Kinetics

Definition Kinetics is the study of reaction rates Reaction Rate is the speed of reaction Reaction rate is measured as the change in concentration of a reactant or product per unit of time Reaction rate can be either a positive or negative number, but will always be shown as positive

Equation RATE =  [A]/  t

Reaction Example 2NO 2  2NO + O 2 As time goes on the concentration of NO 2 decreases and the concentration of NO and O 2 increases

Experimental Data

Example of Average Reaction Rate Calculate the reaction rate for the decomposition of NO 2 for the given time periods a) 0 to 50 sec b) 350 to 400 sec What happens to reaction rate over time? Why?

Reaction Rate From Products Rate of reaction can be determined from product formation as easily as reactant disappearance 2NO 2  2NO + O 2 Rate of NO appearance is 2x the rate of O 2 appearance ½  [NO 2 ]/t = ½  [NO]/t =  [O 2 ]/t

Example Consider the reaction 2N 2 O 5 → 4NO 2 + O 2 If mole of N 2 O 5 is consumed in a 3.0L container each second of reaction, what are the rates of production of NO 2 and O 2 ?

Rate Law To begin we will consider conditions where only the forward reaction occurs at a significant rate Only the concentrations of the reactants are significant in determining the rate Rate law shows how the rate is dependant on the concentration of reactant

Rate = k[NO 2 ] n k – rate constant – different for reactions at different temperatures n – order of reactant – can only be determined experimentally

Example What are the units for k in the rate law shown below? Rate = k [A] 2 [B] 1/2