Atoomstructuur en het Periodiek systeem

Chemische Binding (Bonding)
hfst. 8 Zumdahl
Modellen
Concepten
Inter-moleculaire binding
Intra-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)
Modellen
Algemene Concepten (Ch. 8)
Inter-moleculaire binding
Intra-moleculaire binding
Overzicht:
Chemische Binding (‘Bonding’)
Drie modellen voor binding:
Ionbinding
Covalente binding
Polaire 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:
•
ionic
A nonmetal and representative group metal react
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: H2O, NH3, SO3, CH4, H2S
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)
Terminologie
Ionbinding
Covalente binding
Polaire 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]
161
2 Ionization of the metal atoms
M(g) ⌫ M+(g) + e- [endothermic]
520
3 Dissociation of the nonmetal
½X2(g) ⌫ X(g)
[endothermic]
77
4 Formation of X- ions in the gas phase:
X(g) + e- ⌫ X- (g) [exothermic)
-328
5 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)
Terminologie
Ionbinding
Covalente binding
Polaire 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 reactionenthalpies 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*432
ΔHo (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)
Modellen
Concepten
Inter-moleculaire binding
Intra-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 2
rij 2: Li t.m Ne: 2s, 2p
totaal 8
rij 3: K t.m Ar: 3s, 3p
totaal 8
etc.
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, H2S, NH3, CH3OH, POCl3
Lewis structures /Formal Charge
Bij gebruik van de procedure zijn er soms meerdere
mogelijkheden
De beste Lewis-structures zijn die die de laagste energietoestand 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 atom
2. # VE “belonging” to the atom in the
molecule
Formal Charge
O–C–O
(-1)
(0)
Not as good
(+1)
O=C=O
(0)
Better
(0)
(0)
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.