The next step in kinetics

* Molecules must collide to react. * Concentration affects rates because collisions are more likely. * Must collide hard enough. * Temperature and rate are related. * Only a small number of collisions produce reactions.

* For EVERY reaction, there is a certain minimum energy that is needed for collisions to be effective = E a Activation Energy

Potential EnergyPotential Energy Reaction Coordinate Reactants Products

Potential EnergyPotential Energy Reaction Coordinate Reactants Products Activation Energy E a

Potential EnergyPotential Energy Reaction Coordinate Reactants Products Activated complex

Potential EnergyPotential Energy Reaction Coordinate Reactants Products EE }

Potential EnergyPotential Energy Reaction Coordinate 2BrNO 2NO + Br Br---NO 2 Transition State

* Activation energy - the minimum energy needed to make a reaction happen. * Activated Complex or Transition State - The arrangement of atoms at the top of the energy barrier.

K=zfp z = number of collision occurring/time at a given concentration f = fractional collisions – fraction of collision in which colliding particles energy is equal or greater than E a; f = e -Ea/RT p = steric factor – only certain orientations of colliding molecules are likely to lead to a reaction; this cannot be predicted.

* A is called the frequency factor = zp * E a = activation energy * R = ideal gas constant (in J/K mol) * T is temperature in Kelvin * k = zpe -E a /RT = Ae -E a /RT

* ln k = -(E a /R)(1/T) + ln A * Another line !!!! * ln k vs 1/T is a straight line * With slope E a /R so we can find E a * And intercept ln A

* What are the implications? * What happens to the rate constant as E a increases? * E a = 0 then e 0 = 1 * E a = 1 then e -1 =.37 * E a = 2 then e -2 =.14 * It makes sense that an increase in E a results in a smaller rate constant.

* Said that reaction rate should increase with temperature. * At high temperature more molecules have the energy required to get over the barrier. * The number of collisions with the necessary energy increases exponentially.

* Observed rate is too small * Due to molecular orientation- the have to be facing the right way

O N Br O N O N O N O N O N O N O N No Reaction O N Br O N

* A reaction is found to have a rate constant of 8.60x10 -1 sec -1 at 523 K and an activation energy of kJ/mol. What is the value of the rate constant at 270 K? * Here we will use the 2-point version of the Arrhenius Equation ln (k 1 /k 2 ) = E a /R x (1/T 2 – 1/T 1 ) ln (k 1 /8.6 x ) = J/8.314 (1/523 -1/270) ln (k 1 /8.6 x ) = k 1 /8.6 x = e K 1 = 4.25 x sec -1

Which statement is true concerning the plot of rate constants at various temperatures for a particular reaction? A) A steep slope of the ln k versus 1/T plot is indicative of small changes in the rate constant for a given increase in temperature. B) Different sections of the ln k versus 1/T plot show different Ea values. C) The plot of k versus T shows a linear increase in k as the temperature increases. D) A steep slope of the ln k versus 1/T plot is indicative of a large E a. E) The y-intercept of the ln k versus 1/T plot is the E a value for that reaction

Which statement is true concerning the plot of rate constants at various temperatures for a particular reaction? A) A steep slope of the ln k versus 1/T plot is indicative of small changes in the rate constant for a given increase in temperature. B) Different sections of the ln k versus 1/T plot show different Ea values. C) The plot of k versus T shows a linear increase in k as the temperature increases. D) A steep slope of the ln k versus 1/T plot is indicative of a large E a. E) The y-intercept of the ln k versus 1/T plot is the E a value for that reaction