8.5 Notes PDF

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✨Polarity, Bonds & Enthalpy✨ Mo Molecular lecular Po Polarity larity ● A compound is nonpolar if it is symmetrical and has a VSEPR type AX₂, AX₃, AX₄, AX₅, or AX₆ (all X(s) are the same in the compound) ○ H₂O has polar bonds and is a polar molecule because the net dipole moment is non-zero ● Exceptions ○ AX₂E₃ (linear, nonpolar) ○ AX₂E₄ (linear, nonpolar) ○ AX₄E₂ (flat, nonpolar) Bond Order ● Bond Order: Number of bonding electron pairs (number of bonds) between two bonded atoms Bo Bond nd TType ype

Bond Order

Single Bond

1

Double Bond

2

Triple Bond

3

Resonance Structures

Fractional

Bond Order: Reso Resona na nance nce SStructures tructures ● Bond Order of Each Bond in Resonance Structure = Number of bonds involved in resonance/Number of resonance structures ○ Ex. O₃ ■ O

O

O

O

O

O

● Bond Order = 3/2 = 1.5 Bond Length ● Bond Length: The distance between the atomic nuclei when energy is minimized ○ Potential Energy (PE) of Interaction = Nuclear-Nuclear (repulsion) + Electron-Nuclear (attraction) + Electron-Electron (repulsion) ● Bond length increases with increasing atom size

○ F-H = 92pm < Cl-H = 127pm < Br-H = 142pm < I-H = 161pm ● For series of bonds that differ only in bond order, bond length decreases with increasing bond order ○ C-O = 143pm > C=O = 122pm > C≡O = 113pm Bond Dissocia Dissociation tion Energy ● Bond Energ Energy: y: The energy required to break a chain bond in a gas-phase molecule to individual atoms ○ Always an endothermic (requires energy) process Singl Singlee Bonds

H

H

C

N

O

F

Si

P

S

Cl

Br

I

436

413

391

463

565

318

322

347

432

366

299

346

305

358

485

-

-

272

339

285

213

163

201

283

-

-

-

192

243

-

146

184

452

335

-

218

201

201

155

565

490

284

253

249

278

222

-

293

381

310

234

201

-

326

-

184

226

255

213

-

242

216

208

193

175

C N O F Si P S Cl Br I

151 Mul Multiple tiple Bonds C=C

602

O=O

498

C≡C

835

C=O

732

N=N

418

N=O

607

N≡N

945

C=N

615

Bond Energ Energy y, Bo Bond nd Order & Bo Bond nd Length ● The greater the bond order, the higher the bond strength and the shorter the bond

○ C-O = 143pm > C=O = 122pm > C≡O = 113pm ○ C-O = 358 kJ/mol < C=O = 732 kJ/mol < C≡O = 1,072 kJ/mol Enthalpy of Rea Reaction ction (�rH⁰) ● Enthalpy (H): The sum of the internal energy of a system plus the product of pressure and volume ● The charge of enthalpy, �H, is equal to the heat exchange under constant pressure ○ � rH⁰ = � (Energies of bonds broken) - � (Energies of bonds formed) ○ � rH⁰ = � �H⁰(Bonds broken) - � �H ⁰ (Bonds formed) ■ Combustion of hydrogen ● 2H₂ (g) + O₂ (g) → 2H₂O (g) + Energy ○ 2 H-H + O-O = 2 H-O-H ● Atoms: H, H, H, H, O, O ● Reactants: H₂ (g), O₂ (g) ● Products: 2H₂O (g) ○ � H⁰ H-H = 436 kJ/mol ○ � H⁰ O-O = 498 kJ/mol ○ � H⁰ H-O = 463 kJ/mol ■ (2(436) + 498) - (4(463)) = -482 kJ/mol...