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Understanding Chemical Kinetics: Potential Energy Diagrams

Learn how potential energy diagrams illustrate reactions, activation energy, and kinetics. Explore exothermic and endothermic processes, activated complexes, fast vs. slow reactions, and practice problems.

jameson-carlson
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Understanding Chemical Kinetics: Potential Energy Diagrams

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  1. Potential Energy Diagrams Chemical Kinetics Mrs. Kay

  2. Review of Exothermic • Reactants Ep is higher than Products Ep. • Now, we must consider the activation energy (the energy needed so that the reactants bonds will break and reform to make product)

  3. Review of Endothermic • Reactants Ep is lower than Products Ep. • Need to add more energy to the system for the forward reaction to take place. • Still need to consider activation energy

  4. Activated Complex • Is the short-lived, unstable structure formed during a successful collision between reactant particles. • Old bonds of the reactants are in the process of breaking, and new products are forming • Ea is the minimum energy required for the activation complex to form and for a successful reaction to occur.

  5. Fast and slow reactions • The smaller the activation energy, the faster the reaction will occur regardless if exothermic or endothermic. • If there is a large activation energy needed, that means that more energy (and therefore, time) is being used up for the successful collisions to take place.

  6. Sample Problem • The following reaction has an activation energy of 120kJ and a ΔH of 113kJ. 2NO2 2NO + O2 • Draw and label a potential energy (activation energy) diagram for this forward reaction. • Calculate the activation energy for the reverse reaction (if the reaction went backwards)

  7. Another Problem • The following hypothetical reaction has an activation energy of 70kJ and a ΔH of -130kJ A + B  C + D • Draw and label a potential energy diagram for the reaction • Calculate the activation energy for the reverse reaction.

  8. Watch the following Flash • Review of what is occuring during a chemical reaction for both endothermic and exothermic. • KNOW THIS!! • http://mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf

  9. Practice: • The following hypothetical reaction has an Ea of 120kJ and a ΔH of 80kJ 2a + B  2C + D • Draw and label a potential energy diagram for this reaction. • What type of reaction is this? • Calculate the activation energy for the reverse reaction. • Calculate the ΔH for the reverse reaction.

  10. Analyze the activation energy diagram below. • What is the Ea for the forward reaction? For the reverse reaction? • What is the ΔH for the forward reaction? For the reverse reaction? • What is the energy of the activated complex?

  11. Answer to #2 • The activation energy (Ea) for the forward reaction is shown by (a):Ea (forward) = H (activated complex) - H (reactants) = 400 - 100 = 300 kJ mol-1 The activation energy (Ea) for the reverse reaction is shown by (b):Ea (reverse) = H (activated complex) - H (products) = 400 - 300 = 100 kJ mol-1 The enthalpy change for the reaction is shown by (c): H = H (products) - H (reactants) = 300 - 100 = +200 kJ mol-1 for the forward and reverse reaction.

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