Presentation on theme: "Techniques for measuring rates:"— Presentation transcript:
1 Techniques for measuring rates: Most detection of reactants or products done using some kind of spectroscopy:UV/vis absorption detection via unique electronic excited statesIR 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 spectrometrySimple access for gas-phase reactions, detect reactants and products after their ionizationMonitor reactions in zero-pressure limitAccess to volatile liquids and electrolytes also possible
5 Ultrafast kinetics via “pump-probe” technique Goal:Follow temporal evolution of a molecular system at well-defined times afterphotoinitiation of a reaction.Pump-probe spectroscopy:Excite sample with light pulse #1Wait time DtMeasure system response with light pulse #2DtSystem response12
6 Ultrafast kinetics via “pump-probe” technique Nanosecond pulses: electronic delayPico-/femtosecond pulses: optical delay linefixed distanceTime difference for the pulsesDt = (d2 – d1)/cc = 3·108 m/s For Dd = 0.3 mm Dt = 1 psvariable distance
7 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 lawMethod 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)
9 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 reactionStoichiometric factors of the reactants in some cases determine the order of the reaction Example: NO N2O2Constructing a rate law from a multistep reaction mechanism often requires approximations based on the values of the rates of some steps relative to othersMechanisms can be consistent with kinetic data, but are very difficult to prove!