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Kinetics

Kinetics. Temperature relationships and reaction mechanisms. Activation Energy Reactions occur by collision but not every collision leads to a reaction. Colliding molecules must possess a certain minimum energy to break reactant bonds and bring about reaction.

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Kinetics

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  1. Kinetics Temperature relationships and reaction mechanisms

  2. Activation Energy Reactions occur by collision but not every collision leads to a reaction. Colliding molecules must possess a certain minimum energy to break reactant bonds and bring about reaction.

  3. Effect of temperature on reaction rate Usually an increase in temperature leads to an increase in rate. Rate is approximately doubled for every 10 degree rise in temperature. demo -plop plop fizz fizz

  4. Animation of factors affecting rate http://www.sci.sdsu.edu/mathtutor/kinetics1.html

  5. Relationship between k and T is given by the Arrhenius Equation: R = 8.31 J/K Ea is the activation energy in joules Plot of ln k vs 1/T should yield a straight line

  6. Ex - Suppose rate doubles when T increases from 25oC to 35oC. What is Ea?

  7. Requirements for a successful collision: 1. Ea must be met or exceeded. 2. The orientation of the molecules must be advantageous to the reaction. demo - funneling ahead

  8. Reaction Mechanisms Most reactions take place in a series of steps. To find the expression for a multi-step mechanism: 1. Focus on the slow step. Assume the rate for the slow step is the rate for the reaction. 2. The coefficients in the rate determining step give the order of the reaction. 3. Eliminate any unstable intermediates from the rate expression.

  9. Reaction: A2 + X2 → 2AX mechanism: X2 ↔ 2X fast X + A2 → AX + A slow A + X → AX fast rate = k[X2]1/2[A2] Confirm the rate law:

  10. Catalysis catalyst - increases the rate of a reaction without being used up. The catalyst allows a different path for the reaction that has a lower activation energy.

  11. 2 H2O2 → 2 H2O + O2 The direct reaction is the result of collisions between energetic H2O2 molecules but the activation energy is high. The reaction can be catalzed by I- ions. H2O2 + I- → H2O + IO- H2O2 + IO- → H2O + O2 + I- 2 H2O2 → 2 H2O + O2 The Ea is much lower for this process.

  12. demo - dollar bill catalyst Music video http://www.youtube.com/watch?v=XX9Xo6zm_kM Notes quiz https://docs.google.com/spreadsheet/embeddedform?formkey=dFdmbXJYY0VWTW05M3UzT2oxcm5VY3c6MQ

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