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- Explaining Reaction Rates - 1.  Collision theory states that chemical reactions can occur only if reactants collide with proper orientation and with.

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Presentation on theme: "- Explaining Reaction Rates - 1.  Collision theory states that chemical reactions can occur only if reactants collide with proper orientation and with."— Presentation transcript:

1 - Explaining Reaction Rates - 1

2  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 2

3  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 3

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5  Activation energy (E a or E act ) is the minimum energy that reactant molecules must posses for a reaction to be successful (fruitful) 5

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8  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 8

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12  Collision Theory Collision Theory  5 Factors Affecting Reaction Rates 5 Factors Affecting Reaction Rates 12

13 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 13

14  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 14

15  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. 15

16  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 16

17  An increase in initial concentration increases the rate of a reaction because the total number of collisions possible per unit time is increased proportionately 17  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  A rise in temperature increases the rate of a reaction for two reasons: 1.The total number of collisions possible per unit time is increased slightly 2.The fraction of collisions that are sufficiently energetic to be effective is increased dramatically 18

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20  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 20

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

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