1. - Explaining Reaction Rates -
Chemical Kinetics
- Explaining Reaction Rates -

2. Collision Theory
Collision theory states that chemical reactions can occur only if reactants collide with proper orientation and with enough kinetic energy to break reactant bonds and form product bonds

3. Orientation
Some reactant orientations for collisions between molecules or ions can lead to reactions (called effective collisions)
While other reactant orientations do not (called ineffective collisions)
The study of these collisions is sometimes called collision geometry

4. Orientation (alignment or positioning)

5. Activation Energy
Activation energy (Ea or Eact) is the minimum energy that reactant molecules must posses for a reaction to be successful (fruitful)
The ball leaves point A moving toward the right. As it rises on the uphill portion of the track it slows down: Kinetic energy converts to potential energy. The ball can only successfully overcome the rise of the track and proceed to point B if it has enough initial speed (kinetic energy). We could call this situation an effective trip. The minimum kinetic energy required is analogous to the activation energy for a reaction. If the ball does not have enough kinetic energy it will not reach the top of the track and will just roll back to point A. This is analogous to two molecules colliding without enough energy to rearrange their bonds: They just rebound. Since the activation energy sometimes seems to prevent reaction, it is often called an activation energy barrier.

8. Activation Energy
Activated complex is an unstable chemical species containing partially broken and partially formed bonds representing the maximum potential energy point in the change – also called transition energy

9. Over the progress of an effective collision between molecules in the gas phase, the potential energy increases to a maximum at the point of closest approach, then decreases to a final value higher than the initial energy (as the reaction is endothermic). The potential energy gain of the molecules comes from conversion of kinetic energy. The
overall reaction would lower the temperature of the system and surroundings. activated complex an unstable chemical species containing partially broken and partially formed bonds representing the maximum potential
energy point in the change; also known as transition state

10. Over the progress of this exothermic reaction, the potential energy (or enthalpy) increases to a maximum as the activated complex forms, then decreases to a final value lower than the initial energy. The potential energy lost by the molecules is converted to kinetic energy. The overall reaction would raise the temperature of the surroundings.

12. Videos
Collision Theory
5 Factors Affecting Reaction Rates

13. Factors Affecting Rate and Collision Theory
Each of the five factors that affect rate increases either collision frequency or the fraction of collisions that are effective (or both) to increase rate of reaction
rate = collision frequency X fraction effective
Concentration
Surface area
Temperature
Nature of Reactant
Catalyst
Temperature

14. Theoretical Effect of Chemical Nature of Reactant
For any reactant, the bond type, strength, and number determine the activation energy required for a successful collision
Reactions involving the breaking of fewer bonds per reactant proceed faster than those involving the breaking of a larger number of bonds per reactant
Weaker bonds are broken at a faster rate than stronger bonds

15. Theoretical Effect of Chemical Nature of Reactant
For example, it takes less energy to break a single C – C bond than a double C = C bond
Reactions between molecules are usually slower than reactions between ions because, in molecules, covalent bonds have to be broken and new bonds re-formed. This slows down reaction rates.

16. Theoretical Effect of Chemical Nature of Reactant
The size and shape of a molecule or ion can also affect reaction rate
Some reactions involve complicated molecular substances or complex ions
These are often less reactive than smaller, less complex entities
This is in part because more bonds must be broken. However, it is also less likely that complex molecules or ions will collide in an orientation relative to each other that will be effective allowing a reaction to occur

17. Theoretical Effect of Concentration
An increase in initial concentration increases the rate of a reaction because the total number of collisions possible per unit time is increased proportionately
Theoretical Effect of Surface Area
An increase in reactant surface area increases the rate of a reaction because the total number of collisions possible per unit time is increased proportionately

18. Theoretical Effect of Temperature
A rise in temperature increases the rate of a reaction for two reasons:
The total number of collisions possible per unit time is increased slightly
The fraction of collisions that are sufficiently energetic to be effective is increased dramatically

19. Experiment shows that when the temperature increases from T1 to T2, the shape of the Maxwell- Boltzmann distribution curve flattens and shifts to the right. Note the very large increase in the fraction of molecules able to react at the higher system temperature.

20. Theoretical Effect of Catalysis
A catalyst increases the rate of a reaction by providing an alternative pathway, with lower activation energy, to the same product formation
A much larger fraction of collisions is effective following the changed reaction mechanism
Catalysts are involved in the reaction mechanism at some point, but are regenerated before the reaction is complete

22. The reaction shown here proceeds by a three-step mechanism when a catalyst is present, but nonetheless proceeds much faster than by the one-step uncatalyzed mechanism. The catalyzed mechanism has a lower activation energy, so more collisions are successful.

23. Learning Checkpoint
p. 387 Practice UC # 1 p. 396 – 397 Practice UC # 1, 2, 3, 4

24. (b) The lower curve represents a catalyzed reaction; the upper curve represents the uncatalyzed reaction. (c) The reaction is endothermic so heat flows from the surroundings into the system. (d) (i) is the activation energy for the uncatalyzed reaction; (ii) is the activation energy for the catalyzed reaction; (iii) is the enthalpy change.