Activation_Energy_and_Catalyst.ppt
- 2. Temperature and Rate Generally, as temperature increases, so does the reaction rate. This is because k is temperature dependent.
- 3. The Collision Model In a chemical reaction, bonds are broken and new bonds are formed. Molecules can only react if they collide with each other.
- 4. The Collision Model Furthermore, molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation.
- 5. Activation Energy In other words, there is a minimum amount of energy required for reaction: the activation energy, Ea. Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.
- 6. Reaction Coordinate Diagrams It is helpful to visualize energy changes throughout a process on a reaction coordinate diagram like this one for the rearrangement of methyl isonitrile.
- 7. Reaction Coordinate Diagrams The diagram shows the energy of the reactants and products (and, therefore, E). The high point on the diagram is the transition state. • The species present at the transition state is called the activated complex. • The energy gap between the reactants and the activated complex is the activation energy barrier.
- 8. Maxwell–Boltzmann Distributions Temperature is defined as a measure of the average kinetic energy of the molecules in a sample. • At any temperature there is a wide distribution of kinetic energies.
- 9. Maxwell–Boltzmann Distributions As the temperature increases, the curve flattens and broadens. Thus at higher temperatures, a larger population of molecules has higher energy.
- 10. Maxwell–Boltzmann Distributions If the dotted line represents the activation energy, then as the temperature increases, so does the fraction of molecules that can overcome the activation energy barrier. • As a result, the reaction rate increases.
- 11. Maxwell–Boltzmann Distributions This fraction of molecules can be found through the expression where R is the gas constant and T is the Kelvin temperature. f = e -Ea RT
- 12. Arrhenius Equation Svante Arrhenius developed a mathematical relationship between k and Ea: k = A e where A is the frequency factor, a number that represents the likelihood that collisions would occur with the proper orientation for reaction. -Ea RT
- 13. Arrhenius Equation Taking the natural logarithm of both sides, the equation becomes ln k = - ( ) + ln A 1 T y = m x + b Therefore, if k is determined experimentally at several temperatures, Ea can be calculated from the slope of a plot of ln k vs. . Ea R 1 T
- 14. Catalysts Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction. Catalysts change the mechanism by which the process occurs.
- 15. Catalysts One way a catalyst can speed up a reaction is by holding the reactants together and helping bonds to break.
- 16. Enzymes Enzymes are catalysts in biological systems. The substrate fits into the active site of the enzyme much like a key fits into a lock.