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Chemical Kinetics

Chemical Kinetics. field of chemistry concerned with rates and mechanisms of chemical reactions. Reaction Rate : speed a reaction occurs. Measured by how fast reactants are used up and product produced. Some reactions are fast (ex: firework) Some reactions are slow (ex: iron rusting)

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Chemical Kinetics

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  1. Chemical Kinetics • field of chemistry concerned with rates and mechanisms of chemical reactions.

  2. Reaction Rate: speed a reaction occurs. • Measured by how fast reactants are used up and product produced. • Some reactions are fast (ex: firework) • Some reactions are slow (ex: iron rusting) • Rate of reaction depends on the frequency and effectiveness of the collisions between reactant particles.

  3. Collision Theory: • in order to react, particles must effectively collide with each other. • During an effective collision, electrons shift between atoms and old bonds are broken and new ones form.

  4. What makes a collision effective? • Particles must have the right speed or kinetic energy • Particles must have the correct orientation to each other. http://chem.salve.edu/chemistry/temp2a.asp http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/collis11.swf

  5. The minimum amount of energy needed for a reaction to occur is called the activation energy. http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

  6. Reaction Mechanisms: • Most products don’t form as the result of a single collision, but instead are formed by a series of steps by which particles rearrange themselves to form products. • The rate determining step in a reaction mechanism is always the slowest step. • Usually means that step has the highest activation energy

  7. Factors Affecting Reaction Rate Temperature: Increasing temperature increases reaction rate • The faster particles move, the greater the frequency of collision • There is a greater chance the collision will be effective (particles will have enough energy to overcome activation energy) http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

  8. Factors Affecting Reaction Rate Concentration: • The greater the concentration of reactants, the greater the chance of collision • More concentrated = less space between particles • Ex: Zn in 1M HCl versus Zn in 6M HCl http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

  9. Factors Affecting Reaction Rate Pressure: (gases only) • Compressing a gas will increase the concentration on reactant particles. • This increase in concentration allows for more collisions

  10. Factors Affecting Reaction Rate Surface Area: (of a solid reactant) • The greater the surface area, the more the reactant particles are exposed to each other for possible collisions. • Ex: steel wool vs. steel bar, block of wood vs. sawdust

  11. Ways to increase surface area: • Grind into fine powder • Stir or agitate to disburse particles

  12. Factors Affecting Reaction Rate Nature of Reactants: • Simpler reactants will react faster than ones with many complex bonds. • Fastest to React: • (aq) ionic compounds • have simple ions that are disbursed in solution • Covalents are usually slower to react. • Gases usually react faster than substance in the liquid or solid phases.

  13. Factors Affecting Reaction Rate Catalysts: speed up the rate of a chemical reaction without being altered. (ex: enzymes) Works by providing an alternate reaction pathway that require less energy. Lowers the amount of activation energy needed to start reaction http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.ht m

  14. Inhibitors: slow down chemical reactions • Can tie up reactants in side reactions • Can block active site in enzymatic catalysts • Can provide alternate reaction pathway with higher activation energy requirements.

  15. Sample Regents Questions

  16. Given the balanced equation representing a reaction:2HCl(aq) + Na2S2O3(aq)--> S(s) + H2SO3(aq) + 2NaCl(aq) Decreasing the concentration of Na2S2O3(aq) decreases the rate of reaction because the (1) activation energy decreases(2) activation energy increases(3) frequency of effective collisions decreases(4) frequency of effective collisions increases

  17. As the temperature of a chemical reaction in the gas phase is increased, the rate of the reaction increases because (1) fewer particle collisions occur(2) more effective particle collisions occur(3) the required activation energy increases(4) the concentration of the reactants increases

  18. Why can an increase in temperature lead to more effective collisions between reactant particles and an increase in the rate of a chemical reaction? (1) The activation energy of the reaction increases.(2) The activation energy of the reaction decreases.(3) The number of molecules with sufficient energy to react increases.(4) The number of molecules with sufficient energy to react decreases.

  19. At 20.°C, a 1.2-gram sample of Mg ribbon reacts rapidly with 10.0 milliliters of 1.0 M HCl(aq). Which change in conditions would have caused the reaction to proceed more slowly? (1) increasing the initial temperature to 25°C(2) decreasing the concentration of HCl to 0.1 M(3) using 1.2 g of powdered Mg(4) using 2.4 g of Mg ribbon

  20. 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? (1) a zinc strip and 1.0 M HCl(aq)(2) a zinc strip and 3.0 M HCl(aq)(3) zinc powder and 1.0 M HCl(aq)(4) zinc powder and 3.0 M HCl(aq)

  21. Given the balanced equation representing a reaction: Zn(s) + 2HCl(aq)==>H2(g) + ZnCl2(aq) Which set of reaction conditions produces H2(g) at the fastest rate? (1) 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl at 20.oC(2) 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl at 30.oC(3) 1.0 g powdered Zn(s) in 50. mL of 1.0 M HCl at 20.oC(4) 1.0 g powdered Zn(s) in 50. mL of 1.0 M HClat 30.oC

  22. Thermodynamics • Study of the energy involved in chemical reactions. • Particles must have a certain energy to react • During collisions KE (of motion) is converted to PE (stored energy in chemical bonds)

  23. PE Diagrams • Show how PE changes during a chemical rxn. • PE Reactants • PE Products • Activation Energy: • energy needed to start • energy required to form “activated complex”

  24. Activated Complex: • intermediate between reactant and product • Found at top of energy hill • Must be formed for rxn to occur • Unstable and short lived (very high in energy) • Contain bonds in process of being broken/formed

  25. Heat of Reaction (∆H) • Overall net change in energy (enthalpy) for reaction • Difference between PE of products and reactants ∆H = PE products – PE reactants What is ∆H for this reaction?

