Equilibrium of the Iron Thiocyanate Reaction

Transcription

1 Equilibrium of the Iron Thiocyanate Reaction Many chemical reactions are found to proceed to an equilibrium at which a mixture of both reactant and products is present. The extent to which reactants are converted to products is expressed by the equilibrium constant, K. This experiment will probe the equilibrium of Fe(III) ions reacting with the thiocyanate ion, SCN. The reaction has two possible products given below, in lab this week you will determine which of these two reactions actually occurs. Fe 3+ + SCN FeSCN 2+ Rxn 1 Fe SCN Fe(SCN) 2 + Rxn 2 The nice aspect of this equilibrium is that the reactants are colorless, but the product is a deep red color. The concentration of the products at equilibrium will be measure by observing the absorption of blue light. By measuring the concentration of the product produced from five different reactant concentrations, you will determine which equilibrium above occurs for the iron thiocyanate reaction. Measuring Concentration by Optical Absorption: Along with studying a system at chemical equilibrium, this laboratory will introduce the concept of absorption spectroscopy. The red product of the reaction is absorbing blue light, allowing the compound to transmit red room light to your eye. There is a simple relationship between the amount of light a sample will absorb and the concentration of the absorbing molecule. A lc In this relationship A is the absorbance (read by a spectrophotometer), c is the solution concentration (mol/l), l is the path length of the sample (cm) and is the extinction coefficient of the molecule (in this case it has units of L / mol cm). indicates how easily the molecule absorbs light at a specific wavelength. This relationship between solution absorbance and analyte concentration is known as Beer s law. The important aspect of Beer s law in this instance is that Absorbance is linearly related to the concentration of the absorbing species. This allows the creation of a calibration curve to be constructed between absorbance and known concentrations of the product. In Figure 1 the iron thiocyanate ion absorbance at 460 nm is plotted as a function of concentration. This plot allows the rapid determination of solution concentration from solution absorbance. As you perform these absorbance measurements, consider how long it took to make a concentration measurement using freezing point depression. (The advantage of freezing point depression is you do not need to know the identity of the compound.)

2 [Fe(SCN)? ]?+ calibration curve at 460 nm Abs = [conc] E E E E E-04 [Fe(SCN)? ]?+ mol/l Figure 1. An absorbance vs. concentration plot for the product of the iron thiocyanate equilibrium. Experimental Procedure: Preparing the Five Equilibrium Mixtures: From the front of the room obtain a 5 ml pipet, a graduated pipet, a red pipet bulb and two spectrophotometer tubes. Place the spectrophotometer tubes in your test tube rack. In your smallest beakers obtain from the reagent bottles 30 ml of M Fe(NO 3 ) 3 and 20 ml of M KSCN. The five solutions you will prepare will have equal amounts of iron ions and differing quantities of thiocyanate ions. Each solution will have the same 10 ml volume, made by adding varying amounts of H 2 O. In your test tube rack add five clean test tubes. Label your test tubes 1 5 by placing your rack on a sheet of paper and writing the number on the paper. Fill the test tubes with the reagents indicated in Table 1. It is best to fill all tubes first with the amounts of thiocyante using the graduated pipet. Place the pipet bulb gently over the end of the pipet. The bulb should be on so loosely that it would fall off if you held the pipet upside down. Do not jam the bulb onto the pipet. Allow your partner to hold your reagent beaker. To deliver 1.0 ml of SCN, draw into the pipet 3 or 4 ml of solution. Remove the bulb and allow the solution to drain back into the reagent beaker. Place your index finger over the top of the

3 pipet to control the flow. Slow the flow down with your finger, stopping it at the 1 ml mark (or 9 ml mark depending on the labeling of your pipet). Expect to draw up and drain the solution several times before you get the volume correctly. Transfer the SCN solution to the correct test tube. Repeat the addition for the other four test tubes. (Be sure that all members of the group have the pleasure of pipetting.) After the SCN solutions have been added, use the 5.0 ml pipet to measure the Fe 3+ solutions. The drawing up in this 5 ml pipet is more difficult because there is only a small volume between the 5 ml mark and the top. Have the bulb on loosely, so that it is easily removed as the liquid nears the mark. Try hard not to fill the red bulb with Fe(NO 3 ) 3. If the solution gets on your hands wash them immediately. Expect to perform several extractions before the correct volume is reached. Rinse the graduated pipet with water and add the necessary amounts of water to each test tube. This should take the volume of each test tube to 10 ml. You will need a little beaker of DI water from which you will draw the water into the pipet. ml of 0.002M SCN ml of 0.002M Fe 3+ ml H 2 O Tube Tube Tube Tube Tube Obtain from the front of the room a piece of parafilm so that the test tubes can be shaken. Cover the top of each test tube with the parafilm and press down with your thumb. Invert the tube several times to completely mix the solution. Start with the most dilute SCN solution so that as the next solution is mixed the parafilm does not accidentally contaminate a less dilute solution. Any reagent remaining in your beakers should be placed in the waste-beaker in the hood. Calculate the initial concentrations of Fe 3+ and SCN in each test tube 1-5. This is just a dilution calculation.

