Many chemical reactions reach a state of equilibrium if conditions are right. In an equilibrium system, forward and reverse reactions occur at equal rates so that no net change is produced. When equilibrium is reached by a reaction in a test tube, it appears that changes have stopped in the tube. Once equilbrium has been reached, is it possible to produce further observable changes in the tube? If so, can you control the kinds of changes? If not, why are further observable changes impossible? You will observe several chemical systems in this laboratory activity. A careful study of your observations will enable you to answer these questions.
Procedure
Obtain a test tube rack, six small (13 x 100 mm) test tubes that are clean but don't have to be dry, and a test tube clamp. The test tubes should be placed open end up in the test tube rack. Prepare a hot water bath: Half-fill a 250 mL beaker with tap water. Start to heat the water (as your teacher directs) so that the water will be near boiling when you are ready to use it. Prepare an ice water bath: Fill a 250 mL beaker with crushed ice. Add enough tap water to make "slush". Set up a data table with column headings as indicated below (The last column will be completed after data have been collected.) System Disturbance Observed Change Direction of Shift 1 2
etc. As you set up equilibrium systems and add disturbances to them in the procedure, enter appropriate information in each of the first three columns of your data table. Mix chemicals in test tubes by holding the top of the tube with one hand while you flick the bottom of the tube with your other hand until the tube contents.
System 1: Iron(III) and thiocyanate
Setting Up the Equilibrium
Half-fill the first tube in your rack with distilled water. Add two drops of 0.1 M Fe(NO3)3 and two drops of 0.1 M KSCN to this tube. Mix the contents thoroughly. If the contents of the tube are not red-orange, repeat Step 2 until the solution is red-orange. Divide the red-orange solution in the first tube among six tubes so each tube contains the same volume.
Chemical Equation for the Equilibrium System Fe3+(aq) + SCN-(aq) FeSCN2+(aq) + heat Colorless Colorless Red-orange from Fe(NO3)3 from KSCN
Disturbing the Equilibrium
Leave Tube 1 undisturbed; use it as a control. Use a clean, dry spatula to add a small crystal or two of solid iron(III) nitrate, Fe(NO3)3, to Tube 2. Mix. Under Disturbance on your data table, record what you did or added to the system to cause the change you observed. In this and all other observations, pay particular attention to color and color change. Always compare with the control tube or you may miss slight color changes. Phrase your Observed Change so the kind of change you observe is indicated, e.g., "lighter red" or "from grey to pink." Use a clean, dry spatula to add one or two small crystals of solid potassium thiocyanate, KSCN, to Tube 3. Mix. Record observations. Add 5 drops of 0.1 M sodium hydroxide, NaOH, to Tube 4. Mix, observe, and record.
Use a test tube clamp to place Tube 5 in a hot water bath. When the contents of the tube are hot, observe and record. Use a test tube clamp to place Tube 6 in an ice water bath. When the contents of the tube are cold, observe and record. (Data check: Obtain your teacher's initials.) Discard all test tube contents in the waste container provided by your teacher. Do not pour anything in the sink. Rinse the tubes with tap water; remove as much water as possible by shaking before standing the tubes upright in the test tube rack. Follow these same disposal and rinsing procedures after you complete each system below.
System 2: Bromothymol blue
Setting Up the Equilibrium
Half-fill three test tubes with distilled water. Add three drops of bromothymol blue indicator to each tube. Mix thoroughly.
Chemical Equation for the Equilibrium
Bromothymol blue is a weak organic acid with a complex formula. For our purpose, its formula can be abbreviated to HBb. HBb(aq) H+(aq) + Bb-(aq) Yellow Colorless Blue
(Green can be observed if approximately equal amounts of yellow and blue forms are present.)
Disturbing the Equilibrium
To Tube 2 add two drops of 0.1 M hydrochloric acid, HCl, and mix. Observe and record. To Tube 3 add two drops of 0.1 M sodium hydroxide, NaOH, and mix. Observe and record. Explore what happens when you now add NaOH to Tube 2 or HCl to Tube 3. See whether your observations are in agreement with observations you have already recorded.
System 3: Complex Ions of Copper(II) (Cu2+)
Setting Up the Equilibrium Half fill a test tube with 1.5 M copper(II) chloride, CuCl2, solution. Divide so five tubes contain approximately equal volumes. Equilibrium has already been established in the solution.
Chemical Equation for the Equilibrium CuCl42-(aq) + 4 H2O(l) Cu(H2O)42+(aq) + 4 Cl-(aq) + heat Green soln Colorless Light blue soln Colorless
Disturbing the Equilibrium
To Tube 2 add a small quantity (the size of a rice grain) of solid calcium chloride, CaCl2. Mix to dissolve the solid. Repeat the addition and dissolving of solid CaCl2 until no more solid will dissolve. Observe and record. To Tube 3 add enough ethyl alcohol, C2H5OH, to triple the volume of the solution. Mix, observe, and record. Place Tube 4 in a hot-water bath. When the solution is hot, observe and record. Place Tube 5 in an ice-water bath. When the solution is cold, observe and record.