  26. Endothermic vs. Exothermic Exothermic: (∆H negative) • Products have less energy than reactants • Energy given off Endothermic: (∆H positive) • Products have more energy than reactants • Energy taken in • Usually requires more activation energy than exothermic reactions http://www.youtube.com/watch?v=VbIaK6PLrRM&feature=player_embedded

  27. Determine the following: • PE reactants • PE Products • PE activated complex • Forward activation energy • Reverse activation energy • ∆H for forward rxn • Exo or endo? http://www.kentchemistry.com/links/Kinetics/PEDiagrams.htm

  28. Affects of a Catalyst • Catalysts lower: • activation energy for both the forward and reverse reactions • Energy of the activated complex How much energy was saved by using a catalyst?

  29. Catalysts Don’t Change ∆H • They do NOT EFFECT: • PE of reactants or products. • ∆H (overall heat lost or gained)

  30. Which interval on this diagram represents the difference between the potential energy of the products and the potential energy of the reactants?    (1) 1     (2) 2    (3) 3      (4) 4

  31. Which statement is true about energy in this reaction? • The reaction is exothermic because it releases heat.      • The reaction is exothermic because it absorbs heat. • The reaction is endothermic because it releases heat. • The reaction is endothermic because it absorbs heat.

  32. Which statement best describes how a catalyst increases the rate of a reaction? • The catalyst provides an alternate reaction pathway with a higher activation energy. • The catalyst provides an alternate reaction pathway with a lower activation energy. • The catalyst provides the same reaction pathway with a higher activation energy. • The catalyst provides the same reaction pathway with a lower activation energy.

  33. Which statement correctly describes the energy changes that occur in the forward reaction? (1) The activation energy is 10. kJ and the reaction is endothermic. (2) The activation energy is 10. kJ and the reaction is exothermic. (3) The activation energy is 50. kJ and the reaction is endothermic. (4) The activation energy is 50. kJ and the reaction is exothermic.

  34. In a chemical reaction, the difference between the potential energy of the products and the potential energy of the reactants is defined as the • activation energy • ionization energy • heat of reaction • heat of vaporization

  35. Which information about a chemical reaction is provided by a potential energy diagram? (1) the oxidation states of the reactants and products (2) the average kinetic energy of the reactants and products (3) the change in solubility of the reacting substances (4) the energy released or absorbed during the reaction

  36. Which statement correctly describes an endothermic chemical reaction? (1) The products have higher potential energy than the reactants, and the ΔH is negative. (2) The products have higher potential energy than the reactants, and the ΔH is positive. (3) The products have lower potential energy than the reactants, and the ΔH is negative. (4) The products have lower potential energy than the reactants, and the ΔH is positive.

  37. Given the reaction: S(s) + O2(g) → SO2(g) + energy  Which diagram best represents the potential energy changes for this reaction?

  38. Reference Table I Heats of Reaction Table (∆H ): • Shows various reactions and whether they are endothermic or exothermic. • - ∆H exo • + ∆H endo • The energy involved is for the relative number of moles indicated in the balanced equation

  39. Using Table I • Table shows several types of reactions: • Top 6 reactions are combustions (- ∆H ) • Middle reactions are synthesis • ∆H values are showing ∆H of “formation” of the compound from its elements • Bottom reactions show dissolving of various ionic salts (some are exo, some endo) • Last reaction is neutralization

  40. Using Table I Ex: 2CO(g) + O2(g) → 2 CO2 + 566 KJ • Exothermic reaction • ∆H = -566.8KJ • The reverse of this reaction would be endothermic ∆H = +566 KJ (←) • If just one mole of CO used, half the energy would be produced ∆H = - 283KJ

  41. Given the reaction: 2 H2(g) + O2(g) --> 2 H2O(l) + 571.6 kJ What is the approximate ΔH for the formation of 1 mole of H2O(l)? • -285.8 kJ         • +285.8 kJ   • -571.6 kJ     • +571.6 kJ

  42. Chemical cold packs are often used to reduce swelling after an athletic injury. The diagram represents the potential energy changes when a cold pack is activated. Which lettered interval on the diagram represents the potential energy of the products?  Which lettered interval on the diagram represents the heat of reaction? Identify a reactant listed in Reference Table I that could be mixed with water for use in a chemical cold pack.

  43. According to Table I, which salt releases energy as it dissolves?  • KNO3 • LiBr • NH4NO3 • NaCl

  44. Given the reaction: CH4(g) + 2 O2(g) --> 2 H2O(l) + CO2(g) What is the overall result when CH4(g) burns according to this reaction? • Energy is absorbed and ΔH is negative.       • Energy is released and ΔH is negative. • Energy is absorbed and ΔH is positive.   • Energy is released and ΔH is positive.

  45. According to Table I, which potential energy diagram best represents the reaction that forms H2O(l) from its elements? • (1) • (2) • (3) • (4)

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