Unit 13 Kinetics & Equilibrium Page 1 of 14 Chemistry Kinetics, Entropy, Equilibrium, LeChatelier s Principle, K, Unit 13 Quiz: Unit 13 Test: Final Project: VOCABULARY: 1 Chemical equilibrium 2 equilibrium constant 3 Reversible reaction 4 catalyst 5 Le Chatelier s principle 6 entropy 7 activated complex 8 Chemical kinetic 9 Activation energy 10 collision theory 11 enthalpy 12. Heat of reaction OBJECTIVES: Understand how to measure temperature and heat (including units) Be able to list what affects reaction rate. Be able to explain the collision theory Describe enthalpy and entropy. Be able to label a reaction mechanism, determining if the reaction is endothermic or exothermic Understand how a catalyst affects reaction rates and a reaction mechanism. Be able to describe a reversible reaction. Be able to apply Le Chatlier s principle to a system in equilibrium. Be able to write the equilibrium expression and the equilibrium constant, K. Be able to complete simple calculations for the value of K Understand what information the value of K conveys Heat of reaction Read pgs 514 521 and answer the following questions: 1. What is heat of reaction? 2. How is a thermochemical equation different from a regular equation? 3. What is enthalpy? 4. What is the symbol for enthalpy? 5. What is the sign on H for an exothermic reaction? 6. What is the sign on H for an endothermic reaction? Section 17-2 Driving force of reactions Read pgs 526-530 and answer the following questions: 1. Do reactions tend to go toward lower energy or higher energy states? 2. What is entropy? Its symbol? 3. Do processes in nature go toward more order or more disorder (lower entropy or higher entropy)? 4. Which has the most entropy solids, liquids or gases? 5. On pg 530 do # 8
On Entropy sheet below OMIT # 8 Unit 13 Kinetics & Equilibrium Page 2 of 14
Unit 13 Kinetics & Equilibrium Page 3 of 14 CHAPTER 17 - The Reaction Process Read info below and then answer the questions that follow. Different reactions occur at different rates. A reaction that goes to completion proceeds until all of one of the reactants is used up and the reaction stops. A reaction that can occur in both directions is called a reversible reaction and will eventually reach equilibrium. A reaction mechanism is the step by step sequence of reactions by which the overall chemical change occurs. The slowest step, which will determine the rate is called the rate determining step The set of assumptions regarding collisions and reactions is known as the collision theory. The change in concentration of reactants per unit time is called the reaction rate. The area of chemistry that deals with reaction rates is known as kinetics We can use knowledge of reaction rates to determine whether we will have more reactants or more products at equilibrium. Reaction rate depends on five things: 1. Nature of reactants (ex. State- s,l,or g; size of particles; bond type/strength) 2.surface area 4. concentration 3.temperature 5. catalyst The nature of the reactants and concentration will determine the kind of reaction that occurs. For a reaction to occur between substances their particles must collide and these collisions must result in interactions (this is known as the collision theory). Reaction rate is proportional to the number of effective collisions. When an effective collision occurs, the particles may form an activated complex. The energy required to form this is called activation energy. The nature of the reactants refers to their complexity and the number of bonds that must be broken and reformed in the course of a reaction. The larger the surface area, the faster the reaction rate because a larger surface area will lead to more particles colliding effectively. Temperature also effects rate. Higher temperature usually produce faster rxns. At higher temperatures, particles have more kinetic energy and are more likely to collide effectively. The last thing that affects rate is a catalyst, which increases reaction rate without being permanently changed. A catalyst lowers the activation energy for a reaction (by providing an alternate pathway for the reaction). An inhibitor slows reaction rate by preventing it from reacting. Rate laws relate reaction rate to concentration of reactants; this will be covered in AP Chemistry!
Unit 13 Kinetics & Equilibrium Page 4 of 14 Questions: 1. List and explain (use the idea of collisions in your explanation) the 5 factors that affect reaction rate (you should know these for the test!) A. B. C. D. E. 2. Given: A. hot tea or ice tea and B. sugar cubes or powdered sugar what combination would dissolve together the fastest why? Use the collision theory in your answer. 3. When have you learned about catalysts before? (think Bio) Give an example of a catalyst 4. If we are reacting zinc metal with hydrochloric acid which would react the quickest 1.0 M HCl or 2.0 M HCl why? (use the collision theory in your answer) Energy diagram important parts! At the top of the curve is the this is also where the activated complex is located (where the reactants are being changed to the products. A will lower the activation energy making the reaction occur more quickly. To find the H (heat of reaction) the difference in the enthalpy of the products reactants is found. If the products have more enthalpy the H is + and the reaction is endothermic (heat was gained). If the reactants have more enthalpy the H is (negative) and the reaction is exothermic (heat was released).
Unit 13 Kinetics & Equilibrium Page 5 of 14 1. Is the forward reaction exo or endothermic? 2. What is the PE for the reactants (of the forward rxn)? 3. What is the PE for the products (of the forward rxn)? 4. What is the activation energy of the forward rxn? 5. What is the activation energy for the reverse rxn? 6. What is the heat of reaction (ΔH) for the forward rxn? 7. What is the PE of the activated complex? 8. Draw a dotted line to indicate the addition of a catalyst.