System 4: Dinitrogen tetroxide (N2O4)
Setting Up the Equilibrium
Dinitrogen tetroxide, N2O4, can decompose into nitrogen dioxide, NO2, a reddish brown poisonous gas. So that you may work with these substances safely, your teacher will provide two sealed tubes each containing a mixture of these subtances. Equilibrium between N2O4 and NO2 has already been established in the tubes.
Chemical Equation for the Equilibrium N2O4(g) + heat 2 NO2(g) Colorless Reddish brown
Disturbing the Equilibrium (Caution: N2O4 and NO2 in the sealed glass tubes are poisonous. Handle the tubes carefully to avoid breaking the tubes and releasing the gases.) Place one sealed tube containing the equilibrium system in a hot water bath. When hot, compare to the unheated tube and record. After removing the tube from the hot water bath, cool it under running cold tap water. Then place the tube in an ice-water bath. When cold, compare to the unchilled tube and record.
System 5: Complex Ions of Cobalt(II) (Co2+)
Setting Up the Equilibrium Half-fill a test tube with 1.5 M cobalt(II) chloride, CoCl2. Divide the solution so five tubes contain approximately equal volumes. Equilibrium has already been established in the solution.
Chemical Equation for the Equilibrium heat + Co(H2O)62+(aq) + 4 Cl-(aq) CoCl42-(aq) + 6 H2O(l) Red Colorless Blue Colorless
Disturbing the Equilibrium
To Tube 2 add a small quantity (the size of a rice grain) of solid calcium chloride, CaCl2. Mix to dissolve the solid. Repeat the addition and dissolving of solid CaCl2 until no more solid will dissolve. Observe and record. To Tube 3 add enough acetone, CH3COCH3, to double the volume of the solution. Mix, observe, and record. Place Tube 4 in a hot water bath. When the solution is hot, observe and record. Place Tube 5 in an ice water bath. When the solution is cold, observe and record. Wash hands thoroughly before leaving the laboratory.
Data Analysis and Concept Development
To complete the fourth column on the right side of your data table (headed Direction of Shift), decide whether each disturbance caused the equilibrium system to shift left or right. Record the direction of shift in this column. How do you decide direction of shift? Consider the equilibrium system A B Yellow Green
If a disturbance causes the system to become more yellow, chemists would say that the equilibrium position has shifted to the left because the system must have moved to produce more of the yellow molecules shown on the left side of the chemical equation. If the system shifted to the right you would observe more green in the system. The direction of shift is "right". Use these ideas to decide and record the direction of shift caused by each disturbance. Use your data table to find all cases where a disturbance was caused by heating. After you have found all of these cases, answer the following: How does the direction of shift relate to the side of the chemical equation on which the heat term is written? Write a rule which would allow you to predict how other equilibrium systems would shift when disturbed in this way. Use your data table to find all cases where equilibrium systems were disturbed by cooling.
How does the direction of shift relate to the side of the chemical equation on which the heat term is written? Write a rule which would allow you to predict how other equilibrium systems would shift when disturbed in this way. Use your data table to examine all cases where a disturbance was caused by increasing the concentration of a substance already present in the equilibrium system. Hint: Adding solid Fe(NO3)3 to System 2 increases the concentration of Fe3+(aq) and NO3-(aq) when the solid dissolves. Adding HCl solution to System 3 increases the concentration of both H+(aq) and Cl-(aq) in the system. Write a rule which would explian how the direction of shift relates to the side of the chemical reaction on which the substance with increased concentration is written. In some cases the equilibrium system was disturbed by decreasing the concentration of a substance in the system. Usually this is done by adding another substance not involved in the equilibrium which reacts with a substance in the system, changing it to a different substance. For example, in System 1 you added 0.1 M NaOH (containing aqueous Na+ and OH- ions). OH- reacts with Fe3+ to form the precipitate Fe(OH)3(s). This decreases the concentration of Fe3+(aq) remaining in the solution. Concentration can also be decreased by adding another solvent (acetone or alcohol) to dilute the water in the system. Identify substances whose concentration is decreased in as many cases as you can. For each, explain what causes the concentration of a particular substance to decrease. Write chemical equations where possible.
