Chemische Binding (Bonding) hfst. 8 Zumdahl

ModellenConceptenInter-moleculaire bindingIntra-moleculaire binding

Models (Zumdahl, end § 8.7)

Models are “attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world”(Zumdahl 5th p. 370; 6th p.368; 7th p. 348).

Fundamental Properties of Models

- Models…• are human inventions based on incomplete understanding • and do NOT equal reality• are (over)simplifications, and are therefore often wrong.• become more complicated as they age.

- We must understand the underlying assumptions in a model to prevent misuse.

(Zumdahl 5th p. 372; 6th p.370; 7th p. 350).

What models in Zumdahl Ch. 7?

• Energy and waves

• Atoms:• The Bohr model • Quantum Mechanical model

• Underlying assumption/insight:• all three: quantisation of energy• Bohr vs. Quantum-Mechanical:

• localised vs. delocalised electrons

Chemische Binding (Bonding) (vervolg)

ModellenAlgemene Concepten (Ch. 8)Inter-moleculaire bindingIntra-moleculaire binding

Overzicht: Chemische Binding (‘Bonding’)

Drie modellen voor binding:

IonbindingCovalente bindingPolaire covalente binding

Voorbeelden?

Overview of Bonding-types (8.2)

• Perspectief: • vanuit de atomen in de binding

• Extremen:• Ionbinding = elektronen/lading is geheel verdeeld• Covalente binding: elektronen paar wordt perfect gedeeld

• Polaire Covalent • zit daar precies tussen in, lading is enigzins verdeeld

Overview of Bonding-types (8.2)

• Waarom (verklaring voor dit model)?:

• atomen willen “edelgas” configuratie• die vertegenwoordigen voor elk atoom een bereikbare, lagere energietoestand

• Edelgasconfiguratie:• atoom: buitenste s en p orbitalen zijn gevuld met 8 elektronen (H, He: 2 elektronen)

• Polair Covalent • elektronen zitten wat dichter bij het ene atoom

Achieving Noble Gas Electron Configurations (NGEC)

• General (not always applicable) rule:

• A nonmetal and representative group metal react ionic compound. • The valence electron configuration of the nonmetal are filled

to achieve NGEC.• The valence orbitals of the metal are emptied to achieve

NGEC.

• Two nonmetals react: they share electrons to achieve NGEC (covalent bonding or polair covalent)

NGEC in a Bond: result

• Metal + nonmetal: • Ionic bond

• Two of the same non-metals: • Covalent bond

• Two different non-metals:• polar covalent bond

• Division of charge: 5th fig. 8.11; p. 368; 6th p. 366; 7th Fig. 8.12 p. 346

Bond type

• Kunnen we een eigenschap van elk atoom definiëren en karakteriseren...

• met een voorspellende waarde t.a.v. welke binding zich zal vormen als twee atomen A en B een binding aangaan?

Electronegativity (8.2)Electronegativity: “The ability of an atom in a molecule to attract shared electrons to itself.”

Difference in Electronegativity Bond Type

Zero Covalent

Intermediate Polar Covalent

Large Ionic

Polarity

A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment.

H - F⎯→δ+ δ-

Polar Covalent Bond: dipole moment

• Two different non-metals:• polar covalent bond

• Diatomic molecule: • always

• Polyatomic molecule:• depending on structure • 5th fig. 8.4; p. 355-356; 6th p. 355; 7th p. 336-337

• Examples: H2 O, NH3 , SO3 , CH4 , H2 S

Bond Length

Definition in the context of different models?

“The distance where the system energy is at its minimum”

fig. 8.1: The distance between nuclei where the quantum mechanic probability function is at its maximum

Concepten (vervolg)

TerminologieIonbindingCovalente bindingPolaire covalente binding

Ionic Bonds

- When would these be formed?When a low(er) energy state is achieved

- Whereby determined?

1. Lattice energy (roosterenergie): net energy gain or loss by electrostatic attractions/repulsions of closely packed ions.

see 5th, figure 8.8; 7th figure 8.9 and §8.5

2. Energy involved to lose or gain electron in reaction (electronegativity)

Ionic Bonds

Diatomic molecule:

E = 2.31* 10-19 [J.nm] (Q1. Q2) / r)

Q1 and Q2 = numerical ion charges r = distance between ion centers (in nm)

* with a positive & negative Q, the result is…* negative, thus a lower energy state achieved

Lattice Energy (8.5)

The change in energy when separated gaseous ions are packed together to form an ionic solid

M+ (g) + X-(g) MX(s)

Lattice energy is negative (exothermic)

(energy is released from the system of ions that combine into a lattice)

