#11. Chemical Equilibrium

Transcription

1 #11. Chemical Equilibrium Goal To observe and explain equilibrium shifts based on Le Chatelier s Principle. Introduction In any chemical reaction, reactants are converted to products. In some cases, some of the products are converted back to reactants. Such reactions are called reversible reactions where reactants are converted to products (forward reaction) and products are converted back to reactants (reverse reaction). When the rate of the forward reaction becomes equal to the rate of the reverse reaction, we say that the reaction is at equilibrium. At this point, there is no change in the concentration of the reactants and the products. We can disturb a reaction at equilibrium by changing the concentration, pressure/ volume or temperature. Le Chatelier s Principle states that a reaction (system) at equilibrium will stay at equilibrium until acted upon by some outside force that changes the equilibrium conditions for example, by adding a greater concentration of one of the products or reactants. The system will then adjust to return to equilibrium by counteracting the change that has just been made. Some examples of stresses and how the equilibrium system responds to counteract those stresses are as follows: Change in Concentration If we add a reactant, the equilibrium will shift towards the products. This is because a portion of the added reactant will be used up to form more products, until the equilibrium is re-established. If we remove a reactant, then to compensate for the loss of the reactants, the product will decompose to form reactants and therefore the equilibrium will shift towards the reactants. If we add a product, then the equilibrium will shift to the reactants in order to consume the excess product. If we remove a product, then to compensate for the loss of the products, more product will be formed from the reactants and the equilibrium will also shift towards the products. Changes in Temperature When temperature is increased, the reaction shifts in the direction that absorbs heat (the endothermic direction) to counteract the increase in temperature. When temperature is decreased, the reaction shifts in the direction that releases heat (the exothermic direction) to counteract the decrease in temperature. During this lab you will investigate Le Chatelier s Principle using iron(iii)-thiocyanate equilibrium. You will be combining Fe(NO 3) 3, (iron (III) nitrate) and KSCN (potassium thiocyanate ) to produce a red colored solution of FeSCN 2+, a complex ion (potassium and nitrate are spectator ions). This is the main reaction that you will be considering: (1) Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) yellow colorless red When the system contains mostly of reactants, the solution is yellow. When the system shifts towards products, the color changes to red. By observing the color change, we can see the shift in the equilibrium between reactants and products.

2 Addition of hydrochloric acid provides chloride ions, which produce FeCl 4 - (a complex ion) when combined with iron (III) ions. This is a side reaction that removes the Fe 3+ ions as shown in equation (2). This disrupts the equilibrium established in the main reaction (1). The main equilibrium will shift back to the reactants. (2) Fe 3+ (aq) + Cl - (aq) FeCl 4 - (aq) yellow colorless colorless Laboratory Activity Equipment small beaker 6 x test tubes test tube rack pipettes 2 x 250 ml beakers ice Chemicals 0.01 M Fe(NO 3) M KSCN 1M Fe(NO 3) 3 1 M KSCN 3M HCl Procedure 1. In a small beaker mix 5 ml of 0.01 M Fe(NO 3) 3 and 5 ml of 0.01 M KSCN. This is your stock solution. CAREFULLY CHECK the CONCENTRATION on the bottle there are two different concentrations of both chemicals used in this lab! 2. Set up 6 test tubes on a rack. To each test tube add 1 pipette of the stock solution you just made. 3. Add 10 drops of water to test tube 1. Record the color of the solution. This is your control sample and you will compare the colors of all other test tubes to this test tube. 4. Add slowly 8 drops of 1 M Fe(NO 3) 3 to test tube 2. Be sure to mix the contents by flicking the bottom of the test tube with the tip of one finger while holding the top of the test tube with your thumb and index finger of your other hand. Record the color of the solution and compare it to test tube Add slowly 8 drops of 1 M KSCN to test tube 3. Mix as before and record the color you observe and any change in color. 6. Add slowly, 8 drops of 3 M HCl to test tube 4. Mix as before and record the color observed and any change in color 7. To test tube 5, add slowly 10 drops of water and place this test tube in a hot water bath (a beaker with hot water). After 10 minutes, record the color observed any change in color. 8. To test tube 6, add slowly 10 drops of water and place this test tube in a cold water bath (a beaker containing a mixture of ice and water). After 10 minutes, record the color observed any change in color. 9. Disposal: All the solutions must be disposed of in the properly designated waste container.

3 Name CHM101 Lab Chemical Equilibrium Grading Rubric Team Name To participate in this lab you must have splash-proof goggles, proper shoes and attire. Criteria Points possible Points earned Lab Performance Printed lab handout and rubric was brought to lab 3 Safety and proper waste disposal procedures observed 2 Followed procedure correctly without depending too much on instructor or lab partner 3 Work space and glassware was cleaned up 1 Lab Report Data Sheet observations accurately recorded, direction of shift correctly identified. 2 Post Lab Q1 2 Post lab Q2 (correct stress and direction, explains the system s response to stress clearly and specifically) 5 Post Lab Q3 2 Total 20 Subject to additional penalties at the discretion of the instructor.

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5 Chemical Equilibrium: Data Sheet Name Fill in the following table: Test tube Substance added /Temperature change Color Equilibrium shifted towards Reference color: Reference Q1. Based on test tubes 5 and 6, is the forward reaction of the equilibrium exothermic or endothermic? Hint: add heat to the reaction below as a product and reactant and see which make sense with your results. Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) Report Page 1 of 2

6 Chemical Equilibrium: Post Lab Name Q2. Consider the equilibrium that you studied in this experiment: Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) Test Tube Direction of shift (copy from previous page) Type of stress (ex: reactant added, product removed, temp inc) Explain how the direction of shift relieves the applied stress Q3. Consider the equilibrium below and predict the direction that the equilibrium will shift for each of the following change. C (s) + H 2O (g) + heat CO (g) + H 2 (g) a) Cooling b) Removing CO (g) c) Adding H 2 (g) d) Adding H 2O (g) Report Page 2 of 2