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Unit 13 Kinetics & Equilibrium Page 7 of 14 Chemical Equilibrium Read pages 372-373 and pgs 553-555 Reversible reaction : Chemical equilibrium: Dynamic state: reactions are continually happening. Ex: physical equilibrium: rate of water evaporating and condensing is equal so the overall amount of water (in a closed system) does not change. Equilibrium constant: K or K eq or K c K = [products] coefficients [reactants] coefficients brackets in chemistry represent concentration Why use K? K = 1 K< 1 K> 1 Very Important: K includes only gases and aqueous solutions; liquids and solids do not have a concentration Graphs representing equilibrium: For the first graph together: A. Label the reactant(s) and the product(s) B. Write an equation for the rxn C. label where equilibrium is reached. D. are there more reactants or products at equilibrium? E. What type of value would you expect for K? ********For graph # 2 : answer the same questions as above A - E
Unit 13 Kinetics & Equilibrium Page 8 of 14 Example 1: Write the equilibrium expression for: N 2 (g) + O 2 (g) 2 NO (g) Calculate the value for K if [N 2] =.32, [O 2] = 1.1 and [NO] = 2.1 You try: Given: 2 HF (g) F 2 (g) + H 2 (g) A. Write the K expression B. Calculate the value of K if [HF] = 6.2 [F 2] = 3.1 and [H 2] = 2.2 C. Do you have more reactants or products? PRACTICE/You try: 1. Write the expression for the equilibrium constant for each of the following reactions and then calculate the value of the equilibrium constant for those with values listed. A. 2 CO(g) + O 2 (g) 2 CO 2 (g) B. CaCO 3(g) CaO(g) + CO 2(g) C. 2 SO 2(g) + O 2(g) 2 SO 3(g) D. H 2(g) + Cl 2(g) 2 HCl(g) [SO 2] = 0.001 [O 2] = 0.002 [SO 3] = 0.003 [H 2] = [Cl 2] = [HCl] = 1.0 x10-2 (all the same) E.. N 2(g) + 3 H 2(g) 2 NH 3(g) [N 2] =.044 [H 2] = 0.12 [NH 3] = 0.0034 Acids, Bases, and water There are different Ks for different substances. K a is used for acids (substances that begin with H) K b is used for bases (substances that contain OH-). We will study acids and bases next chapter. Ksp is used for insoluble solids (ppts) and Kw is used for aqueous solutions. K w = dissociation constant for water Does not include H 2O because it is a pure liquid with no concentration! Example: H 2O(l) H +1 (aq) + OH -1 (aq) K w = [H+] [OH-]
Unit 13 Kinetics & Equilibrium Page 9 of 14 Extra PRACTICE/You try: Write the K for the following equations, calculate the value and predict if there are more products or reactants at equilibrium Equation K expression and math work K value More products or reactants 2H 2S (g) 2H 2 (g) + S 2 (g) [H 2S] = 0.25, [H 2] = 0.88, [S 2] = 0.44 2H 2S (g) 2H 2(g) + S 2 (g) [H 2S] =0.18, [H 2 ] = 0.014 [S 2] = 0.035 2N 2O 5(g) 4NO 2(g) + O 2 (g) [N 2O 5] = 0.50, [NO 2] = 0.80, [O 2] = 0.20. Le Chatelier s Principle A system at equilibrium will shift to adjust to changes to stay at equilibrium. Conditions: 1. Concentration: A. Increase/add shifts away (to consume excess) B. Decrease/Remove shifts towards (to replace) 2. Pressure: A. Increase shifts to side with less gaseous moles B. Decrease shifts to side with more gaseous moles 3. Temperature: depends on if rxn is exothermic or endothermic Temperature is the only condition that changes the value of K 4. catalyst does not produce a shift rxn speeds up in both directions! Example: Predict the shift when PCl 5 (g) PCl 3 (g) + Cl 2 (g) exothermic A. add PCl 5 B. Remove PCl 5 C. Increase P D. Decrease T E. Add Cl 2 You try: 2CO (g) + O 2 (g) 2 CO 2 (g) + 167 kj A. Add CO 2 C. Decrease pressure B. Remove O 2 D. Decrease temperature
Unit 13 Kinetics & Equilibrium Page 10 of 14 Le Chatlelier s principle practice! Given: 12.6 kj + H 2 (g) + I 2 (g) 2 HI (g) Stress Shift [H 2] [I 2] [HI] K 1. Add H 2 xxxxxx 2. Add I 2 xxxxx 3. Add HI xxxxx 4. Remove H 2 xxxxx 5. Remove I 2 xxxxx 6. Remove HI xxxxx 7. Increase Temp 8. Decrease Temp 9. Increase Pressure 10. Decrease Pressure
Unit 13 Kinetics & Equilibrium Page 11 of 14 Review Sheet 1. List the 5 factors that affect reaction rate. 2. Processes in nature go toward: higher or lower energy 3. Processes in nature go toward higher or lower disorder (entropy) 4. SKIP! For the following match the correct symbol and definition: 5. H A. Celsius Temperature 6. T B. Free Energy E. Kelvin Temperature 7. S C. entropy F. enthalpy D. reaction rate 9. Which has more entropy an ice cube or liquid water? 10. Does dissolving salt in water cause an increase or decrease in entropy? Use the graph below to answer the following questions: 11. Which letter represents the relative energy of the activated complex? 12. Which letter represents the energy of the reactants? products? 13. Which letter represents the difference in energy between reactants and products? 14. Which letter represents the activation energy for the reverse uncatalyzed reaction? 15. Is this reaction endothermic or exothermic? 16. Which letter represents the activation energy of the forward catalyzed rxn? REVIEW SHEET Equilibrium PRACTICE: N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 1. Write the Keq for the above reaction and determine its value, if [N 2] =.3, [H 2] = 1.7 and [NH 3] = 0.7 2. Are there more products or reactants at equilibrium? 3. Given: 2 NO 3 (g) 2 NO 2 (g) + O 2 (g) + 30 kj How would the system shift if: A. more Nitrogen dioxide is added B. Which of the above changes would affect the value of the equilibrium constant? C. Some of the nitrogen trioxide is removed? D. The pressure is decreased? E. How would the concentration of nitrogen dioxide be affected by B? F. The temperature is increased?
