Electronic Effects

  • Created by: LBCW0502
  • Created on: 15-10-17 09:56
What is the linear combination of atomic orbitals?
Method to express wave functions of MOs as sums and differences of wave functions of AOs
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How can AOs as wave functions be combined constructively?
In phase/added together, results in orbital spread over both atoms
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How can AOs as wave functions be combined destructively?
Out of phase/subtract one from another, results in a nodal plane down the centre between two nuclei, wave functions of two AOs cancel out each other, leading to two regions of opposite plane
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What are the key concepts of MOs? (5)
Electrons in a molecule occupy MOs, two electrons per MO, express MOs as combinations of AOs, electrons in atom occupy AOs, two electrons per AO
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What is the molecular orbital theory?
Used to describe covalent bonds, called MOs as they belong to the whole molecule, MO describes region of space where electrons are located, specific size/shape/energy
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In a hydrogen molecule, two single occupied 1s AOs combine to form how many MOs?
2
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Describe the additive way of combining AOs to form MOs
Formation of MO with lower energy (egg-shaped) - called bonding MO, electrons in region between two nuclei (bonding atoms together)
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Describe the subtractive way of combining AOs to form MOs
Formation of MO with higher energy and a node between nuclei, shape of elongated dumb-bell - called antibonding MO, electrons can't occupy central region between nuclei/node (can't contribute to bonding 2 nuclei repel each other)
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Combination of two s AOs leads to what?
Bonding and antibonding sigma MOs
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Combination of two p AOs leads to what?
Bonding and antibonding pi MOs
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Lower energy pi bond MO has no node between nuclei results from what?
Combination of p orbital lobes with the same algebraic sign
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Higher energy pi bond MO has no node between nuclei results from what?
Combination of p orbital lobes with opposite algebraic signs
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What happens if the antibonding contains electrons?
The molecule will break
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What is sp3 hybridisation?
Combining 1 s orbital and 3 p orbitals to form 4 sp3 orbitals (hybrid orbitals are stronger than unhybridised orbitals)
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Why is the orientation of sp3 orbitals in one direction with an unsymmetrical shape?
Positive p lobe adds to s orbital but negative p lobe subtracts from s orbital. One of the two lobes is larger and overlaps more effectively with an orbital from another atom to form a bond
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What is the shape of methane (with sp3 hybridisation)?
Tetrahedral (bond angle of 109.5)
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What is the bond strength of C-H in methane?
439 kJ/mol (105k cal/mol)
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What is the bond length of C-H in methane?
109 pm (1 pm = 0.001 nm)
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What is the shape of ethane (with sp3 hybridisation)?
Tetrahedral (bond angle of 109.5) - 111.2 degrees
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What is the bond strength of C-C in ethane?
377 kJ/mol (90 kcal/mol)
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What is the bond strength of C-H in ethane?
421 kJ/mol (101 kcal/mol)
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What is the bond length of C-C in ethane?
154 pm
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What is the shape of ethylene (with sp2 hybridisation)?
Planar (3 sp2 orbitals at 120 degrees but remaining p orbital perpendicular to sp2 plane/90 degrees)
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What is sp2 hybridisation?
2s orbital combines with 2 of the 3 p orbitals. Results in 3 sp2 orbitals and 1 p orbital. Unsymmetrical about nucleus
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Two Cs with sp2 hybridisation head on overlap to form which bond?
Sigma bond
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Unhybridised 2p orbitals interact sideways to form which bond?
Pi bond
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Combination of sp2-sp2 sigma bond and 2p-2p pi bond leads to which type of bond?
C=C bond
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Where are electrons located in a sigma bond?
Centred between nuclei
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Where are electrons located in a pi bond?
Electrons in regions above and below a line drawn between nuclei
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What is the bond angle for H-C-H and H-C-C in ethylene?
120 degrees (117.4 and 121.3 degrees respectively)
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What is the bond strength of C-H in ethylene?
464 kJ/mol (111 kcal/mol)
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What is the bond length of C-H in ethylene?
108.7 pm
