Gummy Bear Demonstration:

Transcription

1 Name: Unit 8: Chemical Kinetics Date: Regents Chemistry Aim: _ Do Now: a) Using your glossary, define chemical kinetics: b) Sort the phrases on the SmartBoard into the two columns below. Endothermic Rxns Exothermic Rxns Sketch of Representative Endothermic Potential Energy Diagram Sketch of Representative Exothermic Potential Energy Diagram Remember: Every chemical reaction, whether endothermic or exothermic, requires BOTH an INPUT and an OUTPUT of energy! Gummy Bear Demonstration: 1 P a g e

2 Practice: Write out the chemical equation for each of the following rxns. Determine whether each is endothermic or exothermic. Draw a sketch of the correct potential energy diagram. 1. Methane gas, CH 4, undergoes a combustion reaction to form the products of complete combustion, carbon dioxide and water vapor. (Recall another reactant necessary for combustion!) Chemical Equation: Potential Energy Diagram Sketch Endothermic or Exothermic? Type of Chemical Reaction: 2. Water is decomposed into its elements by the process of electrolysis. Chemical Equation: Potential Energy Diagram Sketch Endothermic or Exothermic? Type of Chemical Reaction: 3. Zinc metal reacts with aqueous hydrogen chloride, HCl (also known as hydrochloric acid). The products of this reaction are aqueous zinc chloride and hydrogen gas, which bubbles out of the solution. The flask that the reaction occurs in gets very hot. Chemical Equation: Potential Energy Diagram Sketch Endothermic or Exothermic? Type of Chemical Reaction: 4. Iron metal combines with oxygen to form rust, or iron III oxide. Energy is released. Chemical Equation: Potential Energy Diagram Sketch Endothermic or Exothermic? Type of Chemical Reaction: 2 P a g e

3 Name: Date: Period: Do Now: Practice Interpreting Graphs & Diagrams Directions: Answer the questions that follow based on the graphs below, which show the energy changes that occur during chemical reactions. 1. Label each graph as either endothermic or exothermic. 2. Compare the energy of the reactants (PE R or H R ) to the energy of the products (PE P or H P ) for each g raph. 3. In the exothermic graph, the reactants are hydrogen and oxygen molecules, and the product is water. Write the thermochemical equation for this reaction. In the endothermic graph, the reactant is water and the products are hydrogen and oxygen molecules. Write the thermochemical equation for this reaction Both graphs have a peak in the middle of the graph. Highlight this peak. The doubleheaded arrow beneath the peak shows the difference in energy between the reactants and the peak. This difference is called the activation energy. Highlight this as well. a. Which graph has a higher activation energy? b. Explain why. 5. Infer: What do you think would happen in a reaction if there was not enough activation energy available to cross the activation energy peak? Which diagram below represents this situation? Diagram A Diagram B 3 P a g e

4 Reviewing Potential Energy Diagrams a) Fill in the blanks using the word bank below. 1. measures the heat content. 2. is the amount of stored energy at the start of the reaction. 3. is the amount of stored energy at the end of the reaction. 4. The minimum amount of energy needed to initiate effective collisions is known as the. 5. The measures the heat content or change of enthalpy in the chemical system. Formula: 6. Highest point on the potential energy diagram that describes the unstable formation of the reacting particles before an effective collision is known as the 7. describes the amount of stored energy at the activated complex. Activated Complex P.E. of reactants P.E. of products Heat of reaction Word Bank: Activation energy (E a ) P.E. of the activated complex H = H products H reactants Enthalpy b) Label the Potential Energy Diagram Below: 4 P a g e

