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Collision Theory

Collision Theory. When and why a reaction happens. General Rule # 1. A collision must occur in order for reaction to occur. A reaction is an exchange/rearrangement of electrons Must come in contact to know that the other atom/molecule is present Must come in contact to exchange electrons.

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Collision Theory

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  1. Collision Theory When and why a reaction happens

  2. General Rule # 1 • A collision must occur in order for reaction to occur. • A reaction is an exchange/rearrangement of electrons • Must come in contact to know that the other atom/molecule is present • Must come in contact to exchange electrons. • BUT!!! Not all collisions result in a reaction.

  3. Corollary # 1 – The Orientation • Observe the demo. When will the car and dart stick together? • Can you shoot the dart at the car in any orientation? • Molecules must also be aligned so that their reactive parts collide in order for reaction to occur?

  4. Corollary # 2 – The Energy • Observe the demos. Do all rollings of the ball result in it making it over the hill? • There is a minimum energy needed for a collision to cause a reaction – called the Activation Energy. • Energy Diagrams show this activation energy:

  5. The Explanation • What factors affect reaction rates? • Concentration • Surface Area • Catalysts • Temperature • Can we explain why they are true with collision theory?...(yes we can…or I wouldn’t have asked! )

  6. Concentration • Remember the tenets: more collisions means more likely that a reaction will occur. • Higher concentration means more molecules crammed together. • This increased concentration leads to more collisions and more favorable collisions.

  7. Surface Area • The molecules must be able to get to each other to collide. • If the molecules are not stacked on top of each other, all molecules can collide, not just the ones on the exterior. • The increased number of collisions will create more reaction.

  8. Catalysts • Catalysts hold molecules in certain orientations. • This can increase the frequency of a favorable collision orientation. • This can also give the reaction a different pathway that can lower the activation energy.

  9. Temperature • Increasing temperature increases molecular speed. • Increasing the speed will increase the number of collisions. • Increasing the speed will increase the severity of the collisions (giving more the needed activation energy).

  10. Quantifying Collision Theory • Can we take the ideas of collision theory and put them into a mathematical equation? • Can we predict the value of the rate constant as it changes with temperature and other factors? • Would I have asked?

  11. The Arrhenius Equation Frequency factor Activation Energy rate constantk = A e –Ea/RT Ideal gas constant Absolute Temperature (8.31 J/mol K)

  12. The Math Trick • If you take the natural log of both sides: ln k = ln A + ln e –Ea/RT ln k = ln A - Ea/R (1/T) y = b + m x

  13. What can we do? ln k = ln A - Ea/R (1/T) y = b + m x So if we graph ln k on the y-axis and 1/T on the x-axis, we can calculate the activation energy!

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