CHEM 211 Lecture 1: Study Note at UMich

60 views1 pages

azuremammoth471

8 Jan 2019

School

Department

Course

Professor

For unlimited access to Class Notes, a Class+ subscription is required.

Document Summary

The purpose of this lab was to determine if lowering the concentration by one half for the potassium permanganate reaction would yield comparable results to the original non green experiment. Lowering the concentration by a half is considered to be a greener reaction because you are not using as much reagents. From the results, one can conclude that lowering the concentration still yields similar results, and the only difference is a slightly lower amount of product formed. These results prove that greener reactions can also give you the desired results, and lowering the concentration is an effective way of conducting a green experiment that produces similar results to the. This implies that greener reactions such as lowering concentration can be done instead of using excessive amounts of reagents.

Related Questions

INTRODUCTION

Many transition metal cations can have more than one possible charge (Worksheet 1). Differently charged species of the same element can have different chemical and physical properties. Knowing the charge can allow separation of the different ions based on their properties. For example, Cu²⁺ is blue and stable in solution, while Cu⁺ is colorless and reacts with oxygen. Hemoglobin contains iron as Fe²⁺, but it is inactive at binding oxygen when the iron gets oxidized to Fe³⁺. There are many similar situations in which it is important to always properly indicate charges of ions.

This laboratory experiment uses the difference in chemical properties of Fe²⁺, iron(II), and Fe³⁺, iron(III), to quantify the amount of each present in a solution of unknown concentrations of each. You will first determine the amount of iron(II) in your sample by titrating with a reagent that does not react with iron(III). You will then titrate a second sample after converting all the iron(III) to iron(II) in order to determine the total iron present. The percent composition of Fe²⁺ in the unknown mixture will be calculated.

BACKGROUND

The concentrations of both iron(II) and iron(III) ions in a solution can be determined by titration with potassium permanganate. The reaction is an oxidation-reduction reaction, or redox, for short. A brief introduction to oxidation-reduction reactions is given in the background of the âAcids and Basesâ experiment. The permanganate ion effectively oxidizes iron(II) to iron(III) in an acidic solution while being reduced to manganese(II) ion. The balanced net ionic reaction is:

5 Fe²⁺(aq) + MnO4^â(aq) + 8 H⁺(aq) â 5 Fe³⁺(aq) + Mn²⁺(aq) + 4 H₂O(l)

Iron(III) is not oxidized by the permanganate ion, and thus does not react. Thus in order to determine the concentration of iron(III) ions, they must first be reduced to iron(II) ions. Zinc metal is used as the reducing agent, also under acidic conditions.

Zn(s) + 2 Fe³⁺(aq) â 2 Fe²⁺(aq) + Zn²⁺(aq) (reduction of the Fe3+)

Zn(s) + 2 H⁺(aq) â Zn²⁺(aq) + H₂(g) (a side reaction that oxidizes excess Zn)

The Zn2+ formed does not interfere with the subsequent titration because Zn2+ is not oxidized by permanganate. Notice that converting all the iron(III) to iron(II) means that the total iron concentration ([Fe2+] + [Fe3+]) is titrated, not just the iron(II) originally present.

Techniques

This is a lab will utilize several of the techniques introduced in the experiment âLaboratory Techniques and Measurements,â the procedures for which are given in Appendix B. Poor laboratory technique and misuse of the glassware will result in poor data results, which will affect the grade you receive on the laboratory report. In order to use your time efficiently in lab, review how to properly:

rinse glassware prepare a standard solution use a volumetric pipette

weigh by difference set up and use a burette use a graduated cylinder

heat samples on a hot plate

Analysis of Iron by Oxidation-Reduction Titration

This lab uses the common quantitative technique of titration. The only difference between the titration you will be doing in this lab and that done in the Molar Mass of a Known Acid lab is the type of reaction used: this lab will use a redox reaction in place of the acid-base reaction performed in the previous experiment.

In order to determine the concentrations of iron(II) and iron(III) ions in a solution containing both ions, two portions of the same sample must be titrated separately. This first titration will determine only the iron(II) concentration. A second sample will be treated with zinc metal in order to reduce the iron(III) to iron(II). The resulting solution will be titrated with potassium permanganate. The result of this titration will give the total iron concentration in the sample. The difference between the first and second titration is the equal to the iron(III) concentration.

