Lewis Structures & Enthalpy Change Exercises

Topics being Covered in the Worksheet:
1. Lewis Structures
2. Bond Order, Bond Length, and Bond Energies
3. Enthalpy Change via Bond Energy
4. Polarity
5. VSEPR
6. Formal Chargeshe questions include tables to fill them the worksheet

Instructions for Working out the Worksheets:

Work in groups of 4 on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help. Your TA will come around when needed to help guide you if needed.

Round Robin Instructions:

1. Each group member will be assigned a number, starting with #1 and ending with the number of people in the group.
2. Student #1 will read the question aloud and define the information needed to solve the problem, create an outline on how you can solve the problem.
3. When the group agrees that the necessary information is complete, student number two will do the first step. When the group agrees that the step is correct, student number three will do the next step. Continue this way until the group agrees that the given answer had been correctly drawn and identified.
4. Student #2 will start the next question by reading it aloud as in #1. Follow this pattern for all the questions in the session.

Topics being Covered in the Worksheet:

 1.     Lewis Structures2.     Bond Order, Bond Length, and Bond Energies3.     Enthalpy Change via Bond Energy 4.     Polarity5.     VSEPR6.     Formal Charges
1. Complete the table which will go over topics 1 and 2. Using the covalent radii table.
 Molecule Lewis Structure Bond ofInterest BondOrder BondLength BondEnergy(kJ/mol) H2 H-H 436 Cl2 Cl-Cl 243 H2O O-H 464 H3CCH3 C-C 346 H2CCH2 C-C 615 H2CCH 2Molecule Lewis Structure BondC-C ofInterest Bond  Order Bond  Length BondEnergy 615(kJ/mol) HCCH C-C 835 CO2 C-O 804 H2CO C-O 799 N2 N-N 945 O2 O-O 498
1. In the following problems you will be solving mathematical problems to help you understand topic 3. Make sure you have the correct significant figures for each problem

Average Bond Energies (kJ/mol)

 H–H 436 kJ/mol C–H 413 kJ/mol C=C 614 kJ/mol H–Cl 431 kJ/mol C–C 348 kJ/mol C!C 839 kJ/mol H–F 567 kJ/mol C–N 293 kJ/mol C=O 799 kJ/mol N–H 391 kJ/mol C–O 358 kJ/mol O=O 495 kJ/mol N–O 201 kJ/mol C–F 485 kJ/mol C!O 1072 kJ/mol O–H 463 kJ/mol C–Cl 328 kJ/mol C=N 615 kJ/mol O–O 146 kJ/mol C–S 259 kJ/mol N=N 418 kJ/mol F–F 155 kJ/mol Cl–Cl 242 kJ/mol N!N 941 kJ/mol C!N 891 kJ/mol

Estimate the enthalpy change (!Hrxn) of the following reactions using the bond energies above.

Cl                            O

 1.             H H + Cl Cl ! H Cl + H ClH H 4.          H C Cl + O !                   C Cl +Cl                                 ClH H Cl

2  .        HH  C C HH  + F F ! H CF  CF  H                     5.       H CH  H + OO  OO  !  O C O +    HH  OO  HH

H H                              H       H

1. H C O H + H C O H ! H C O C H + H O H

H H                                  H       H

1. In the following problems you will be determining which is most polar and explain the reasoning for the choice, to help you understand topic 4.
• carbon disulfide                  OR             sulfur difluoride
• nitrogen trichloride    OR             oxygen dichloride
• boron trihydride          OR             ammonia
• chlorine                   OR             phosphorus trichloride
• silicon dioxide          OR             carbon dioxide
• methane                  OR             CH2Cl2
• silicon tetrabromide   OR             HCN
• nitrogen trifluoride     OR             phosphorus trifluoride

1. In the following problems draw and name the Lewis structure and VSPR structure of the following. This will help with topics 1 and 6.
 Molecules Lewis Structure & Name VSEPR Structure & Name Formal Charge of Central Atom H2CS N2O5 BF3 NO2- CCl4 PF5 ICl3 XeF2
 Molecules Lewis Structure & Name VSEPR Structure & Name Formal Charge of Central Atom ClF5 SF6 XeF4 BrF5 XeCl3– C2H6O C6H6(C’s form a ring) CH2O