CHEM1602

Question 1

What are the four quantum numbers?

Answer 1

n = the shell (1, 2, 3...)
l = angular momentum, the shape of the orbital (0 = s, 1 = p, 2 = d, ... n-1)
ml = magnetic quantum number, the orientation of the orbital (-l, 0, +l)
ms = spin (-0.5, +0.5)

Question 2

What is the Pauli principle and what are the implications?

Answer 2

No two electrons can share the same four quantum numbers

-> a maximum of two electrons can share each orbital, with the same values of n, l, ml but different values of ms

Question 3

What is Hund’s Rule?

Answer 3

Hund’s rule of maximum multiplicity =
if two or more orbitals have the same energy,
then the electrons will spread out to occupy the maximum possible number of these,
maximising the number of parallel spins

Question 4

How do wave functions explain the relative order of the energies of the orbitals?

Answer 4

Wave functions can be used to determine the radial distribution function of the orbitals (how the probability varied of finding an electron at different distances from he nucleus)

the distributions functions can be calculated for all the orbitals and can explain the relative energies of them

the energy of an electron is related to distance of the electron from the nucleus

Question 5

Why does maximising the number of parallel spins reduce the energy of the electrons?

Answer 5

Hund’s Rule
if two electrons occupy different regions of space there is less electrostatic repulsion, which is a lower energy arrangement

the electrons have lower energy if their spins are parallel as a consequence of SPIN CORRELATION
parallel spins will stay further away from each other and their repulsions are therefore reduced

Question 6

What is a covalent bond according to the Lewis model?

Answer 6

When valence electrons are shared between two atoms

Question 7

What is the octet rule?

Answer 7

Each atoms acquires shares in its electrons until its valence hell achieves eight electrons
and maximum stability is achieved when each atom has a ‘filled valence shell’

Question 8

What advantages does the Lewis Model have?

Answer 8

It explains why double and triple bonds are shorter and strong - they have more shared pairs of electrons

Question 9

What are resonance hybrids?

Answer 9

a resonance hybrid is an average of more than one satisfactory Lewis model structure for a particular molecule (e.g. CO3^2-)

Question 10

What are hypervalent compounds and why do they deviate from the Lewis model ‘octet rule’ give an example and how can they be explained?

Answer 10

Hypervalent compounds (such as this in period 3) require more than 8 electrons in their outer shell in order to draw a Lewis structure of the molecule
they are explained by low-lying energy d-orbitals that accommodate the extra electrons
e.g. PCl5

Question 11

What are the assumptions of the VSEPR theory?

Answer 11

1. electrons in bonds and lone paris are ‘charge clouds’ that repel each other
2. the lowest energy arrangement is when they are arranged as far away from each other as possible - this determined the equilibrium molecular shape
3. lone pairs repel more than bonding pairs
4. lone pairs will occupy the equatorial position over the axial position as the repulsion is less from the axial bonding pairs, then the equatorial bonding paris if it was in the axial position (e.g SF4)

Question 12

Why do we not have to consider resonance structures in the VSEPR theory?

Answer 12

Multiple bonds are treated like single bonds (e.g. PO4 is tetrahedral)

Question 13

What are the disadvantages of the Lewish model?

Answer 13

1. hypervalent compounds

2. does not predict the unpaired electrons of O2, which give oxygen its paramagnetism properties

Question 14

What is a covalent bond according to Valence Bond theory?

Answer 14

A bond is created when two half-filled orbitals on two atoms overlap to create a bonding orbital containing paired electrons

Question 15

What are the advantages and disadvantages of Valence Bond Theory?

Answer 15

Explains the relative bond strength go diatomic molecules by the degrees of overlap of the atomic orbitals

the observed geometry of polyatomic molecules does to match the spatial arrangements of the constituent atoms

Valence bond theory/hybridisation and Lewis model all are localised models and focus only one a particular bond in a molecules - bond length, strength and force constant
but it does not tell us about the electron distribution over the whole molecule

Question 16

What is bonding according to molecular orbital theory?

Answer 16

The wave functions of electrons can combine constructively or destructively

• if they combine constructively the electron density is increased between the two atoms = bonding molecular orbital
• -> a bonding molecular orbital is lower in energy than the original atomic orbitals
• if they combine destructively, the electron density is decreased between the two atoms = anti-bonding molecular orbital (*)
• -> the anti bonding molecular orbitals are higher in energy than the original atomic orbitals

Question 17

What is the first law of thermodynamics?

Answer 17

Energy cannot be created or destroyed, it is transformed into different types of energy

Question 18

What is 1cal?

Answer 18

= 4.2J

it is the amount of energy required to raise the temperature of 1g of water by 1C

Question 19

What is specific heat capacity?

Answer 19

It is the enthalpy change in Joules required to raise the temperature of 1kg of a substance by 1C

Question 20

What is ‘latent heat’/ enthalpy of fusion/metling?

Answer 20

The enthalpy change in Joules required to convert 1kg of a material from solid to liquid at the same temperature