Techniques for measuring rates: Most detection of reactants or products done using some kind of spectroscopy: UV/vis absorption  detection via unique electronic excited states IR absorption or Raman scattering  detection via unique vibrations (“fingerprint” region of IR spectra around 500 – 2000 cm-1) NMR  analysis via unique responses regarding environment of 1H, 13C, ... EPR  detection of unpaired electrons (radicals) and their interactions … Alternative: mass spectrometry Simple access for gas-phase reactions, detect reactants and products after their ionization Monitor reactions in zero-pressure limit Access to volatile liquids and electrolytes also possible

CHAPTER 20: FIGURE 20A.1 PHYSICAL CHEMISTRY: THERMODYNAMICS, STRUCTURE, AND CHANGE 10E | PETER ATKINS | JULIO DE PAULA ©2014 W. H. FREEMAN AND COMPANY PHYSICAL CHEMISTRY: QUANTA, MATTER, AND CHANGE 2E| PETER ATKINS| JULIO DE PAULA | RONALD FRIEDMAN ©2014 W. H. FREEMAN D COMPANY

Microfluidic “alcove” type mixing device Inlet 1 Inlet 2 Mixing time (flow at 120 ml/min) : ~100 ms Mixing time by diffusion: ~15 ms

CHAPTER 20: FIGURE 20A.2 PHYSICAL CHEMISTRY: THERMODYNAMICS, STRUCTURE, AND CHANGE 10E | PETER ATKINS | JULIO DE PAULA ©2014 W. H. FREEMAN AND COMPANY PHYSICAL CHEMISTRY: QUANTA, MATTER, AND CHANGE 2E| PETER ATKINS| JULIO DE PAULA | RONALD FRIEDMAN ©2014 W. H. FREEMAN D COMPANY

Ultrafast kinetics via “pump-probe” technique Goal: Follow temporal evolution of a molecular system at well-defined times after photoinitiation of a reaction. Pump-probe spectroscopy: Excite sample with light pulse #1 Wait time Dt Measure system response with light pulse #2 Dt System response 1 2

Ultrafast kinetics via “pump-probe” technique Nanosecond pulses: electronic delay Pico-/femtosecond pulses: optical delay line fixed distance Time difference for the pulses Dt = (d2 – d1)/c c = 3·108 m/s  For Dd = 0.3 mm  Dt = 1 ps variable distance

Strategies for measuring reaction rates: Isolation method: If all reactants except [A] are in large excess, their concentrations are essentially constant.  Rate only depends on [A] Example: vr = kr [A] [B]2, but [B] is in large excess  vr = kr’ [A] where kr’ = kr[B]2 in this case, we speak of “a pseudo 1st order” rate law Method of initial rates: measure vr at various concentrations of one species, measure initial rate with high time resolution. Often used combined with isolation method, so other reactants are in excess. Example: vr = kr [A]a [B]b A in excess  vr = kr’ [B]b  log vr = log kr’ + b log [B]  vr = kr” [A]a  log vr = log kr” + a log [A] Plotting log vr as a function of log[A] (or log[B]) gives linear plots with slopes a (or b) (see Figure 20A.4)

CHAPTER 20: FIGURE 20A.4 PHYSICAL CHEMISTRY: THERMODYNAMICS, STRUCTURE, AND CHANGE 10E | PETER ATKINS | JULIO DE PAULA ©2014 W. H. FREEMAN AND COMPANY PHYSICAL CHEMISTRY: QUANTA, MATTER, AND CHANGE 2E| PETER ATKINS| JULIO DE PAULA | RONALD FRIEDMAN ©2014 W. H. FREEMAN D COMPANY

Note: Initial rates method does not always work, since products could affect the rate as well, particularly for composite reactions. The reaction mechanism is defined through the series of elementary steps that describe the molecular transformation: collisions, making and breaking bonds, etc. Elementary steps add up to the overall balanced reaction Stoichiometric factors of the reactants in some cases determine the order of the reaction Example: 2 NO  N2O2 Constructing a rate law from a multistep reaction mechanism often requires approximations based on the values of the rates of some steps relative to others Mechanisms can be consistent with kinetic data, but are very difficult to prove!

CHAPTER 20: FIGURE 20B.1 PHYSICAL CHEMISTRY: THERMODYNAMICS, STRUCTURE, AND CHANGE 10E | PETER ATKINS | JULIO DE PAULA ©2014 W. H. FREEMAN AND COMPANY PHYSICAL CHEMISTRY: QUANTA, MATTER, AND CHANGE 2E| PETER ATKINS| JULIO DE PAULA | RONALD FRIEDMAN ©2014 W. H. FREEMAN D COMPANY