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Chemistry 12
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Purpose : Establish and investigate the chemical equilibrium reaction, and LeChatelier's Principle. Introduction : In some chemical reactions, the product remain in contact with each other. If the energy requirement low enogh, product reast to form the reatants again. When the rates of this forward and reverse reaction are equal, chemical equilibrium exist. A mathmatical relationship based on concentration. The equation for the Keq of reaction which is mA + nB sP + rQ would be Keq = ([P]s[Q]r) / ([A]m[B]n) . LeChatelier's Principle says that if a chemical system at equilibrium experiences a change in concentration, temperature or total pressure the equilibrium will chift in order to minimize that change. Just like a seesaw. If one side go to up, another side go to down. Apparatus and Materials : Test tube, syringe, 100ml beaker, NaCl, HCl, K2CrO4, H2SO4, NaOH, Fe(NO3)3, KSCN, Water Procedure : Part A. Solubility Equilibrium for NaCl 1. Obtain 5 ml of saturated NaCl solution in a clean test tube. 2. Drop few concentrated HCl, and observe. 3. Clean up. Part B. Chromate-Dichromate Equilibrium 1. Obtain 4 ml of 1M K2CrO4 soluion. 2. Drop 3M H2SO4 until solution turns to orange. 3. Add NaOH and shake gently. 4. Again, add H2SO4. 5. Note the change. 6. Clean up. Part C. Iron(III)-Thiocyanate Equilibrium 1. Take 2ml of 0.2M Fe(NO3)3 Solution in a 100 ml beaker, note the color. 2. Add 4 ml of 0.1M KSCN and about water(as much as you need to make it light color). 3. In separate 3 test tube, place 4 ml of this solution for each test tube. 4. Add 10 to 15 drops of Fe(NO3)3 to one test tube. 5. Add 10 to 15 drops of KSCN to one another test tube. 6. Add a few drops of 6M NaOH to the last one. 7. Observation each tube and record the color change. 8. Clean up. Results and Conclusions : 1. About Part A NaCl+(s) Na+(lq) + Cl-(eq) HCl which added to this equilibrium makes more Cl- and it reacts with more Na+ and it product more NaCl+. By this stress(Cl), the equilibrium shifts so as to relieve the stress. 2. About Part B By adding H2SO4 to the K2CrO4 solution, H+ reacts with 2CrO4- and it forms water, and then, Cr2O7 appears. K2CrO4 is yellow, Cr2O7 is orange, so in this process, solution turns its color from yellow to orange. -And by adding NaOH to that solution, OH makes more H2O and that H2O reacts with Cr2O7, then it became 2CrO4 + H+ solution. Again, Cr2O7 is orange, K2CrO4 is yellow, so in this process, concentration of K2CrO4 became greater than Cr2O7, so solution turn to yellow from orange. 3. About Part C By adding KSCN to Fe(NO3)3, SCN- reacts with Fe3+(pale brawn) and it forms Fe(SCN)++ which is red. -By adding Fe(NO3)3 to that solution(light red), concentration of Fe increaces and it become greater than concentration of Fe(SCN) so solution turn to yellow(pale brown). -By adding KSCN to that solution(light red), it forms more Fe(SCN), so solution become red(color gets deep). -By adding NaOH to that solution(light red), it forms Fe(OH)3 and it is quite insoluble. Soluttion looks dark purple. Questions : 1. the equilibrium constant expression for the reaction in; partB, Keq = [Cr2O7] / ([CrO4]2[H]6) partC, Keq = ([Fe][SCN]) / [Fe(SCN)] 2. Fe = 0.1M, SCN = 0.2M, Fe(SCN) = 0.3M (0.1 x 0.2) / 0.3 = 0.06666.... 100X-10X=6.666...-.66666 90X=6 X=6/90=1/15 Keq = 1/15 or 0.0666... 3. An increase in pressure does not have any effect on any equilibrium systems that observed in this experiment. Because pressure is only effect on gas. Conlusion : In this experiment, we could see that shifts occur as the color change because there were difference between the color of reactant and the color of product. Increasing one's consentration(add more) is one of stress. System at equilibrium is subjected to a stress, and the equilibrium shifts so as relieve the stress.
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