5 Name Date Practice Reading and Drawing Potential Energy Diagrams 1. Answer the following questions based on the potential energy diagram shown here: a. Does the graph represent an endothermic or exothermic reaction? b. Label the potential energy of the reactants and products, the activation energy of the forward reaction, H, and the activated complex. c. Determine the heat of reaction, H, (enthalpy change) for this reaction. d. Determine the activation energy, Ea, for this reaction. e. How much energy is released or absorbed during the reaction? f. How much energy is required for this reaction to occur? 2. Sketch a potential energy curve that is represented by the following values of H and Ea: H = -100 kj and Ea = 20 kj You may make up appropriate values for the y-axis (potential energy). Is the curve endothermic or exothermic? 3. Sketch a potential energy diagram for a general reaction A + B C + D iven that H = 10 kj and Ea = +40 kj. You may make up appropriate values for the y-axis (potential energy). Is the curve endothermic or exothermic? 5 P a g e

6 Name: Date: Period: Collisions Drive Reactions Collision Theory Background Information: In order for a reaction to occur, particles of the reactant must collide. Not every collision will do. The colliding particles must approach each other at the proper angle, the proper amount of energy and appropriate speed in order for a reaction to occur. Such a collision is called an effective collision. The greater the rate of effective collisions is, the greater the reaction rate is. If all the particles shown on the reactant side of a balanced equation had to collide in order for a reaction to occur, the reaction would not take place. As a result, chemical reactions occur by a series of intermediate steps between the initial reactants and final products, each of which probably involves a collision of only two particles. This series of steps is called a reaction mechanism. The slowest step of the reaction mechanism is called the rate-determining step. Answer the questions below based on the reading above and on your knowledge of chemistry. 1. You are playing pool. You line up your cue, pull back, and shoot. Complete the following statement: Nothing will happen at all if the cue ball. 2. How is this similar to the requirements for the molecules of reactants to react with each other? 3. Examine the picture at right. Why isn t the procedure pictured a good way to chop down a tree? How is this similar to the requirements for a chemical reaction to occur? 4. Hydrogen gas and iodine vapor combine to form hydrogen iodide gas, as shown in the equation: H + I 2 HI 2 2 Draw a diagram to show an orientation for the reactant molecules that WOULD produce an effective collision and an orientation that would NOT produce an effective collision. Orientation for Effective Collision Orientation for Ineffective Collision 6 P a g e

7 Collision Theory: How Do Reactions Occur? Reactions only occur when there are EFFECTIVE COLLISIONS between reacting particles. Effective Collisions Require: Practice Regents Questions 1. What is required for a chemical reaction to occur? a. standard temperature and pressure b. a catalyst added to the reaction system c. effective collisions between reactant particles d. an equal number of moles of reactants and products 2. A reaction is most likely to occur when reactant particles collide with a. proper energy, only b. proper orientation, only c. both proper energy and proper orientation d. neither proper energy nor proper orientation 3. Two reactant particles collide with proper orientation. The collision will be effective if the particles have a. high activation energy b. high ionization energy c. sufficient kinetic energy d. sufficient potential energy 7 P a g e

8 Name: Date: Aim: What factors affect the rate of a chemical reaction? Do Now: A rate is a measure of how much something changes within a specified amount of time. 1. What units can be used to measure each of the following rates? a. Car speed: per b. Plant growth: per c. Rusting: per 2. Which of the above has the fastest reaction rate? 3. Which of the above takes the longest reaction time? 4. Based on the diagram below, is the rate of conversion of reactant to product constant throughout the reaction? How can you tell? Factors Affecting Rate of Reaction (6. ) 8 P a g e

9 Name: Chemical Kinetics Aim: Do Now: Which reaction rate factor does each diagram below represent? 1. In terms of collision theory, increasing will the rate of reaction because 2. In terms of collision theory, increasing will the rate of reaction because 9 P a g e

10 3. Larger particles, less surface area, fewer frequent effective collisions Smaller particles, greater surface area, more frequent effective collisions In terms of collision theory, increasing (which is the same as decreasing ) will the rate of reaction because 4. Pressure only affects the state of matter, because and are not compressible. In terms of collision theory, increasing on a gaseous sample (which is the essentially increasing ) will the rate of reaction because 10 P a g e

