Flask (initial) b. Flask & Zn c. Flask & Zn & I 2 d. Initial Zn (b-a) e. Initial I 2 (c-b) f. Flask & Zn h. Beaker (initial) i. Beaker + product j. Product (i-h) k. Excess Zn (f-a) L. Zn used (d-k) moles Zn (use mass L) * moles I (use mass e) ** Formula (4 s.f.) *** Empirical Formula (rounded subscripts) * Convert I 2 mass to moles of atomic I by dividing by g/mol. ** Example: Zn 1045I 2100 this answer is close to the ideal answer, a 1:2 ratio, but is displayed with 4 S.F. in each subscript. *** Use formula subscripts simplified to single digits. (e.g. Cu 1Cl 2)!!!NOTE: I will grade your data and calculations by entering your data table measurements into an Excel Spreadsheet. You will not receive full credit for your calculations unless your results match mine exactly. Observations: Record your experimental observations in the space below.

6 Answer the following questions: Answers must be readable and make sense for credit. Copied/duplicate answers will result in all involved students receiving a zero score 1. Why is the exact amount of methanol used in this experiment not important? (i.e. why was only approximately 15 ml used?) 2. What is responsible for the color of the solution before you started heating? 3. What is the purpose of rinsing the Zn (s) with methanol several times and adding the rinse solution to the beaker? 4. What is the limiting reactant in this experiment? Provide two experimental observations that support your conclusion? 5. Calculate the total mass reactants by adding masses e and L from your data table. Subtract this value from mass product (j) 6. If mass was conserved, these two masses (Question 5) should be exactly equal and their calculated difference equal to zero. Give two specific explanations for why the mass product does not equal the mass reactants in your experiment. a. b.