4 Measurement of the Product Concentration: Take your solutions to the spectrophotometer (Spec 20) to measure the absorbance of the FeSCN ion-complex. Since you have the absorbance vs. concentration curve at a wavelength of 460 nm, dial your spectrometer to that wavelength. Examine the spec 20 schematic in Figure 2 to see the location of the control knobs. Sample Compartment Wavelength Control 0 % Transmission Control: Adjust to 0% T when no sample present. On/Off Switch 100 % Transmission Control: Adjust to 100% T when blank present. Figure 2. Schematic of a Spec 20 Turn on the Spectronic 20 or spectrometer by turning the zero control knob (left front) clockwise, close the sample compartment door, and allow the instrument to warm up at least 10 minutes. After the instrument has warmed up, adjust the wavelength control to 460 nm and adjust the zero control knob until the meter reads 0% T. Pour a small amount of the blank solution or DI water into the spectrometer tube. Rotate the spectrometer tube so the solution coats the wall of the spectrometer tube. Discard DI water down the sink. Refill the spectrometer tube to ¾ full with DI water. Wipe the outside of the spectrometer tube with a Chemwipe or paper-towel. Insert the cuvette into the Spectronic 20. Line up the white hash mark on the spectrometer tube with the raised mark on the sample compartment. Close the sample compartment door. Slowly rotate the light control knob (right front) clockwise until the meter reads 100% T. Be careful not to change the zero control knob during this process. The

5 Spectronic 20 is now internally calibrated (meaning the instrument is properly adjusted for operation). After the spectrometer has been calibrated, measure the percent transmittance (%T) for each FeSCN solution. Fill the second spectrometer tube with solution #1 (the most dilute). Place the solution in the spectrometer and record the absorbance. After the %T is recorded the solution can be returned to the test tube and the second most concentrated solution can be measured. Continue in this way until all solution percent transmittances have been measured. Convert all recorded %T to absorbance (A) using the following equations: A = - log T T = %T 100% Determine the concentration of the FeSCN ion-complex in each reaction tube by converting all calculated absorbance to concentration using the linear equation in the Figure 1 calibration curve. All of the test tube solutions can be placed in the waste beaker. The Spec 20 tubes should be rinsed thoroughly with DI water and returned to the front of the room. To prepare for next week s lab on Acid-Base Titration, prepare approximately 500 ml of 0.1 M NaOH solution from a stock NaOH solution of 6 M. This requires you to do a dilution calculation. The 500 ml 0.1M NaOH solution can be stored in a corked flask until next week s lab.

6 Determining the Equilibrium Constant for the Two Proposed Reactions: In this experiment data collection is rather quick and data analysis is rather lengthy. As you did in the pre-lab assignment, determine K for each proposed reaction using all five initial concentrations. Recall that you have measured the [FeSCN] eq concentration and you will have to use the reaction stoichiometry to calculate [Fe 3+ ] eq and [SCN ] eq from the initial concentrations of [Fe 3+ ] 0 and [SCN ] 0. It is probably a good idea to write the ICE tables for each reaction and create a data table that contains the components needed to calculate K (ex. [Fe 3+ ] eq, [SCN ] eq and [Fe(SCN)??+ ] eq ) for each test tube. From the reproducibility of the equilibrium constant, which equilibrium reaction Fe 3+ + SCN FeSCN 2+ Rxn 1 Fe SCN Fe(SCN) 2 + Rxn 2 occurs in the reaction of Fe 3+ with SCN? Report the equilibrium constant, K, as an average of the five experimental measurements. The uncertainty in your measurement can be given as the standard deviation in your measurement. If you are unfamiliar with standard deviation calculations, ask the TA to show you how this can quickly be calculated in Excel. In your abstract briefly describe the system studied and the experimental method used to determine K. Also report the equilibrium constant and its uncertainty. Turn in your lab sheets before you leave lab. Place your 0.1 M NaOH solution at the front of the room. It will be stored until next week.