11 5. a) Which metal would react most vigorously with water? Na (s) Fr (s) Ca (s) Sr (s) Li (s) b) Which substance would react fastest? C 6 H 12 O 6(s) C 3 H 8(g) NaCl (s) NaCl (aq) 6. Presence of a Catalyst Draw the effect of a catalyst on the potential diagram below: Define catalyst using your glossary: 11 P a g e

12 Practice Regents Questions Collision Theory & Factors Affecting Rxn Rate 1. For a reaction at equilibrium, which change can increase the rates of the forward and reverse reactions? a) a decrease in the concentration of the reactants b) a decrease in the surface area of the products c) an increase in the temperature of the system d) an increase in the activation energy of the forward reaction 2. Which statement explains why increasing the temperature increases the rate of a chemical reaction, while other conditions remain the same? a) The reacting particles have less energy and collide less frequently. b) The reacting particles have less energy and collide more frequently. c) The reacting particles have more energy and collide less frequently. d) The reacting particles have more energy and collide more frequently. 3. A 5.0-gram sample of zinc and a 50.-milliliter sample of hydrochloric acid are used in a chemical reaction. Which combination of these samples has the fastest reaction rate? a) a zinc strip and 1.0 M HCl(aq) b) a zinc strip and 3.0 M HCl(aq) c) zinc powder and 1.0 M HCl(aq) d) zinc powder and 3.0 M HCl(aq) 4. If the pressure on gaseous reactants is increased, the rate of reaction is increased because there is an increase in the a) activation energy b) volume c) concentration d) heat of reaction 5. Given the reaction: CuSO 4(s) Cu 2+ (aq) + SO 2 (aq) The CuSO4(s) dissolves more rapidly when it is powdered because the increased surface area due to powdering permits a) increased solvent contact b) increased solute solubility c) the equilibrium to shift to the left d) the equilibrium to shift to the right 12 P a g e

13 6. Which statement explains why the speed of some chemical reactions is increased when the surface area of the reactant is increased? a) This change increases the density of the reactant particles. b) This change increases the concentration of the reactant. c) This change exposes more reactant particles to a possible collision. d) This change alters the electrical conductivity of the reactant particles. 7. For a given chemical reaction, the addition of a catalyst provides a different reaction pathway that a) decreases the reaction rate and has a higher activation energy b) decreases the reaction rate and has a lower activation energy c) increases the reaction rate and has a higher activation energy d) increases the reaction rate and has a lower activation energy 8. Which potential energy diagram represents the change in potential energy that occurs when a catalyst is added to a chemical reaction? a) b) c) d) 9. If a catalyst is added to a system at equilibrium and the temperature and pressure remain constant, there will be no effect on the a) rate of the forward reaction b) rate of the reverse reaction c) activation energy of the reaction d) heat of reaction 13 P a g e

14 Name: Date: Pd: Chemical Kinetics Aim: What are two characteristics of a spontaneous reaction? Two factors are favored in nature to make a reaction occur spontaneously: THE DRIVE TOWARD MINIMAL POTENTIAL ENERGY (ENTHALPY, H) Spontaneous Reactions represent a drive toward potential energy & stability. Potential Energy Diagram Sketch Endothermic or Exothermic? H Value Practice Regents Questions: 1. Given the reaction at STP whereby solid NaOH (s) dissociates into its ions, the heat of reaction ( H) is a. negative and the reaction is spontaneous b. negative and the reaction is not spontaneous c. positive and the reaction is spontaneous d. positive and the reaction is not spontaneous 2. According to Reference Table I, which gas is formed from its elements by a spontaneous reaction? a. NO (g) b. NO 2(g) c. CO 2(g) d. C 2 H 2(g) 14 P a g e

