Collision Theory and Rate of Reaction. Sunday, April 15, 18

Transcription

1 Collision Theory and Rate of Reaction

2 Collision Theory System consists of particles in constant motion at speed proportional to temperature of sample Chemical reaction must involve collisions of particles with each other and with the walls of the containers

3 Collision Theory effective collision : has sufficient energy and correct orientation for bonds can be broken and new bonds form ineffective collision : particles rebound from collision, essentially unchanged

4 Collision Theory Rate of reaction depends on frequency of collisions and the fraction of collisions that are effective

5 Correct Orientation Particles must collide with correct orientation for the collision to be effective

6 Activation Energy Activation Energy: The minimum increase in potential energy of a system required for molecules to rearrange their structure and result in an effective collision

7 Transition State Theory Activated Complex: an unstable molecule containing partially broken and partially formed bonds representing the maximum potential energy point in the change

8 Transition State Theory Transition State: energy maximum where the activated complex is formed

9 Maxwell-Boltzmann Distribution show the kinetic energy of the particles in a sample the area under the entire curve represents all of the particles in the sample

10 Maxwell-Boltzmann Distribution the area under the curve to the right of the activation energy represents the particles that have sufficient energy to react

11 Reaction Mechanisms Most chemical reactions actually occur in a sequence of elementary steps called the reaction mechanism Reaction mechanisms may only be determined by experimentation For a reaction mechanism to be possible, the elementary steps must add to give the balanced equation.

12 Example Consider the reaction NO2 (g) + CO (g) NO (g) + CO2 (g) This reaction occurs in two steps: 1) NO2 + NO2 NO3 + NO (slow) 2) NO3 + CO NO2 + CO2 (fast) NO3 is considered a reaction intermediate

13 Reaction Mechanisms The slow step in the mechanism is the rate determining step it determines the rate of the overall reaction. To increase the rate of the overall process, the rate of the slow step must be increased.

14 Factors Affecting Reaction Rate

15 There are five factors that affect the rate of reaction chemical nature of reactants concentration of reactants temperature presence of a catalyst surface area

16 Chemical Nature of Reactants Similar elements (such as those in the same group of the periodic table) tend to react similarly, but at different rates Example: Sodium and other alkali metals react so quickly they are never found alone in nature Example: Gold and silver react so slowly that they are often used for jewelry

17 Concentration of Reactants If the initial concentration of the reactant is increased, the rate of the reaction is increased If twice the number of particles are present, there should be twice the number of effective collisions, increasing the rate of reaction.

18 Temperature If the temperature of the reaction is increased, the rate of the reaction is increased Increasing the temperature increases the kinetic energy of the particles causing them to collide more often and with more force.

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20 Presence of a Catalyst Catalyst: a substance that alters the rate of a chemical reaction without being permanently changed itself A much larger fraction of collisions become effective.

21 Surface Area The amount of exposed surface area affects the reaction rate. The reaction rate increases with the increase in surface area.

22 Homework Read and summarize p 370 Representing a Reaction with Potential Energy Diagrams p. 371 #11-14