Formation ionic solid

Total enthalpy change = state property

! define suitable steps (see 5/6th Figure 8.8; 7th Fig. 8.9 )• Sublimation of the solid metal Li F [kJ/mol]

M(s) ⌫ M(g) [endothermic] 1612 Ionization of the metal atoms

M(g) ⌫ M+(g) + e- [endothermic] 5203 Dissociation of the nonmetal

½X2 (g) ⌫ X(g) [endothermic] 774 Formation of X- ions in the gas phase:

X(g) + e- ⌫

X- (g) [exothermic) -3285 Formation of the solid MX

M+(g) + X-(g) MX(s) [quite exothermic] -1047

Total -647

Lattice Energy

E = k [J.nm] (Q1. Q2) / r)

Q1 and Q2 = numerical ion charges r = distance between ion centers (in nm)

Concepten (vervolg)

TerminologieIonbindingCovalente bindingPolaire covalente binding

Bond Energy

- The net energy-input to a molecule required to break a particular bond.

- It gives us information about the strength of a bonding interaction

Hess’s Law (Ch. 6)

The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps.

⇒ Also to be used for calculation of reaction- enthalpies from bond energies!

⇒ Why: Enthalpy = a state property!

Reaction Energies

Bond breaking requires energy (endothermic)

Bond formation releases energy (exothermic)

ΔH = Σ ΔH (bonds broken) – Σ ΔH (bonds formed)

energy required energy released

Born Haber cycle for ammonia synthesis

• Reaction: N2 + 3 H2 ⇔

2 NH3

• Enthalpy change: ΔHo = - 92 kJ/mol N2

• Enthalpy change equals also= ΔHo (break N2 ) = 941

ΔHo (break H2 )*3 = 3*432ΔHo (formation N-H)*6 = - 6*391

= -109 kJ/mol

Any difference between outcome and actual (measured) value is caused by effects not described by this simple model (e.g. polarity)

Ammoniak - produktie

Source: http://www.greener-industry.org/pages/ammonia/6AmmoniaPMHaber.htm

source: http://www.linde-anlagenbau.de/process_plants/hydrogen_syngas_plants/gas_products/ammonia.php

The production of the N2 /H2 mixture: Reforming + CO-shift

Chemische Binding (vervolg)

ModellenConceptenInter-moleculaire bindingIntra-moleculaire binding

Intermoleculaire Binding

Gelokaliseerde electronmodellen- Ionbinding- Lewis structuren•

VSEPR- Valence Bond model•

Hybridisatie (Hfk. 9)Gedelokaliseerde electronmodellen (hfk. 9)

- MO theorie- Metaalbinding

Lewis Structure

- Shows how valence electrons are arranged among atoms in a molecule.

- Reflects central idea that stability of a compound relates to NGEC - noble gas electron configuration.

Valence electrons

- De elektronen in de orbitalen die het laatst gevuld worden, I.e. met het hoogste quantumgetal voor het bereiken van NGEC

- Periodiek systeem:rij 1: H en He: 1s totaal 2rij 2: Li t.m Ne: 2s, 2p totaal 8rij 3: K t.m Ar: 3s, 3p totaal 8etc.

Comments About the Octet Rule

- 2nd row elements C, N, O, F observe the octet rule.- 2nd row elements B and Be often have fewer than 8 electrons around

themselves - they are very reactive- 3rd row and heavier elements CAN exceed the octet rule using empty

valence d orbitals.- When writing Lewis structures, satisfy octets first, then place electrons

around elements having available d orbitals.

Lewis-structuren schrijven

• Standaard procedure (8.10):1. Tel alle valentie electronen op van alle atomen, het gaat om

het totaal!2. Teken een −

tussen elk paar verbonden atomen (dus geen : of .. !!!)

3. Verdeel de overige electronen, zo dat de duet regel voor waterstof, en de octet regel voor overige wordt nageleefd. (dit vergt ‘trial and error’)

• Probeer HF, H2 S, NH3 , CH3 OH, POCl3

Lewis structures /Formal Charge

Bij gebruik van de procedure zijn er soms meerdere mogelijkheden

De beste Lewis-structures zijn die die de laagste energie- toestand weergeven

> Molecules: Formal charge on each atom = 0;

> Ions: Formal charge on least/most electronegative atom (positive/negative ion)

Formal Charge

The difference between the number of valence electrons (VE) on the free atom and the number assigned to the atom in the molecule.

We need:1. # VE on free neutral atom2. # VE “belonging” to the atom in the

molecule

O – C – O O = C = O(-1) (0) (+1) (0) (0) (0)

Not as good Better

Formal Charge

Resonance

Occurs when more than one valid Lewis structure can be written for a particular molecule.

These are resonance structures. The actual structure is an average of the resonance structures.