Lecture 15 Molecular analysis 2 Molecular geometry bonding 1 Read HW BLB 3 3 3 5 9 1 9 2 BLB 3 21a c e f 25 29 37 49 51 53 Sup 3 1 8 Know molecular empirical formulas formula molecular weights VSEPR Exam 2 March 2 Need help Get help TAs in CRC 211 Whitmore and Supplemental Instruction SI hours on Chem 110 website Sheets office hours Mon 12 30 2 Tue 10 30 12 in 324 or 326 Chem Bldg Keep reviewing what we ve covered previously in class We will cover the same concepts over over again in different contexts and build on what was covered previously BST 7 Molecular formulas structure Feb 19 Sheets Page 1 Lecture 15 Important connections Avogadro s number NA connects molecules or atoms to moles connects microscopic properties to macroscopic properties e g g mL that are experimentally measurable review Lecture 2 Conservation of energy energy is not created or destroyed it just changes its form review Lecture 2 Conservation of mass mass of products mass of reactants balance the reaction Balanced chemical equation connects moles molecules of reactants with moles molecules of products related to conservation of mass Formula weight connects mass to moles connects a property that can be measured determined experimentally to moles or molecules of substance given in the balanced reaction Empirical formula tells relative number of atoms in a molecule obtained from mass or molecular formula Sheets Page 2 Lecture 15 Example What is the mass of one molecule of chlorophyll C55H72MgN4O5 Sheets Page 3 Lecture 15 Example Police officers confiscate a packet of white powder that they believe contains heroin Purification by a forensic chemist yields a 38 70 mg sample for combustion analysis This sample gives 97 46 mg CO2 and 20 81 mg H2O A second sample is analyzed for its nitrogen content which is 3 8 Is this sample really heroin C21H22NO5 Sheets Page 4 Lecture 15 VSEPR molecular geometry shapes of molecules molecular shape spatial arrangement of atoms valence shell electron pair repulsion VSEPR model predicts molecular shape 3D from Lewis structures 2D model predicts shape based on minimization of electron pair repulsion lowest e pair interactions yields best shape Lewis structures only show 1 of bonds 2 types of bonds example CCl4 Sheets Page 5 Lecture 15 Electron pair geometry VSEPR concept 1 first consider molecules with a single central atom A e pairs around the central atom other maximally due to electrostatic repulsions each e must stay near nucleus but otherwise they try to get as far apart from each other Sheets Page 6 Lecture 15 Determining electron pair geometry draw Lewis structure count electron domains around central atom electron domains fall into two classes 1 pairs connect 2 atoms 2 pairs non bonding arrange electron pairs in one of the five geometries see BLB Table 9 1 arrangement of e pairs of e domains 2 3 4 5 6 Sheets Page 7 Lecture 15 Determining molecular shape 1 draw the Lewis structure of the molecule 2 determine the electron pair geometry 3 count electron domains lone pairs single bonds double bonds triple bonds each count as one domain sets baseline geometry 4 to determine molecular geometry focus on e pairs ONLY Sheets Page 8 Lecture 15 Determining molecular shape example molecules with 4 e domains e pair geometry will be angles near placement of bonded atoms determines geometry of bonded atoms lone pairs 4 0 placement of bonded atoms molecular geometry 3 1 2 2 1 3 Sheets Page 9 Lecture 15 VSEPR concept 2 1 unshared e pairs lone pairs need more space than shared pairs why 2 multiple bonds repel other electron domains in a similar fashion as lone pairs take up more space more electron pairs consequences bond angles H C H 109 5 o H tetrahedral Sheets N H H 107 o H H trigonal pyramidal Page 10 O H 104 5o bent Lecture 15 H Before next class Read HW BLB 9 1 9 3 BLB 9 16 17 21 23 25 27 Sup 9 1 7 Know VSEPR theory electron pair electron domain geometry molecular geometry molecules with more than one central atom molecules with multiple bonds Answers p 3 1 48 10 21 g p 4 yes it is heroin Sheets Page 11 Lecture 15
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