In this titration, potassium permanganate serves as its own indicator. The intense purple color of the permanganate solution becomes colorless when the permanganate is reduced to manganese(II). Thus, as iron(II) is oxidized with the addition of the permanganate solution from the burette it will impart a purple color that will fade. However, once there is no iron(II) left in solution a faint purple color will remain, marking the endpoint of the titration. This color can be slightly altered by the presence of iron(III), which is yellow in solution. Remember that iron(III) is generated by the reaction between iron(II) with permanganate, increasing the yellow appearance of the solution and which can interfere with the detection of the endpoint. Phosphoric acid is added to help minimize this effect.

pre lab questions:

Using this information: I used 4.00 g potassium dichromate. The amount of Iron ll is 4.00 gram in 2nd flask. Used grey moose vodka 2.00 ml and filled flask with water. This was the 2% vodka solution. For the experiment I used 5.00ml of the vodka and added 35.00 ml of more water to make it 1/8 of 2% or 0.25% of the original. With this information and the answer's i have placed in the table please help with remainder of questions. Need to work to understand answers so I will get correct on upcoming test. Thanks.

Calculate the concentration of the dichromate ion in the first volumetric flask.

Calculate the concentration of the iron (II) ion in the second volumetric flask.

Experiment 2: Titrate the Vodka Sample

Lab Results

Record the following lab data in the table below. If you had to repeat one of the titrations, disregard the value that was different.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL	5.00ml
(b) coarse titration volume of iron (II) solution range in mL	14.10 -14.15 ml
(c) volume of iron (II) solution delivered from the burette in mL during the first fine titration	14.13ml
(d) volume of iron (II) solution delivered from the burette in mL during the second fine titration	14.11ml
(e) average volume of iron (II) solution used in the fine titrations	14.12ml
(f) the color of the analyte solution at the end point of the titration	purple
(g) the color of the analyte solution after adding the indicator	dark green

Data Analysis

Record and calculate the quantities in the table below using the data from your dichromate titrations. Use an average value for the volume of iron (II) solution used in the titration. If one of your values is very different, and you had to perform the titration three times, disregard the value that was very different when computing the average.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL 5.00ml
(b) moles of dichromate ion added to the Erlenmeyer flask
(c) average volume of iron (II) solution delivered from the burette in mL 14.12ml
(d) moles of iron (II) ions delivered from the burette
(e) moles of excess dichromate ions that reacted with the iron (II) ions (remember that the ratio in which they react is 1 dichromate : 6 iron (II))
(f) moles of dichromate that reacted with the ethanol in the vodka (Subtract excess dichromate ions that reacted with the iron (II) ions from the original moles of dichromate ion present.)
(g) moles of ethanol in the 5 mL diluted vodka sample according to the stoichiometric ratio of 2 dichromate ions to 3 ethanol molecules

The amount of alcohol in a drink is typically reported as percent alcohol by volume. Volume percent or volume/volume percent (% v/v) most often is used when preparing solutions of liquids. Volume percent is defined as:
% v/v = V_solute/V_solution Ã 100
Find the percent alcohol (ethanol) by volume for the vodka used in the lab by following the steps outlined in the table below.

(a) given the molar mass of ethanol of 46.07 g/mol, calculate the mass of alcohol (ethanol) in the tested sample solution
(b) given the density of ethanol of 0.7893g/ml, find the volume in mL of ethanol present in the diluted vodka solution
(c) record the volume of vodka used in the experiment in mL
(d) find the percent alcohol by volume (% v/v) in the diluted vodka solution
(e) the diluted vodka solution was prepared by diluting 2.00 mL vodka to 100.00 mL. Calculate the dilution factor used (N:1)
(f) multiply the percent alcohol by volume in the diluted vodka solution by the dilution factor to obtain the % v/v alcohol in the original vodka solution

Conclusions

The Grey Moose vodka tested in this lab reports a percent alcohol by volume of 40.0% on its label. How does your value compare to the reported one? If the values are different, give one possible experimental error that might have contributed to the difference.

Potassium permanganate is another strong oxidizing substance similar to potassium dichromate. An acidic solution of purple permanganate ions can get reduced to colorless Mn²⁺ ions in the presence of ethanol. Write down the redox reaction between permanganate and ethanol, and balance it using the half-reaction method.

Besides vodka, there are other colorless alcohol-containing beverages that can be titrated following the procedure in your lab. Given the average values for the percent alcohol by volume listed in the table below, which beverage do you expect to use the least amount of iron (II) standard solution during the titration? Assume all lab procedures stay the same.

	% alcohol by volume
White rum	37.0%
Vermouth	18.0%
White whine	12.0%

Equipment Materials

50.00 ml buret, 250.ml volumetric flask, 1.00 ml volumetric pipet 10.0 mL volumetric pipet, 25.00 mL volumetric pipet, pipettor, 250 mL Erlenmeyer flask, 10.0 mL graduated cylinder, several beakers, several watchglasses, unkown bleach solution, 10% potassium iodide solution, 2M hydrochloric acid solution, 0.150 M sodium thiosulfate solution, 1% starch solution.

