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Mechanisms and Activation Energy

The lizard in the photograph is not simply enjoying the sunshine or working on its tan. The heat from the sun’s rays is critical to the lizard’s survival. A warm lizard can move faster than a cold one because the chemical reactions that allow its muscles to move occur more rapidly at higher temperatures. A cold lizard is a slower lizard and an easier meal for predators. From baking a cake to determining the useful lifespan of a bridge, rates of chemical reactions play important roles in our understanding of processes that involve chemical changes.

Figures (10)

Henry Taube's experiment establishing the role of a bridging ligand in inner sphere electron transfer.
Benzoin condensation reaction mechanism. Cyanide ion (CN−) acts as a catalyst here, entering at the first step and leaving in the last step. Proton (H+) transfers occur at (i) and (ii). The arrow pushing method is used in some of the steps to show where electron pairs go.
Example of an enzyme-catalysed exothermic reaction
SN2 reaction mechanism. Note the negatively charged transition state in brackets in which the central carbon atom in question shows five bonds, an unstable condition .
The Michael addition reaction mechanism with ammonia's lone pair as a Michael donor, using curved arrows (electron or arrow pushing method)
The sparks created by striking steel against a piece of flint provide the activation energy to initiate combustion in this Bunsen burner. The blue flame sustains itself after the sparks stop because the continued combustion of the flame is now energetically favorable.
Figure 12.13 Illustrated are two collisions that might take place between carbon monoxide and oxygen molecules. The orientation of the colliding molecules partially determines whether a reaction between the two molecules will occur.
Figure 12.14 Reaction diagram for the exothermic reaction A+B⟶C+D.A+B⟶C+D.
Figure 12.15 Molecular energy distributions showing numbers of molecules with energies exceeding (a) two different activation energies at a given temperature, and (b) a given activation energy at two different temperatures.
Figure 12.16 This graph shows the linear relationship between ln k and 1T1T for the reaction 2HI⟶H2+I22HI⟶H2+I2 according to the Arrhenius equation.

Key Points

  • The activation energy (Ea) of a reaction is measured in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).
  • In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
  • When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate.
  • A catalyst is able to reduce the activation energy by forming a transition state in a more favorable manner.
  • A substance that modifies the transition state to lower the activation energy is termed a catalyst; a catalyst composed only of protein and (if applicable) small molecule cofactors is termed an enzyme.
  • In the Arrhenius equation, the term activation energy (Ea) is used to describe the energy required to reach the transition state, and the exponential relationship k = A exp(−Ea/RT) holds.

Terms

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