Experiment no. 9 Chemical Equilibrium Agustin, Agustin, Victoria Victoria T. T. Crisostomo, Jan Christine R. Morales, Jessica Christine C Group # 9, Chem. 14.1, WEG1, Mr. Ralph Julius L. Mendoza March 18, 2009 Chemical equilibrium is an extremely important process in nature particularly in many industrial (e.g. production of ammonia) and biological processes (production of hemoglobin in relation to altitude). Experiment 9, chemical equilibrium, will determine how various stresses, according to Le Chatelier’s Priciple, being introduced in a system at equilibrium can alter the system’s equilibrium position by shifting in direction to counteract the effect of the stress. These stresses include increase or decrease in concentration, temperature, and pressure. To know the effect of the change in concentration, different reactants are added in the initial mixture and for the determination of the effect of temperature change, two mixtures with the same components are used: the temperature in one of the mixtures mixtures is increas increased ed while while the temperatur temperature e in the other other mixture mixture is decrease decreased. d. For both set-ups set-ups,, an undisturbed mixture is used as reference for the comparison in the color of the disturbed mixtures. A darker disturbed mixture will mean a forward reaction, conversely, a lighter disturbed mixture will mean a backward reaction. Again, the significance of this color change together with its corresponding shift in direction is to tell that the reaction is going towards its natural state which is to be in equilibrium. Keywords: reversible reactions, equilibrium, chemical equilibrium, Le Chatelier’s principle, stress
Introduction: Equilib Equilibriu rium m is a state state where where there there are no observable changes as time goes by, that is, it is constan constantt in time time and space. space. Chemic Chemical al reactio reactions, ns, particularly reversible reactions, have the tendency to alter alter its conditi conditions ons to achieve achieve equili equilibriu brium. m. At this chemical chemical equilib equilibrium rium,, the rates rates of the forwar forward d and reve reverse rse reac reacti tion ons s are are equal equal.. Furth Furtherm ermor ore, e, the the concentrations of the products and reactants remain const constan ant. t. Chem Chemica icall equi equilib libri rium um is impo import rtant ant in understanding understanding biological and industrial industrial process such as produc production tion of hemogl hemoglobin obin in relatio relation n to higher higher altitudes and production of ammonia. Le Chatelier’s principle is used to predict the directi direction on or shift shift of the equilib equilibrium rium position position when when stress stress such such as change change in concen concentrat tration ion,, pressur pressure, e, volume, or temperature occurs in the reaction. This This expe experi rim ment ent will ill focu focus s more more on the the evaluation of the effects of changes in concentration and temper temperatu ature re on the equilib equilibrium rium,, explain explain these these effects on the equilibrium system, and interpret these consequences through the Le Chatelier’s principle. Experimental: 20 drops of 1 M Fe(NO3)3 solution and 20 drops of 1 M KCNS solution were mixed in a 10-ml test tube, and 7 ml of water was added. The mixture was then shook and observed. After the solution was prepared, eight test tubes with labels were filled with ten drops of the solution. 10 drops of each of the followi following: ng: distill distilled ed water, water, 0.1 M Fe(NO Fe(NO3)3, 0.1 0.1 M KCNS, 0.1 M KCl, 0.1 M AgNO 3, and a pinch of NaF, was add added to test est tub tubes 1,2 1,2,3,4,5, and and 6 respectively. Observations were made on test tubes 2 Chem. 14.1, Chemical Equilibrium
to 6 using test tube 1 as reference. The changes in the intensity of color or appearance of the mixture were noted. noted. The observation observations s recorded recorded after each addition of reagent and the reactions involved were analyzed. On the other hand, 10 drops of distilled water was added to test tubes 7 and 8. Test tube 7 was placed in ice water and was then compared to test tube 1. The solution in test tube 8 was heated over a low Bunsen flame. Results: Given the following reaction: Fe3+ + CNS FeCNS2+ orange orange colorl colorless ess blood-r blood-red ed Reag Reagen entt /Tre /Treatm atmen entt
Obser Observa vatio tion n (com (compa pare red d to test tube #1) 0.1 M Fe(NO3)3 Became lighter 0.1 M KCNS Became darker 0.1 M KCl Became lighter 0.1 M AgNO3 Became lighter Pinch of NaF Became lighter Increase in temp. Became lighter Decrease in temp. Became darker Table 1: Results gathered from the reactions
Direction of Shift to the left to the right to the left to the left to the left to the left to the right
Discussion: Le Chat Chatel elier ier’s ’s Princ Princip iple le stat states es that that if an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is part partia iall lly y offs offset et as the the syst system em reac reache hes s a new equilib equilibrium rium positio position. n. Change Change in concent concentrat ration ion and change in temperature are the most common forms of stress that affect a system in a state of equilibrium. It should be noted that a system has a tendency to go Page 1 of 4
