CHEM 112 Lecture Notes - Lecture 34: Royal Aircraft Factory F.E.2

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2 May 2016

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Chem112 lecture 34 complex redox reactions, batteries, and fuel cells. Sometimes water, protons, or hydroxide, need to be added when polyatomic oxo- cations or oxo-anions are present. Since the oxidation half is less complicated, do it first! Mn2: you should get fe2+ fe3+ + e- Now comes the complex part: first, figure out the charges. )(-8)-1 2: thus, the charge on mn must be +7 in order to make sense of mno4, now, balance the atoms. Mn2+ + 4 h2o: oxygen is now balanced, but now we introduced hydrogen. That is okay, because we can balance hydrogen with h+ Mn2+ + 4 h2o: now that the mass is balanced, balance the charge, since we are going from +7 to +2, there must be 5 electrons applied to the reactants. + 5 e- mn2+ + 4 h2o: now put everything together. + 5 e- mn2+ + 4 h2o: get the same number of electrons.

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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:

HERE IS HOW THIS IS GOING TO WORK! IF YOU ANSWER THE QUESTIONS IWILL GIVE YOU THE 3K POINTS + MORE AFTER I REPOST IT.

2. The amount of heat involved in the synthesis of 1 mole of acompound from its elements, with all substances in their standardstates at 25*C, is called _________.

3. What is a valid unit for specific heat?

4. Use Hess's Law to find the heat of reaction in the examplebelow.

N2O4 (g) ----> 2NO2 (g)

Given that:

N2O4 (g) ----> N2 (g) + 2O2 (g) (Delta H =-9.7kJ)

2NO2 (g) ----> 2O2(g) + N2(g) ( Delta H= -67.7kJ)

5. Use Hess's Law to find the heat of reaction in the examplebelow. (Example doesn't exist btw.)

6. (Delta H) for the formation of rust (Fe2O3) is -826 kJ/mol. Howmuch energy is involved in the formation of 5 grams ofrust?

7. How many kilocalories of heat are required to raise thetemperature of 50g of aluminum from -5*C to 35*C? (Specific heat ofAluminum = 0.21 cal/g*C)

8.Calculate the (Delta H) for this reaction:

Zn(s) + Cu (2+ ion)(aq) ---> Zn(2+) + Cu(s)

(Delta H) for Cu(2+) = +64.4 kJ/Mol; (Delta H) for Zn(2+) = -152.4kJ/mol

9. The following equation shows the reaction that occurs whennitroglycerine explodes.

4C3H5O9N3 ----> 12CO2 + 6N2 + O2 +10H2O + 1725 Kcal

Is this reaction exothermic , endothermic, a combustion reaction ,or a combination reaction?

10. What does it mean for ammonia, (NH3) to have a (Delta H f) =-46 kJ/mol?

(for example: Define heat of formation. What does a negative heatof formation mean? what do the units of kJ/mol mean?)

11. What is the maximum amount of KCl that can dissolve in 500g ofwater?

(solubility of KCl is 34 g/ 100g of water at 20*C)

12.List the ways(operations) that can help make a substancedissolve faster in a solvent?

13. If a crystal added to an aqueous solution causes many particlesto come out of solution, the original solution was (saturate,unsaturated, supersaturated).

14. What is the number of moles of solute in 165 mL of a 0.82 Msolution?

15. What mass of NaCl is needed to make 350 mL of a 1.00 Msolution?

16. Colligative properties depend upon...

17. If one mole of each of these solutes in added to the sameamount of water, which solution has the highest boiling point?(glucose, aluminum sulfate, magnesium acetate, copper (I)chloride)

18. When an acid reacts with a base, what compounds areformed?

19. If the [H+] in a solution is 1x10^(-13) mol/L, what is the[OH-]?

20. If the [OH-] = 1x10^-6 mol/L, what is the pH of thesolution?

21. For a solution to be considered acidic what must be thehydrogen-ion concentration?

22. Which of the following represents a Bronsted-Lowry conjugateacid-base pair?

a) SO3(2-) and SO2 b) CO3(2-) and CO c) H3O and H2 d) NH4(+) and NH3

23. With solutions of "strong" acids and "strong" bases, the word"strong" refers to ...

24. What are the products of Al(OH)3 + H2CO3?

25.What is the concentration of hydrochloric acid if 5.0 mL of acidis neutralized by 30.0 mL of 2.00M sodium hydroxide.

26. At equilibrium, what is the rate of production of reactantscompared with the rate of production of products?

27. What is the equilibrium constant for the following reaction2SO3 ---> 2SO2 + O2

28. How does an increase in pressure affect the following reaction?2SO3 (g) --> 2SO2(g) +O2(g)

29.What stresses would shift the reaction to the right?

2SO3(g) ==> 2SO2(g) + O2(g)

30. The Keq of a reaction 4.9 x 10^(-7). Are products or reactantsfavored at equilibrium?

31. In which of these systems is the entropy decreasing?

a) air escaping from a tire b) snow melting c) a gas condensing d) salt dissolving inwater

32. If an atom is reduced in a redox reaction, what must occur toanother atom in the system?

33. Identify the oxidizing agent in the following reaction: 2Na + S---> Na2S

34. What are transferred in an oxidation -reductionreaction?

35. Give the oxidation number for each atom in KMnO4.

36. Which atom has a change in oxidation number of +4 in thefollowing reaction?

K2Cr2O7 + H2O + S ----> KOH + Cr2O3 +SO2

37. What is the oxidation number of sulfur in each of the following:

a. SO3 c.SO2(2-)

b. Na2SO4 d. S2O4 (2-)

38.Fe(3+) ----> Fe represents oxidation, reduction, orhydrolysis?

39. When the half reactions Br2+ 2e- -----> 2Br- and Na---->e- + Na(+) are combined the balanced redox equation is_____________________ ( WRITE the BALANCED REDOX eq)

40. To balance the oxygen and hydrogen for a redox reaction thattakes place in basic solution, it is necessary to use whichsubstance?

41. What particle completes this nuclear equation? 55/25 Mn + 2/1 H====> _____ +2 1/0 n

42. The half-life of tungsten-182 (182/74 W) is 1.1x10^(17) years.How long would it take for a 376.2 kg sample to decay until only123.0 grams remain?

43. After 3 days, the quantity of certain radioisotope is reducedfrom 13.6 grams to 1.9 grams. Calculate the half-life of theisotope.

44. What is the name of the compound having the formulaC8H18.

45. What is the name of the following compound:

CH3---CH2----CH---CH2--C

I \

CH3 OH

46. What is an isomer? Give an isomer of hexane (C6H14) and drawit's structure.

47. What is the name of the hydrocarbons with double covalentbonds?

48. Draw the stucture for 3-methylbutanoic acid.

49. Draw the structure for methylbenzene?

50. What is the name of the following compound:

CH3----C-----CH3

CH3

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CHEM 112 Lecture Notes - Lecture 34: Royal Aircraft Factory F.E.2

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