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Unit 13 Kinetics & Equilibrium Page 13 of 14 Le Chatelier s lab Do NOT write on this sheet In this experiment, you will investigate what happens when the conditions of equilibrium are changed. The equilibrium you will be investigating can be described by the following equation: CuCl 4 - (aq) Cu 2+( aq) + 4Cl - (aq) + heat Green blue colorless Materials: 1.5 M CuCl2.05 M AgNO3 salt solution small test tubes wellplates safety glasses test tube rack Procedure Caution: You must wear safety glasses at all times during this experiment. Part A: How do temperature changes affect the equilibrium? 1. Place 1-3ml of 1.5 M copper(ii) chloride, CuCl2, in a clean, labeled test tube. Place another 1-3 ml of 1.5 M CuCl2 in a second and third test tube. 2. Place one of the tubes in a test tube rack or empty beaker. This will be used as a reference for comparison. 3. Heat the second tube of copper (II) chloride solution in a boiling water bath for several minutes. Carefully record any observations you make. (Be sure to compare it to the reference solution.) 4. Cool the second tube of copper (II) chloride solution in an ice water bath for several minutes. Carefully record any observations you make. (Be sure to compare it to the reference solution.) 5. Look at the equation for the equilibrium. Try to identify the substances that have increased in concentration and the substances that have decreased in concentration 6. Remove the tubes and place them in your test tube rack or in an empty beaker. Unplug the hot plate but leave hot and cold water for the next class! Part B: How do concentration changes affect the equilibrium? 1. Make sure your well plate is clean. In each of two wells, place 3 drops of room temperature copper (II) chloride solution. One well will be used as a reference to compare with the other. Put a piece of white paper under the well plate to make it easier to see. Caution: When using pipettes, leave the pipette for each solution in the test tube with the solution so they do not get mixed-up. 2. Add 2 drops of sodium chloride solution, a drop at a time, to one of the wells. (This increases the concentration of chloride ions, Cl-.) Carefully record your observations. 3. Now add one drop of silver nitrate solution to same well that you added the sodium chloride solution to. (This decreases the concentration of chloride ions because the white solid that forms is silver chloride, AgCl.) Carefully record your observations. 4. Look at the equation for the equilibrium. Try to identify the substances that have increased in concentration and the substances that have decreased in concentration. 5. Clean up by rinsing off the well plate dump the CuCl 2 that was cooled or heated back into the original beaker (do not dump the well plate chemicals into the original beaker). 6. Unplug hot plate but leave hot water on the plate. Leave ice water for next class!
Unit 13 Kinetics & Equilibrium Page 14 of 14 Le Chatelier s lab Answer sheet Name: Data table: Copy the equation (including colors) : Change observations Shift (left or right?) 1. heating solution 2. cooling solution 3. adding NaCl 4. adding AgNO 3 (removing Cl -1 ) Questions: 1. Which way did the system shift when it was heated? 2. how do you know? 3. Which way did the system shift when it was cooled? 4. How do you know? 5. What happened to the concentration of the Cl - when heat was added? 6. What happened to the concentration of the Cl - when heat was removed? 7. Which way did the system shift when NaCl was added? 8. Why did this shift occur? 9. Which way did the system shift when AgNO 3 was added? 10. Why did this shift occur? 11. Predict the shift that would occur if Cu 2+ was added Why? 12. Predict the shift that would occur if the pressure was decreased 13. Why would this occur? Test review 14. A. Write the K expression for the equation: 2SO 3 (g) 2SO 2 (g)+o 2 (g) B Calculate the value of K if [SO 3] = 4.2 [SO 2] = 1.2 and [O 2] = 1.4 Show work!