back to its natural state which is the state of equilibrium. In the experiment, the initial mixture was composed of Fe(NO3)3 (orange) and KCNS(colorless) dissolved in 7 ml of water. After the solute had completely dissolved, a dark-colored mixture was obtained. Each of the eight labelled test tubes was filled with 10 drops of the initial mixture. In test tube # 1, 10 drops of distilled water was added. Upon adding, no change occurred because water as a solvent did not alter the concentration of the mixture. Test tube # 1 was used, too, as reference for comparison, in terms of color of other disturbed mixtures. In test tube # 2, 10 drops of 0.1M Fe(NO 3)3 was added in the mixture, experimentally producing a lighter colored product as compared to test tube # 1, but theoretically the product should have been darker. Keeping in mind that moles are additive, the addition of 0.1 M of Fe(NO3)3 to the initial mixture having 1 M of Fe(NO3)3 means that the reagent increased the concentration of the reactant side causing a shift to the right. Meanwhile, 10 drops of 0.1 M KCNS was added in test tube # 3, resulting to a darker-colored product. The same concept is observed for test tube # 3. In test tube # 4, 0.1 M KCl was added in the mixture which yielded a lighter-colored product than that of the test tube # 1. This means that there was a shift to the left. Theoretically, since KCl is a salt, adding it to the initial mixture will cause it dissociate into K+ and Cl-. These ions did not interact with Fe 3+ and CNS-. This non-interaction means that the dissociation of KCl in this case will yield spectator ions, or ions that are not involved in the overall reaction. Therefore, same color should have been observed which means that no shift occurred. In test tube # 5, 0.1 M of Ag 3NO3 was added in the mixture producing a lighter mixture with a precipitate causing it to shift to the left. This reaction can be described by the following equation: Ag3NO3 + Fe3+ + SCN- AgSCN + Fe(NO3)3. Applying the solubility rules, the nitrate salt [in this case] Fe(NO3)3 is soluble in the mixture while salts with a transition metal [in this case] AgSCN is insoluble in the mixture, making AgSCN the precipitate. In test tube #6, a pinch of NaF was added producing a lighter colored solution that caused a shift to the left. In addition to this, white substances were formed but these are not what we call precipitate. Instead, these substances are complex ions? bat xa ndi nagdissolve? ang complex ions ba insoluble? The reaction is described by the equation NaF + Fe3+ + SCN- FeF3+ + NaSCN. NaSCN, a soluble salt and FeF3+, a complex ion (an ion containing a central metal cation bonded to one or more molecules or ions), were
Chem. 14.1, Chemical Equilibrium
formed. The formation of FeF3+ increased the solubility of NaF. In test tube # 7, 10 drops of distilled water was added, in addition to that, heat was also introduced in the system. After increasing the temperature to 67o C, the product yielded was lighter as compared to test tube # 1 and this means that the shift was to the left. This implies that heat was added in the product side making the reaction exothermic. It should be noted that adding heat is similar to the concept of adding concentration. Meanwhile in test tube # 8, 10 drops of distilled water was added in the mixture and was subjected to a significant decrease in temperature. Since the reaction is exothermic, “removing” that is, decreasing the temperature [in the product side] would cause a shift to the right accounting for the observed darker solution. It formed a darker solution as compared to test tube # 1 indicating that the shift was to the right. *ndi na ata endothermic kasi ang tanong ay kng exo or endo since pinili na natin ung exo, ndi na pedeng endo. Conclusion and Recommendations: Chemical equilibrium is achieved when the reaction in the reactant side, as well as in the product side proceeds at the same rate and the concentration of products and reactants ceases to change. The experiment showed how changes in concentration and temperature affect the direction of equilibrium. The La Chatelier’s Principle which states that if a chemical system is at equilibrium and we add a substance (either a reactant or a product), the reaction will shift so as to re-establish equilibrium by consuming part of the added substance. Conversely, removing a substance will cause the reaction to move in the direction that forms more of that substance described the shifting taking place in a system at equilibrium. Changing the concentration of a substance in equilibrium will cause a shift in the system in the direction which is opposite to the side where the stress is applied to counter act the effect of the said stress. Furthermore, same principle is applied when there is a change in the temperature since heat is treated as either an agent which may be added in the product or in the reactant side. One must be very careful in observing the changes in color that occurred during the experiment because these changes crucial in determining the shift in the experiment. References: - Chemistry: The Central Science (Ninth Edition) by Brown, Lemay, and Bursten - Chemistry (Ninth Edition) by Chang - http://pages.towson.edu/ladon/solprod.html
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http://www.chem1.com/acad/webtext/chemeq/Eq01.ht ml - The Advanced Placement Examination in Chemistry by the College Entrance Examination Board, Princeton, NJ - http://www.public.asu.edu/~jpbirk/CHM115_BLB/Chpt17/ I hereby certify that I have given substantial contribution to this report. ________________________ Agustin, Victoria T. ________________________ Crisostomo, Jan Christine R. ________________________ Morales, Jessica Christine C.
Chem. 14.1, Chemical Equilibrium
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Fe (NO3)3
H2O
KCNS
Initial mixture: shake, color? Into 8 test tubes TT #1: + distilled H 2O [reference test tube] TT #2: + Fe(NO3)3 TT #3: + KCNS TT #4: + KCl TT#5: + AgNO3 TT#6: + a pinch of Na F TT #7: +distilled H2O+ decrease in temperature TT#8: + distilled H2O+ increase in temperature
Observe changes in intensities of colors and compare with TT#1