Background information regarding this experiment is as follows:

An aqueous solution of sodium hypochlorite (NaOCI) is a clear, slightly yellow liquid, and is commonly known as bleach. Aside from its uses as a bleaching agent, sodium hypochlorite solutions are also used as sterlizing agents and in water treatment. Industrial uses include agricultural, food handling, paper production, and textile production. Sodium hypochlorite is also added to waste water to reduce odors.

The concentration of sodium hypochlorite in bleach solutions can be determined by titration. Therefore, we must use a two-step method to titrate sodium hypochlorite. In the first step, sodium hypochlortie, hydrochloric acid, iodide ion, and starch are combined to form a starch-triiodide complex. In this step there are 3 reactions that take place:

The result of these 3 reactions is that when sodium hypochlorite is present the starch-triiodide complex is produced.

In the second step the starch-triiodide product is titrated by sodium thiosulfate to form a colorless solution of iodide, dithionate.

I do not know how to answer the following on the data sheet: (? means I do not know how to answer)

Table 1:

Mass of 1.00 ml of unknown (by tare): 10.552 g

Density of bleach: (g/ml) : 1.055 g/ml

Volume of diluted bleach used in each titration: 10.00

Concentration of sodium thiosulfate: ? (M)

Titration Data:

Initial buret reading (ml): trial1: 0.000 ml Trial 2: 0.00 ml Trial 3: 0.000ml

Final buret reading (ml): trial 1: 13.50ml Trial 2: 13.30ml Trial 3: 13.40ml

Volume delivered(ml: trial 1: ? Trial 2: ? trial 3:?

Calculations: (show clearly)

For trial 1,2,3 ???

1.Moles of Na₂S₂O₃ in titration: ??? trial 1,2,3

2.Moles of NaOCl: ???trial 1, 2,3

3.Average moles of diluted NaOCl: (one answer) ?

4.Average moles of undiluted NaOCl: (one answer.. mol) ?

5.Standard Deviation: (one answer) (mol)

6.Average mass of NaOCl in 25.00mL of undiluted bleach: ? (g)

7.mass of 25.00 mL of undiluted bleach: ? (g)

8. Mass % of NaOCL from experiment: ? (%)

9.Actual mass % of NaOCL (from prof; student does not make an entry):? (%)

More questions all need to be answered please:

1. Calculate the volume of the reagent thiosulafte solution which would be required if you titrated a 25ml sample of the original commercial bleach instead of 25ml of the diluted solution.

2. Why would we choose a graduated cylinder to measure the HCl and KI solutions instead of a volumetric pipette? When would we use a volumetric pippette?

3. You calculated the mass percent of sodium hypochlorite in the bleach( dilute and undiluted). Now calculate the molarity of the sodium hypochlorite in the diulte and undiluted solution/

4. Use oxidation states to show that equations (2) and (4) in the introduction are redox reactions and that equation 1 is not a redox reaction. :

-(1) NaOCl(aq) + HCl(aq) yields HOCl(aq)+NaCl(aq)

-(2) HOCL (aq) + HCl(aq) + 3I(aq) yields I3(aq) + 2CL(aq) + H₂O(l)

-(4) (I₃) (Starch) + 2S₂O₃ yields 3I + S₄O₆ + starch

5. combine equation 1-3 to show the overall reaction for the products of the starch-triiodide complex.

-(1) NaOCl(aq) + HCl(aq) yields HOCl(aq)+NaCl(aq)

-(2) HOCL (aq) + HCl(aq) + 3I(aq) yields I3(aq) + 2CL(aq) + H₂O(l)

-(3) I₃ + Starch yields (I₃) (Starch)

6. What standard was used in this experiment?

7. Describe the naming of NaClO and HClO. What are the names of the following compounds? NaClO₂ , NaClO_3, NaClO₄ , HClO₂, HClO₃, HClO₄

8 Submit a one paged (typed double spaced) report that discusses the following topics:

a. the goals or objectives of the experiment

b.your results as they relate to the goals and objectives

c. the quality of your results.

CHEM 211 Lecture 1: Study Note at UMich

Document Summary

Get access

Related textbook solutions

Chemistry: Structure and Properties

Basic Chemistry

Principles of Chemistry Molecular Approach

Chemistry: Structure and Properties

Principles of Chemistry Molecular Approach

Chemistry: A Molecular Approach

Chemistry: A Molecular Approach

Principles of Chemistry: A Molecular Approach

Chemistry: The Central Science

Related Documents

BIOL 1208 : Biology Lab Study Guide

CHEM 245 Lecture Notes - Lecture 8: Aromaticity, Lidocaine, Broad Peak

BIOLOGY 173 Lecture Notes - Lecture 4: Absorbance, Flowchart, Non-Competitive Inhibition

Related Questions