Chemical Bonds

Question 1

Quantum Mechanics

Answer 1

• particles = waves, ie. they can be described as a wave function
• wave functions of electrons in atoms can be described by PSI x,y,z

Question 2

Electron Density

Answer 2

electron distribution in space for an orbital

Question 3

Coulomb’s Law

Answer 3

• electrostatic potential energy

- closer an electron is to the nuclear the greater energy it has (greater attraction)

Question 4

Aufbau Principle

Answer 4

electrons placed in orbitals starting with lowest energy and working up

Question 5

Pauli Exclusion Principle

Answer 5

2 electrons per orbital with spins paired

Question 6

Hunds Rule

Answer 6

when multiple orbitals of the same energy are available electrons are distributed amongst them

Question 7

Molecular Orbitals

Answer 7

combining atomic orbitals to form molecular orbitals (sigma and pi)
bonding orbitals: wave functions together. high probability electron is between nucleus to give a favorable electrostatic attraction
- favorable coloumb interaction with both nuclei
anti bonding orbitals: wave functions opposite no electron density between nuclei, ie. energetically unfavorable

Question 8

Bonding and Antibonding Orbitals

Answer 8

• bonding orbitals filled first
• when anti bonding orbitals are filled, the energy comes from putting electron in bonding orbital
• this cancels out gain in energy and determines whether two atoms will bond spontaneously

Question 9

Hybridization

Answer 9

the idea that atomic orbitals fuse to form newly hybridised orbitals, which in turn, influences molecular geometry and bonding properties
- sigma: hybridized
- pi: unhybridized
repositioning of orbitals so the electrons are in optimal locations for being shared in covalent bond formation

Question 10

Configuration

Answer 10

conversion between isomers needs a single bond to break

Question 11

Enantiomers

Answer 11

Non superimposable mirror images of each other

Question 12

Diastereoisomers

Answer 12

Not mirror images but are non superimposable

Question 13

Glucose Polymers

Answer 13

Amylose vs Cellulose

• amylose has alpha linkages with bends and twists
• celulose has beta linkages and is flat with straight chains

Question 14

Features Defining Molecular Shape and Structure

Answer 14

1. bond length
2. bond angles (VSEPR theory)
3. bond rotation

Question 15

Conformation

Answer 15

how atoms are joined, is fixed
molecules will adopt a configuration minimizing repulsion forces
- Eclipsed: same orientation of groups
- Staggered: opposite orientations of groups (preferred because of steric hindrance)

Question 16

Isomers

Answer 16

Same molecular formula but different structural formulas

• structural: different bonding
• constitutional: geometrical isomerism

Question 17

Constitutional isomers

Answer 17

You can’t interconvert them without breaking a covalent bond. Configurational change needed. Cis Trans nomenclature needed

Question 18

Types of staggered conformation

Answer 18

Gauche: R groups next to each other

Anti (favored): R groups opposite

Question 19

Conjugated Double bonds

Answer 19

Different types of double bonds
The single and double bonds alternate. These enables the electrons to be delocalised over the whole system and so be shared by many atoms. This means that the delocalised electrons may move around the whole system.

Question 20

Aromatic Compounds

Answer 20

Hydrocarbons which contain benzene, or some other related ring structure. These pi-bonds are delocalized around the ring, leading to an unusual stability for the benzene ring compared to other alkenes.

Question 21

Heterocyclic Aromatic Systems

Answer 21

Pyridine
Pyrimidine
Pyrrole
- carbon in ring replaced by one or more nitrogens

Question 22

Wavelength and Frequency

Answer 22

increase in frequency = energy increase

smaller wavelength = energy increase

Question 23

Electron Orbital Movement

Answer 23

Energy goes into moving electrons between orbitals
Electrons use energy to move to antibonding orbitals
sigma to antisigma = highest energy
pi to anti pi = lowest energy

Question 24

Molecular Vibrations

Answer 24

Electrons vibrate within an orbital/energy level
Stretching, rocking, scissoring, wobbling, etc
Absorbance vs wavelength graph gives a broad peak causes by this vibration

Question 25

Beer Lambert Law

Answer 25

A = E x c x l | Absorbance proportional to concentration

Question 26

Fluoresence

Answer 26

When going back to the ground state, electrons move down to lower energy molecular vibrator levels releasing heat When going back to bonding orbitals they drop from lowest level of anti sigma to highest sigma level We see this lowest energy difference

Question 27

Electronic Properties of Phosphate

Answer 27

- hydridize orbitals to make 4 sp3 orbitals and one 3d electron - tetrahedral geometry with all oxygens being equivalent and having partial negative charge - phosphate ion is very stable and exists in different resonance forms - this stability means it can pull reaction forwards of ATP hydrolysis

Question 28

Delocalisation

Answer 28

- resonance is a type of delocalisation where electrons move around - conjugation is another type of delocalisation where multiple double bonds are allowing electrons to move around in molecular orbitals beyond the 2 atoms

Question 29

Electronic Properties of Oxygen

Answer 29

- 2p orbitals form bond - 1s/2s orbitals non bonding - filled sigma, pi, and anti pi orbitals - anti pi bond cancels out one of the bonding pairs

Question 30

Bond Order

Answer 30

electrons in bonding orbitals - electrons in anti bonding orbitals / 2 Gives net number of shared electron pairs

Question 31

Oxygen Paramegnetism

Answer 31

- electrons in antibonding orbitals line up their spins with each other so have paramagnetism - explains oxygen's reactivity

Question 32

Electronegativity

Answer 32

- attraction between electron and nucleus and the ability of the nucleus to pull electrons - increases across periods (increased nuclear charge) - decreases down groups (electron shielding) - leads to partial charges on atoms

Question 33

Steric Clash

Answer 33

Two atoms closer than the sum of their VDW radii

Question 34

VDW Attraction

Answer 34

Little more than sum of VDW radii - transient dipoles created by random change - weak interactions

Question 35

Hydrogen Bonds

Answer 35

- linear bonds - dipoles in H bonds align (directionality) - essentially a form of dipole dipole interaction only between H and C,N,O,F

Question 36

Hydrophobic Effect

Answer 36

- water molecules form organised clathrates when in contact with non polar structures - this organised structure is energetically unfavorable - bringing non polar faces together relaxes the organised water

Question 37

Bond Energies

Answer 37

``` Covalent bonds: 350 kj H bond: 5-20 kj VDW: 0.2-2 kj hydrophobic: entropy rotational conformations: 10 kj ```