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What is the bond length of C=C in ethylene?
134 pm
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What is the bond strength of C=C in ethylene?
728 kJ/mol (174 kcal/mol)
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Why is C=C less than twice as strong as a single C-C bond?
Due to sideways overlap in the pi part of the double bond - not as great as the head-on overlap in the sigma part
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What is the shape of acetylene (with sp hybridisation)?
Linear shape with 180 degrees for 2 sp orbitals but 90 degrees for the remaining 2 p orbitals
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What is the bond angle for H-C-C in acetylene?
190 degrees
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What is the bond length of C-H in acetylene?
106 pm
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What is the bond strength of C-H in acetylene?
558 kJ/mol (133 kcal/mol)
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What is the bond length of C-C in acetylene?
120 pm (shortest/strongest carbon-carbon bond)
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What is the bond strength of C-C in acetylene?
965 kJ/mol (231 kcal/mol)
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What is the H-N-H bond angle in methylamine?
107.1 degrees (close to tetrahedral)
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What is the C-N-H bond angle in methylamine?
110.3 degrees (close to tetrahedral)
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N forms how many sp3 hybridised orbitals?
4 (one contains 2 non-bonding electrons and the other three have one electron each). Unshared lone pair in 4th sp3 orbital of N occupies as much space as N-H does
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What is the hybridisation of methanol?
sp3 hybridisation
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What is the C-O-H bond angle in methanol?
108.5 degrees (close to 109.5 degrees) - two of the four sp3 hybrid orbitals on O are occupied by non-bonding electron lone pairs and two are used to form bonds
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What is the O-P-P bond angle in methyl phosphate?
110-112 degrees (sp3 hybridisation of phosphorous orbitals)
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What is the hybridisation of dimethyl sulfide?
sp3 hybridisation (significant deviation from 109.5 degrees - tetrahedral)
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In thiols, what is the S attached to?
CH
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In sulfides, what is S attached to?
2 Cs
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Other examples for hybridisation
BH3, CH3+, NH3, H2O, carbonyl groups
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What is the resonance/mesomeric effect?
More than one Lewis structure can be drawn to show delocalisation of electrons (cannot be expressed as a single structure)
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What is the bond length of C-O in acetate?
127 pm (midway between length of typical C-O single bond of 135 pm)
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What is the bond length of C=O in acetate?
120 pm
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Which type of arrow is used between resonance forms?
Double headed arrow
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What is the difference between resonance forms?
Placement of pi and nonbonding valence electrons (3D shapes, connections and position of atoms are the same)
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Mesomerism for benzene (5)
2 resonance forms, alternating single and double bonds, 6 C-C equivalent bonds, symmetrical distribution
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What are the rules for resonance forms? (5)
Forms are imaginary/not real, forms differ in placement of pi or nonbonding electrons, different forms don't have to be equivalent (e.g. charges), forms obey normal rules of valency, resonance hybrid is more stable than any individual resonance form
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What is conjugation?
Series of overlapping p orbitals, allows delocalisation of pi electron density over whole system, sum of contributing resonance hybrids
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What do curly arrows represent?
The movement of electrons
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How are curly arrows used?
Begin at electron rich source and point towards electron-deficient site
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In resonance, how is a double bond formed? (negative)
Electrons move away from negative region to create new double bond (two arrows used)
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In resonance, how is a double bond formed? (positive)
Electron pair moves towards this charge to form a new double bond (one arrow used)
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Other cards in this set

Card 2

Front

How can AOs as wave functions be combined constructively?

Back

In phase/added together, results in orbital spread over both atoms

Card 3

Front

How can AOs as wave functions be combined destructively?

Back

Preview of the front of card 3

Card 4

Front

What are the key concepts of MOs? (5)

Back

Preview of the front of card 4

Card 5

Front

What is the molecular orbital theory?

Back

Preview of the front of card 5
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