Molecular Spectroscopy: Understanding Ozone's Electronic and Vibrational Transitions - Pro, Study notes of Chemistry

Download Molecular Spectroscopy: Understanding Ozone's Electronic and Vibrational Transitions - Pro and more Study notes Chemistry in PDF only on Docsity! H (nm) a WAVCLEN Molecular Spectroscopy (zumaani 14.7) ULTRAVIO! FT - > Violet = w w = 450 _ ¥ Bl ue _ ut [10 —— | 3 Grean 4 br 570 —__ co be 590 - wi 61oe Lange in S Red > = 740 INFRARED Zach color that makes up vis ole ignt corresponds to a specific wavelength and energy range. a = Electronic transitions tI —_—__— Nee Sw Rotational Vibrational transitions transitions Figure 14.52 bw 1 0 4 3 2 | 0 First excited electronic level Ground-state | electronic level Ozone – The Important Molecule πxy πx πz πz∗ UV strong absorption at 230 nm weak absorption x y z = O3 18 valence electrons 9 molecular orbitals empty hν N free atoms: 3N independent motions = 3N degrees of freedom Molecular Vibrations x y z Molecule: N bound atoms 3 translational 3 rotational (non-linear molecule) 3N – 3 – 3 = 3N – 6 degrees of freedom for vibrations x y z x y z x y z Ozone: N = 3 3 x 3 – 6 = 3 vibrational degrees of freedom Acetonitrile – An Important Solvent CH3CN N = 6 3 x 6 – 6 = 12 vibrational degrees of freedom Maximum of 12 vibrational absorptions in the infrared (IR) spectrum 5 bonds = 5 bond-stretching vibrational modes 12 - 5 = 7 bending (deformation) vibrational modes Rotational Spectroscopy é k 2675.34 2677.73 Excited vibrational state Energy { Ground vibrational stale P branch, As=—4 vicmr i Obranch. (A= 0) is missing Albranch, Asa +1 ee K 3056.97 3059.32 o-miwo b i om ch 8a Bb oon Figure 4.1: Term diagram for sponding spectrum for HCl. rovibrational transitions in diatomic molecules and the corre- The Rate is the Change in Concentration per Unit Time Rate of consumption of NO2 = -∆[NO2]/∆t Rate of production of NO = +∆[NO]/∆t Rate of production of O2 = +∆[O2]/∆t As ∆t approaches zero, the instantaneous rate becomes the tangent: d[NO2]/dt 2NO2(g) 2NO(g) + O2(g) Typically, we’re interested in the rate of the reaction itself. For the reaction: 2NO2(g) 2NO(g) + O2(g) Rate = -(1/2)d[NO2] = (1/2)d[NO] = d[O2] dt dt dt The rate of change of concentration of each species is divided by its coefficient in the balanced chemical equation. Rates of change of reactants appear with negative signs, product rates with positive signs. The Differential Rate Law Rate = -d[A] = k[A]n dt Assumes reverse reaction is negligible (i.e. forward reaction is irreversible) k = rate constant n = order of reaction in [A] k and n are determined experimentally! Reaction: aA products Example of a 1st-order reaction 2N2O5 (soln) 4NO2 (soln) + O2 (g) Rate = -d[N2O5]/dt = k[N2O5] (differential rate law) ln[N2O5] = -kt + ln[N2O5]0 (integrated rate law) Data: Figure 15.2 Plotting ln[N2O5] vs time gives a straight line Figure 15.3 Linear! First order in N2O5 Slope = -k = -6.93 x 10-3 s-1 Half life of a 1st-order reaction Half life of a reaction t1/2 • time for a reactant to reach half of its initial concentration • t = t1/2 when [A] = 1/2[A]0 • Since t1/2 = (ln2)/k = 0.693/k • t1/2 is independent of starting concentration ln = -kt[A][A]0