1. Which describes a solution that contains a system at equilibrium? One in which the color of the solution is changing slowly, or one in which the color is not changing. Explain your answer. A solution that contains a system at equilibrium is one in which the colour of the solution is not changing. At equilibrium the concentrations of reactants and products will not change because the rates of forward and reverse reactions become equal. At this state, the physical properties such as density, state and color become constant. This shows that the solution in which color is not changing is at equilibrium and indicating that there is no net change in the system. 2. The following equilibrium is established when copper ions and bromide ions are placed in solution. heat + Cu (H2O)6+2(aq) + 4 Br-(aq) ------ 6 H2O (l) + CuBr4-2 (aq) The tube on the left contains only copper sulfate dissolved in solution. The tube on the right is the result of adding some potassium bromide solution. Given that the Cu (H2O)6+2 ion is blue and that the CuBr4-2 ion is green, answer the questions below. a) What happened to the concentration of each of the ions when the KBr was added? When KBr is added, more Br- is produced hence disturbing the equilibrium. As a result, the forward reaction is favoured. So, the concentration of Br- and CuBr4-2 will be increased The concentration of Cu (H2O)6+2 will get decreased b) Explain why the solution changed color. The color of the solution changed as the Br^- ions were added on the reactants side. So, the concentration of blue Cu (H2O)6+2 ions decreased, while that of green CuBr4-2 ions will get increased and therefore the change in color of the solution will be observed with naked eyes. c) Would the tube feel hot or cold, or the same, when the KBr was added? Adding KBr favours’ forward reaction. The tube will feel cold because heat will be used up. 3. Consider the following equilibrium: Mg (OH)2(s) - Mg+2(aq) + 2 OH- (aq) The tube on the left contains Mg (OH)2(s) and water. A chemical has been added to cause the change shown in the tube on the right. Suggest a possibility for what chemical could have been added. Explain your answer. The removal of OH- and Mg2+ from the reaction would make it proceed to the right. In this case, adding an acid will remove the OH-. As a result, magnesium hydroxide will be forced to dissolve to replace some of the OH- which had been removed by the acid. This is according to Le Chatlier's principle which states that if a system of equilibrium is subjected to some changes, the equilibrium will be shifted towards the side as a way of opposing the change. 4. Methanol has the formula CH3OH and can be produced by the reaction of carbon monoxide with hydrogen gas as follows: CO + 2 H2CH3OH + heat In an attempt to maximize the yield of methanol (amount of methanol produced), a chemist would try to shift the equilibrium as far to the right as possible. Which of the following would accomplish this? Explain your answer. (a) heating the mixture No change in the equilibrium (b) adding an excess of carbon monoxide The equilibrium shifts to the right because of increasing the product side. (c) removing the methanol as it is formed Yes, the reaction shifts to the right as it seeks to reach equilibrium (due to the removed methanol formed) (d) adding a substance that reacts with carbon monoxide No change in the equilibrium C. Post lab Questions: Answer these questions after completing the experiment using your observations to help. You can either write your answers using your MS-word editor (or equivalent), in the relevant spaces below OR print this page, write your answers by hand, and take a picture of your answers to submit on Canvas. 1. Explain the effect of AgNO3 on the cobalt system (experiment I). What did you observe and why? AgNO3 will provide Ag+ which will react with Cl- from the original reaction and form AgCl, which is a white precipitate. Therefore, the equilibrium will be pushed to the right. 2. Predict what would happen if the ammonium system (experiment II) was heated. What would you observe and why? The equilibrium would shift to the right so as to dissipate the heat. This makes the concentration of H2O and that of NH3 to increase, while that of OH- and NH4- to decrease. 3. Predict what would happen if you cooled the iron thiocyanate system (experiment III). What would observe and why? Cooling iron thiocyanate systems would shift the equilibrium to the right. The concentration of the product would be higher, while that of the reactants would be lowered. Cooling means a heat deficit is created, and therefore the forward reaction would be favoured to replace the lost heat. 4. When the chromate system (experiment IV) is heated the tube is observed to go pale yellow. What does this tell you about the enthalpy of the forward reaction as written below? 2 CrO4-2 + 2 H+ ---- Cr2O7-2 + H2O In this case, the enthalpy of the reaction would be positive. 5. For experiment V., when the pressure of the tube containing gaseous NO2 and N2O4 is increased the tube goes colourless. Explain why. An increase in pressure favours’ the forward reaction. Increasing pressure shifts the reaction to the side with a smaller number of moles. N2O4 has less moles as compared to NO2. 6. Consider the following equilibrium. Cu+2(aq) + 4 NH3(aq) Cu (NH3)4+2 (aq) Look at the images of a test tube below. The copper ion in solution is light blue. The Cu (NH3)4+2 ion is a deep blue. The tube on the left contains only copper ions. (a) Identify what chemical was added to the tube on the left to produce the results shown in the centre tube. Explain your answer in terms of how this affected the equilibrium. The chemical added was excess ammonia. Addition of one of the reactants shifts the equilibrium towards the right side. (b) Identify what chemical may have been added to the centre tube to produce the results shown in the tube on the right. Explain your answer. The chemical added is EDTA. The chemical forms (Cu (EDTA)2-, which is pale blue. 7. The barium ion, Ba2+, is toxic to humans. However, barium sulfate is commonly used as an image enhancer for gastrointestinal x-rays. What does this imply about the position of the equilibrium shown here: BaSO4(s) Ba+2(aq) + SO4-2(aq) Explain your answer. Since barium sulfate is non-toxic, the equilibrium should always be towards the left. Ba2+ is toxic and this means that if the equilibrium shifts to the right, Ba 2+ would lead to death of people. 8. Haemoglobin (Hb) and oxygen gas form a complex (HbO2) that carries oxygen throughout the human body. Unfortunately, carbon monoxide also binds to haemoglobin so that an equilibrium is established as follows: HbO2 + CO HbCO + O2 Carbon monoxide poisoning occurs when the concentration of HbO2 in the blood is reduced. The first aid for a person suffering from carbon monoxide poisoning is to (1) move them to an area of fresh air, and (2) administer oxygen. Using the principles of equilibrium, explain how each of these helps to restore the HbO2concentration. The administration of oxygen leads to an increase of the level of oxygen in the body and therefore shifting the reaction to the reactants side. Also, removing them in areas with fresh air helps in reducing the CO in the system hence shifting the reaction to the reactants side.