Potential Energy Diagrams

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Potential Energy Diagrams

• Chemical Kinetics
• Mrs. Kay

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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)

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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

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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.

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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.

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Sample Problem

• The following reaction has an activation energy
  of 120kJ and a ?H 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)

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Another Problem

• The following hypothetical reaction has an
  activation energy of 70kJ and a ?H of -130kJ
• A B ? C D
• Draw and label a potential energy diagram for the
  reaction
• Calculate the activation energy for the reverse
  reaction.

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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/essentialchemist
  ry/flash/activa2.swf

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Practice

• The following hypothetical reaction has an Ea of
  120kJ and a ?H 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 ?H for the reverse reaction.

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

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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.