The equation for the example above is: Fe3+(aq) + 3 OH-(aq) Fe(OH)3(s) For each case involving a decrease in concentration, identify the substance that is decreased in concentration, on which side of the equation this substance is found, and which way the equilibrium is observed to shift. Consider cases where equilibrium was disturbed by decreasing the concentration of a substance in the equilibrium system. How does the direction of shift relate to the side of the chemical equation on which the substance with altered concentration is written? Write a rule which would allow you to predict how other equilibrium systems would shift when disturbed in this way. Write a general rule that would cover all of the types of disturbances you have observed. Write your rule so it can be used to predict the effect of any temperature or concentration disturbance on an equilibrium system
Manfred Eigen was born in Bochum on 9 May 1927, the son of the chamber musician Ernst Eigen and his wife Hedwig, n?e Feld. He received his schooling at the Bochum humanistic Gymnasium.
In the autumn of 1945 he commenced the physics and chemistry course at the Georg-August University in G?ttingen and obtained his doctorate in natural science in 1951. He wrote his dissertation on the specific heat of heavy water and aqueous electrolyte solutions under the guidance of Arnold Eucken. After two years as an assistant lecturer at the physical chemistry department of the university under Ewald Wicke, he transferred to the Max-Planck Institut f?r physikalische Chemie, which had moved to G?ttingen under the Directorship of Karl Friedrich Bonhoeffer. The influence of Bonhoeffer, who provided him with magnificent working conditions at the Institut, is reflected in his later work in the field of biophysical chemistry.
Eigen began his work on the problem of fast ionic reactions in solution in the period 1951-1953, encouraged to do so by the ultrasound absorption measurements carried out by his colleagues Konrad Tamm and Walter Kurtze. During the following years he developed a series of measuring techniques involving times down to the order of a nanosecond. He developed many of these techniques with Leo de Maeyer, who joined him in the autumn of 1954, and with whom he is still collaborating closely at the G?ttingen MaxPlanck-Institut. The Max-Planck-Gesellschaft appointed Eigen a Scientific Member in 1957 and head in 1964. In 1967 he was elected Managing Director of the Institute for a period of three years. At the same time he was appointed to the Scientific Council of the German Federal Republic.
Eigen's scientific development is reflected in the close on 100 papers he has published. The subject matter of these works ranges from the thermodynamic properties of water and aqueous solutions, and the theory of electrolytes, through thermal conductivity and sound absorption, to fast ionic reactions.
In the years 1953-1963 followed the description of a series of novel measuring techniques used for the study of very fast reaction in the range from one second to one nanosecond. The gap between the region of classical reaction kinetics and spectroscopy was thus closed. Eigen was particularly interested in proton reactions: together with De Maeyer he was the first to determine the neutralization rate and found the anomalous conduction characteristics of protons in ice crystals. The development of the theory of relaxation of multi stage processes was followed by studies on metal complex reactions, in which the fast reactions of a large number of metal ions were investigated in relation to their position in the periodic table. Around 1960 the emphasis in his work shifted towards physical-organic chemistry. The individual steps of a series of reaction mechanisms were elucidated, and a general theory of acid-base catalysis was verified experimentally.
At the same time, however, his attention turned also to biochemical questions, which now claimed his chief interest. These questions ranged from hydrogen bridges of nucleic acids, through the dynamics of code transfer, to enzymes and lipid membranes. Biological control and regulation processes, and the problem of the storage of information in the central nervous system also occupy his attention. Practically every year he travels together with his friend and colleague Leo de Maeyer to Boston to discuss topics of common interest with American neurologists, biochemists, and biophysicists.
Eigen holds the following honours and distinctions: Bodenstein prize of the Deutsche Bunsengesellschaft, 1956; Otto-Hahn Prize for Chemistry and Physics, 1962; Kirkwood Medal (American Chemical Society), 1963; Harrison Howe Award (American Chemical Society), 1965; Andrew D. White Professor at large at Cornell University, Ithaca, N.Y., 1965; Honorary Professor at the Technische Hochschule, Braunschweig, 1965; Foreign Honorary Member of the American Academy of Arts and Sciences, 1964; Member of the "Leopoldina", Deutsche Akademie der Naturforscher in Halle, 1964; Member of the G?ttingen Akademie der Wissenschaften, 1965; Honorary Member of the American Association of Biological Chemists, 1966; Honorary degree of doctor of science at Harvard University, U.S.A., 1966; Honorary degree of doctor of science at Washington University, U.S.A., 1966; Foreign Associate of the National Academy of Sciences, Washington, U.S.A. 1966; Honorary degree of doctor of science, University of Chicago, U.S.A., 1966; Carus Medal of the Deutsche Akademie der Naturforscher "Leopoldina", Halle, 1967; Linus Pauling Medal of the American Chemical Society, 1967.
Manfred Eigen is married to Elfriede, n?e M?ller. They have two children, Gerald (born 1952) and Angela (born 1960). In his free time he is a keen amateur musician. His favorite holiday pastime is mountaineering.
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