Sheets Page 1 Lecture 15 Lecture 15: Molecular analysis 2; Molecular geometry & bonding 1 Read: BLB 3.3–3.5; 9.1–9.2 HW: 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 19Sheets Page 2 Lecture 15 Important connections!!! Avogadroʼs 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 formulaSheets Page 3 Lecture 15 Example: What is the mass of one molecule of chlorophyll, C55H72MgN4O5?Sheets Page 4 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 5 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: CCl4Sheets Page 6 Lecture 15 Electron pair geometry: VSEPR concept 1 • first consider molecules with a single central atom (A) • e– pairs around the central atom each other maximally due to electrostatic repulsions • e– must stay near nucleus but otherwise they try to get as far apart from each otherSheets Page 7 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) # of e– domains arrangement of e– pairs 2 3 4 5 6Sheets Page 8 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 ONLYSheets Page 9 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) placement of bonded atoms (molecular geometry) 4 (0) 3 (1) 2 (2) 1 (3)Sheets Page 10 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 tetrahedral trigonal pyramidal bent CHHHH109.5oNHHH107oOHH104.5oSheets Page 11 Lecture 15 Before next class: Read: BLB 9.1–9.3 HW: 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
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