15 2.THE DRIVE TOWARD MAXIMUM DISORDER (ENTROPY) Spontaneous reactions represent a drive toward maximum disorder. Entropy is a measure of the or of a system. Spontaneous reactions favor entropy. Positive S represents an in entropy. Describing Entropy ( S) 1. Temperature At higher temperatures, kinetic energy and randomness (entropy). Ex: Which has higher entropy: CO 2 at 27 o C or CO 2 at 80 o C? 2. Phases of Matter Most entropy Least entropy > > > Ex: For each phase change below, is entropy positive or negative? Phase Change Entropy increase or decrease? Sign of S: (+) or (-) (g) (aq) (s) (aq) (l) (g) (s) (l) 3. Type of Matter (Mixtures vs. Pure Substances) Mixtures have entropy than pure substances. Ex: Which has higher entropy: NaCl (s) or NaCl (aq)? 4. Type of Matter (Free Elements vs. Compounds) Compounds have entropy than the free elements of which they re composed. Ex: In terms of valence electrons, explain why sodium atom and chlorine atom have higher entropy than sodium chloride compound. _ 5. Organization of Matter (# of Moles of Reactants & Products) The side of the equation with the number of has the greater amount of entropy. Ex: 2CO (g) + O 2(g) CO 2(g) S = CaCO 3(s) CaO (s) + CO 2(g) S = 15 P a g e

16 Review: Analyzing the Entropy of Different Reactions Entropy is the degree of randomness in a substance. The symbol for change in entropy is S. Solids are very ordered and have low entropy. Liquids and aqueous ions have more entropy because they move about more freely, and gases have the highest amount of entropy. According to the Second Law of Thermodynamics, nature is always proceeding to a state of higher entropy. When S (entropy) is positive, entropy is increasing and the products have a greater degree of randomness/disorder than do the reactants. Likewise, when S (entropy) is negative, entropy is decreasing and the products occupy less randomness or disorder. Determine whether the following reactions show an increase or decrease in entropy. Also, indicate whether the S would be positive (+) or negative (-). Chemical Reaction Entropy (Increase/Decrease) S (+ or -) Reason (#1-5 from notes) 1. 2KClO 3(s) 2KCl (s) + 3O 2(g) 2. H 2 O (l) H 2 O (s) 3. N 2(g) + 3H 2(g) 2NH 3(g) 4. NaCl (s) Na + (aq) + Cl - (aq) 5. C (s) + O 2(g) CO 2(g) 6. 2N 2 O 5(g) 4NO 2(g) + O 2(g) 7. KCl (s) KCl (l) 8. 2Al (s) + 3I 2(s) 2AlI 3(s) 9. 2NO (g) N 2(g) + O 2(g) 16 P a g e

17 Using your knowledge of chemistry and Reference Table I, answer the following questions: 1. Which requires less effort to maintain: [ messy room / organized room ] [ orderly classroom / chaotic classroom] [ raked pile of leaves / scattered leaves ] 2. Based on the exercise on the previous page, a(n) [ increase / decrease ] in entropy is favored for most chemical or physical changes. 3. Take a look at Table I. What is mostly favored, [ endothermic / exothermic ] reactins. Therefore, in nature, an [ endothermic / exothermic ] reaction is favored. 4. To summarize, reactions tend to proceed towards [ lesser / greater ] enthalpy ( H), and [ lesser / greater ] entropy ( S). This results in a [ spontaneous / non-spontaneous ] chemical reaction. 5. Which term is defined as a measure of the disorder of a system? (1) heat (2) entropy (3) kinetic energy (4) activation energy 6. Which of the following processes has a negative S? (1) evaporating 50 grams of a liquid (2) raising the temperature of water from 250K to 383K (3) freezing 75 ml of a liquid (4) none of the above 7. A reaction will be spontaneous if it results in products that have (1) lower potential energy and less randomness (2) lower potential energy and more randomness (3) greater potential energy and less randomness (4) greater potential energy and more randomness 8. Which statement describes this phase change? C 6 H 4 Cl 2(s) + energy C 6 H 4 Cl 2(g) (1) It is endothermic, and entropy decreases. (2) It is endothermic, and entropy increases. (3) It is exothermic, and entropy decreases. (4) It is exothermic, and entropy increases. 17 P a g e

