st CHEM 1312H 1 Editon Lecture 15 These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 23 6 CRYSTAL FIELD THEORY Recall many magnetc propertes and colours of transiton metal complexes are related to the presence of d electrons from the metal cation the attracton between a ligand and metal ion is a Lewis acid base interacton o the ligand base donates a pair of electrons to an empty orbital on the metal ion o the attractve interactons ion dipole between the two is due largely to the electrostatc field between the positve and negative charges although the metal ion is attracted to the ligand s electrons o we consider the ligands to be points of negatve charge that repel the negatvely charged d orbital electrons from the metal ion o octahedral crystal field illustration found on pg 1022 o crystal field splitng energy the energy gap between two sets of orbitals on diferent levels this energy gap accounts for the diferent colours and magnetsm propertes observed in transiton metal complexes ligands at the low end of the spectrochemical series are weak feld ligands ligands at the high are called strong feld ligands spin pairing energy the diference between the energy required to pair an electron in an occupied orbital and the energy required to place that electron in an empty orbital up top high spin complex electrons are arranged so that they remain as unpaired as possible low spin complex electrons are arranged so that they remain paired as much as possible while stll following Hund s rule tetrahedral and square planar complexes o crystal field splitng energy is much smaller for tetrahedral complexes than it was for comparable octahedral complexes this is because there are fewer ligand point charges in the tetrahedral geometry all tetrahedral complexes are high spin aka crystal feld splitng energy is never large enough to overcome the spin pairing energies square planar complexes are characteristcs of metal ions with a d8 electron confguraton nearly always in low spin nearly always with eight d electrons spin paired to form a diamagnetc complex 4 1 CHEMICAL KINETICS Chemical kinetics the area of chemistry concerned with the rates of reactons Reacton rate the speed at which a chemical reacton occurs Reacton mechanism step by step molecular level view of the pathway from reactants to products Factors that afect rxn rates 1 physical state of reactants homogenous reactons involving either all gases or all liquids heterogeneous reactons where a reacton is limited by the area of contact of the reactants 2 reactant concentratons most chemical rxns proceed more quickly if the concentraton of one or more reactants is increased 3 reacton temperature rxn rates generally increase as temperature increases because as the kinetc energy of the molecules increases so do the number of collisions Keep in mind that a collision must occur with suficient energy to break bonds and with the reactants in the right positon for new bonds to form in the proper locatons for it to be efectve 4 presence of a catalyst catalysts are agents that increase reacton rates without being used up themselves they afect the kinds of collisions that happen and therefore alter the mechanism 14 2 REACTION RATES speed of an event is defined as the change that occurs in a given tme interval units for reacton rates are usually M s it is typical for rates to decrease as a rxn proceeds because concentraton is decreasing as reactants are used up instantaneous rate the rate at a partcular instant during the reacton 14 3 CONCENTRATION AND RATE LAWS Rate laws are equations that show how rate depends on reactant concentraton For the general reacton aA bB cC dD rate law k A m B n where m and n are called reacton orders overall reacton order is the sum of the orders with respect to each reactant in the rxn so something has a second order overall if m and n are both 1 Reacton orders are typically 0 1 or 2 and the rate law must be determined experimentally for each reactn Magnitudes and Units of Rate Constants generally a large value of k about 109 or higher means a fast reacton and a small value of k 10 or lower means a slow reacton rate constants and hence the reacton rate are afected by temperature and presence of a catalyst rate constant units are M 1s 1 Using Inital Rates to Determine Rate Laws We can use the response of a reacton rate to a change in inital concentration to determine reacton order 14 5 TEMPERATURE AND RATE The Collision Model The Arrhenius Equaton k Ae actvaton energy RT where R is the gas constant 8 314 J molk reacton rates decrease as Ea increases this makes sense since if you need more energy to initiate chemical reactions between molecules it s going to take more tme for the reacton to run its course