chemische binding (bonding) hfst. 8 zumdahl binding (bonding) hfst. 8 zumdahl modellen concepten...
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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.