18 Name: Date: Pd: Chemical Kinetics Aim: How do enthalpy and entropy determine reaction spontaneity? Whether a reaction proceeds spontaneously or not depends on the balance between two natural tendencies: [1] The drive toward greater stability (reduced potential energy), i.e. (-) H; [2] The drive toward less organization (increased entropy), i.e. (+) S High entropy is favored by increased temperature. The Gibbs free energy change ( G) predicts whether or not a reaction is spontaneous. It takes into acount the change in enthalpy and the change in entropy. The Gibbs free energy change is the difference between the energy change ( H) and the product of the absolute or Kelvin temperature (T) and the entropy change ( S), according to the equation below: For a system at equilibrium, G = 0. In order for a system to change spontaneously, the resulting G must be negative. If the drive toward lower energy and higher entropy cannot be satisfied at the same time, the type of change that will be favored will depend on the temperature. At low temperatures, the term T S will be small, and H will have the greatest effect on the free energy. At high temperatures, the term T S will be large, and S will have the greatest effect on the free energy. Based on the reading above, fill in the table below and answer the questions that follow Reaction Conditions Is the reaction spontaneous? [- G] Temperature H S (Always, Never, At High Temps, or At Low Temps) High + - High + + High - - High - + Low + - Low + + Low - - Low - + Try This: 1. The entropy change for the following reaction at 298 K is 3.0 J/mol K, and the enthalpy change is 394 kj/mol. Calculate the Gibbs free energy change and determine whether the reaction will occur spontaneously. C(s) + O 2 (g) CO 2 (g) 18 P a g e

19 Transition State Theory What is the Activated Complex (a.k.a. The Transition State)? 19 P a g e

20 During any chemical reaction, particles (atoms, molecules, and/or ions) are rearranged to form new substances by breaking old bonds and forming new bonds. Bond breaking requires energy, while bond making releases energy. Even exothermic reactions, such as burning wood, need energy to get started to break existing bonds in reacting particles. The energy required to initiate a chemical reaction is called activation energy. Activation energy comes from the collisions between reacting molecules, as well as additional input of energy from sources such as a spark, a match, or other heat source and is used to form an unstable, high-energy activated complex. The high-energy activated complex is so unstable, it quickly falls apart to form the products. Because the activated complex lasts only a short time, it is also called a transition state complex. According to transition state theory, during a chemical reaction, intermediate products, known as the activated complex, form that exist for only brief periods of time while the atoms rearrange themselves. Answer the questions below based on the reading above and on your knowledge of chemistry. 1. Why is it necessary to supply energy in the form of a spark to ignite the gasoline in an automobile engine if gasoline releases energy when it burns? 2. According to collision theory, where does the activation energy during a reaction come from? 3. According to transition state theory, what is activation energy used for? 4. What is an activated complex? a) Why does it last only a short time? b) Why it is it considered unstable? c) What forms from the activated complex? 20 P a g e

21 d) How does the potential energy of the activated complex compare to that of the reactant or the product? Practice Regents Questions 1. A piece of Mg(s) ribbon is held in a Bunsen burner flame and begins to burn according to the equation: 2Mg(s) + O 2 (g) 2MgO(s). The reaction begins because the reactants a. are activated by heat from the Bunsen burner flame b. are activated by heat from the burning magnesium c. underwent an increase in entropy d. underwent a decrease in entropy 2. The energy needed to start a chemical reaction is called a. potential energy b. kinetic energy c. activation energy d. ionization energy 3. In the potential energy diagram below, which letter represents the potential energy of the activated complex? 21 P a g e