Oxidation-Reduction Chemistry

[Pages:4]Oxidation-Reduction Chemistry

Introduction

Oxidation-reduction reactions occur when electrons are transferred between reactants. Such transfers are reflected by changes in the oxidation numbers associated with the substances taking part in the oxidationreduction reaction. A detailed discussion of oxidation-reduction reactions is given in Chapter 12 of your textbook (B. B. Laird, University Chemistry). Redox processes are ubiquitous in the environment and are crucial in all living systems. There are four types of redox reactions:

a. combination reactions 2 Fe + 3 Br2 2 FeBr3 b. decomposition reactions 2 HgO 2 Hg + O2 c. displacement reactions Zn + CuSO4 Cu + ZnSO4 and d. disproportionation reactions Hg2+2 Hg + Hg+2 Probably the most familiar example of a redox reaction you use every day is the one occurring in the internal combustion engine of your car. Hydrocarbon fuels, such as the octane in gasoline, combine with oxygen to produce carbon dioxide (CO2) and water (H2O).

2 C8H18 + 25 O2 16 CO2 + 18 H2O

Similarly, when you burn propane in a gas barbeque, it undergoes oxidation. The equation shown below represents the burning of propane. Which substance is reduced? (Hint: there are only two reactants...one undergoes oxidation, and the other undergoes reduction!)

C3H8 + 5 O2 3 CO2 + 4 H2O + heat

To learn more about the combustion of fuels, visit this web site:



Environmental Redox Cycle Site

Oxidation-reduction reactions are extremely important to the environment. While they are always at work around you, they may go unnoticed. Important examples include the carbon, nitrogen, and sulfur cycles. The transformation of carbon dioxide into organic compounds, which in turn produces oxygen needed by plants as well as animals to metabolize carbon compounds, is particularly integral to all life on the planet. Learn more about the chemical reactions involved in these processes at the following web site.



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Active Metal Series How can scientists predict if an oxidation-reduction reaction will occur? Check the web for the answer.



Life-Cycle Redox Processes Without oxidation-reduction reactions, living organisms would be...dead! To learn more about these very important processes, visit the web site below.



Pre-lab

Safety: Always wear your safety goggles in the laboratory. Your TA will ask you to leave the laboratory unless you are wearing proper protective eyewear.

Caution! Acids and bases are hazardous if splashed on clothing, exposed skin or in the eyes. Prolonged exposure of the skin to even dilute solutions of acid and base can cause serious burns. If acids or bases splash on skin or clothes, remove the affected clothing and flush the affected areas thoroughly with cold water. Make sure you are familiar with the hazards associated with all reagents used in this experiment before you begin your work.

Dispose of all solutions in the hazardous waste container.

Pre-lab Assignment: Please write out the following in your lab notebook. This assignment must be completed before the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and accepted by your TA. 1) Briefly describe the objectives of this experiment. 2) Create a list of the chemicals to be used; identify any safety concerns or hazards associated with each. 3) Answer the questions below as part of your prelab.

In addition to these pre-lab requirements, a short quiz may be given at the beginning of lab based on the material in this lab write-up.

In a previous lab, we performed an acid-base titration to determine the concentration of an unknown acetic acid solution (vinegar). In the following prelab, you will use titration data to calculate the concentration of an unknown sulfuric acid solution, H2SO4.

A 0.0100 L sample of H2SO4 was titrated with 0.150 M NaOH. It took 23.3 mL of NaOH from the buret--that's 0.0233 L--to reach the titration endpoint.

A) Balance the chemical equation for this titration by placing coefficients in the spaces below. (If you need to learn or review how to balance equations, visit this link for a tutorial or see section 0.4 in your Laird University Chemistry textbook.)

____ H2SO4(aq) + ____ NaOH(aq) ____ H2O(l) + ____ Na2SO4(aq) 2

B) Based on the coefficients of the equation that you just balanced, how many moles of NaOH would be consumed by one mole of H2SO4?

C) How many moles of NaOH are contained in the 0.0233 L volume of 0.150 M NaOH that is delivered from the buret?

