Kinetics

KineticsTemperature relationships and reaction mechanisms

Activation EnergyReactions 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.

Effect of temperature on reaction rateUsually 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

http://www.sci.sdsu.edu/mathtutor/kinetics1.htmlAnimation of factors affecting rate

Relationship between k and T is given by the Arrhenius Equation:

R = 8.31 J/KEa is the activation energy in joulesPlot of ln k vs 1/T should yield a straight line

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

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

Reaction MechanismsMost 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.

Reaction:A2 + X2 2AX

mechanism:X2 2XfastX + A2 AX + AslowA + X AXfast

rate = k[X2]1/2[A2]

Confirm the rate law:

Catalysiscatalyst - 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.

2 H2O2 2 H2O + O2The 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.

demo - dollar bill catalysthttp://www.youtube.com/watch?v=XX9Xo6zm_kMMusic video

Notes quizhttps://docs.google.com/spreadsheet/embeddedform?formkey=dFdmbXJYY0VWTW05M3UzT2oxcm5VY3c6MQ