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13 Dec 2019

(b) If 3.0% of the heat being measured was being lost to the universe, determine whether they could they distinguish between iron (s = 0.444 J/g Â· Â°C) and Ni (s = 0.449 J/g Â· Â°C).

What is the percent difference between the specific heats of the two metals? Use the specific heat of iron as the theoretical value.
%

(b) Is this difference large enough to differentiate between the metals?

Yes

No

This cannot be determined with the information given.

It depends on how hot the metal is before it is added it to the calorimeter.

It depends on the amount of water used in the calorimeter.

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Sixta KovacekLv2

17 Dec 2019

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A coffee-cup calorimeter contains 130.0 g of water at 25.3 âC . A 124.0-g block of copper metal is heated to 100.4 âC by putting it in a beaker of boiling water. The specific heat of Cu(s) is 0.385 J/gâ K . The Cu is added to the calorimeter, and after a time the contents of the cup reach a constant temperature of 30.3 âC .

Part A

Determine the amount of heat, in J , lost by the copper block. (3350 J)

Part B

Determine the amount of heat gained by the water. The specific heat of water is 4.18 J/gâ K . (2720 J)

Part C

The difference between your answers for (a) and (b) is due to heat loss through the StyrofoamÂ® cups and the heat necessary to raise the temperature of the inner wall of the apparatus. The heat capacity of the calorimeter is the amount of heat necessary to raise the temperature of the apparatus (the cups and the stopper) by 1 K . Calculate the heat capacity of the calorimeter in J/K .

Express your answer using two significant figures.

Part D

What would be the final temperature of the system if all the heat lost by the copper block were absorbed by the water in the calorimeter?

21. If you have one gram of each of the following metals, which one would you expect to heat up the fastest? A) Silver B) Lead C) Aluminum D) Iron 22. If it requires 67 J of heat to r aise the temperature of a pi ece of iron by 5.0 o C, calculate the mass of the iron. The sp ecific heat of iron is 0.46 J/g o C. A) 150 g B) 100 g C) 29 g D) 706 g 6 Lab Quiz # 2, 3 23. Inside a typical calorimeter the sample releases energy and heat flows into the water. In order to calculate the speci fic heat of an unknown metal, which of the following measurements must be made? A) Mass of the unknown metal, mass of the water, initial tempera ture of the metal, final temperature of the water B) Mass of the unknown metal, mass of the water, initial tempera ture of the metal, initial and final tempe ratures of the water C) Mass of the calorimeter, initial temperature of the metal, in itial and final temperatures of the water D) Mass of the calorimeter, mass of the water, initial and final temperatures of the water 24. In lab a student builds an a luminum âtentâ around the calori metry apparatus. The purpose of the âtentâ is to A) cause the system to heat up. B) allow heat to escape into the surroundings. C) limit the amount of oxygen in the system. D) prevent too much loss of heat to the air. 25. Specific heat can be used, a long with other observations, to determine the identity of a substance because it is A) a physical property. B) a chemical property. C) a measure of heat. D) both a physical and a chemical property. 26. During an experiment to determin e the specific heat of a met al, the student did not fully immerse the hot metal object in t he water bath. How would this error change the calculated specific heat? A) The calculated specific heat would be too low because not all of the heat is transferred to the water. B) The calculated specific h eat would be too high because not al l of the heat is transferred to the water. C) The calculated specific heat would be too high because the ma ss submerged is low. D) The calculated specific heat would be unchanged because it is not dependent on the heat transferred

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.

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