D) How many moles of H2SO4 must be present to react completely with the NaOH delivered from the buret?

E) What is the molarity (M) of the H2SO4 solution titrated?

Procedure

Part 1 ? Balancing the Redox Equation

Reagents: 0.020 M KMnO4, 0.20 M FeCl2, and 6 M H3PO4

In a previous lab period, you performed a vinegar titration to determine the concentration of acetic acid. Phenolphthalein was used as an acid-base indicator to visualize the endpoint. The color of your solution was dependent on the form of the indicator species present. In the case of phenolphthalein, the acid form is colorless and the basic form is pink. The titration was complete at the point when approximately half a drop of additional NaOH caused the solution to remain slightly pink. At this point, the moles of OH- ions from the NaOH were equal to (or in very slight excess of) the moles of protons from the acetic acid. This 1:1 stoichiometry is shown below.

NaOH + HC2H3O2 NaC2H3O2 + H2O

In the acid-base titration, you titrated an acid against a base utilizing an acid-base indicator to find the equivalence point. Today, you will be investigating an oxidation-reduction reaction and it would be convenient if there were some indicator to help you find this equivalence point. Fortunately, the color of a solution is frequently due to the oxidation state of a species in solution. Some commonly used redox reagents that are themselves redox indicators are potassium permanganate (KMnO4) and potassium dichromate (K2Cr2O7). These are internal indicators because of large differences in the colors of their oxidized (MnO4-/purple, Cr2O72-/orange-yellow) and reduced forms (Mn2+/light pink, Cr3+/green).

Solution KMnO4 FeCl2 H3PO4

Conc 0.020M 0.20 M

6 M

Trial 1 5 mL 10 mL 10 mL

Trial 2 10 mL 10 mL 10 mL

Trial 3 15 mL 10 mL 10 mL

Trial 4 20 mL 10 mL 10 mL

Trial 5 25 mL 10 mL 10 mL

Your lab group will investigate each of the solution mixtures above. In the laboratory, you will find prepared solutions of KMnO4, FeCl2, and H3PO4. (In this reaction, phosphoric acid is necessary to provide the acid for this reaction, as seen in the reaction below, as well as prevent interference from the

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chloride ions.) Using the chart above, place appropriate amounts of each reagent in a small beaker. Use volumetric pipets for the iron solution, and auto-pipetters for the acid and the permanganate solution. Determine how many moles of each reagent are present in your solutions and report those amounts. Record the final colors of each of these solutions. From your observations, you should be able to determine which of these solutions has added excess KMnO4 and which have not yet gotten to that point. Use this to determine the stoichiometry of the reaction listed below. Confirm that your stoichiometry makes sense by balancing the redox reaction as discussed in your textbook and in class.

Fe2+(aq) + MnO4-(aq) + H+(aq) Fe3+(aq) + Mn2+(aq) + H2O(l)

Part 2 ? Determining the Unknown FeCl2 Concentration

Reagents: 0.020 M KMnO4, 6 M H3PO4, and FeCl2 solution (unknown concentration) 1. Using the information you obtained in Part 1, plan an investigation that will allow you to determine the concentration of a FeCl2 solution of unknown concentration that your TA provides. (Hint: If you need to review an example titration, you may wish to consult the write-up, procedure, and/or data from your "Determination of Acetic Acid in Vinegar" experiment.) 2. Complete your investigation and determine the concentration of the FeCl2 solution. Show balanced equations and calculations in your notebook, and be certain that all your numerical data is expressed with the proper units and significant figures.

Report

Your lab report should be a formal, individual report prepared according to the "Guidelines for Laboratory Reports" you have been given. In addition to the categories discussed in these guidelines you should provide answers to all the questions posed in this laboratory experiment write-up. You should discuss the colors observed for your different solutions and what those colors are due to. You should also discuss the redox reaction that is occurring in this system and the balanced redox equation that you derive from your experiment. You should also discuss what is happening in your titration experiment and how this titration allows you to determine the concentration of the unknown. How is this different and how is it similar to the acid-base titrations you have carried out previously this semester?

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