FIC May 23 workshop questions

Question 1

Calculate the wavelength of an electron which has a velocity of 7.34 x 108 cm s-1
.
The mass of an electron is 9.109 x 10-28 g.

Answer 1

Wavelength = h /mv

6.626 X 10-34 / (7.34 x 108) (9.109 x 10*-28) = 9.9103 10 *-16

Question 2

Calculate the energy change in J associated with the line which represents the transition from n1 = 2 to n2 = 4 ( RH = 3.291 X 10*15)

Answer 2

ΔE = RH h c ( 1/n12 - 1/n22)

(3.291x 1015) (6.626 x 10_34) (3 x 108) (1/22 - 1/42) = 1.23 x 10*-10

Question 3

Calculate the uncertainty in momentum of a neutron moving at 6.00x107 m/s. The mass of a neutron is 1.67510 x 1027.

Answer 3

ΔP = m x Δv = (1.67510 x 1027)(6.00x107) = 1.01 x 10*-19

Question 4

A proton with a mass of 1.67 x 10-27 kg traveling at 4.7 x 105 m/s has an uncertainty in its velocity of 1.77 x 105 m/s. Determine its uncertainty in position.

Answer 4

1.8 x 10*-13

Δx x m x Δv = h/4pi

Δx = (h/4pi) / (1.67 x 10-27) x (1.77 x 105)

Question 5

Which of the quantum numbers govern the shape of an orbital and what are its allowed values ?

Answer 5

l

allowed values are 0 => n-1

Question 6

Which of the quantum numbers govern the energy of an orbital and what are its allowed values ?

Answer 6

n

allowed values are any positive integer

Question 7

Which of the quantum numbers govern the spin properties of an electron and what are its allowed values ?

Answer 7

Ms

+1/2 or -1/2

Question 8

Which of the quantum numbers govern the spatial orientation of an orbital and what are its allowed values ?

Answer 8

Ml

allowed values are -l => +l

Question 9

Write the principle and azimuthal quantum numbers for 4p and 4d orbitals.

Answer 9

4p

n = 4
l = 1

4d

n = 4
l = 2

Question 10

Sketch the plot of the radial wave function against distance and the plot of the radial distribution against distance for a 3d orbital

Answer 10

Check sketches

Question 11

State the Aufbau principle

Answer 11

In the ground state of an atom or ion, electrons fill subshells of the lowest available energy, then they fill subshells of higher energy.

Question 12

State Paulis exclusion principle

Answer 12

No two electrons in the same atom can have identical values for all four of their quantum numbers, two electrons in the same orbital must have opposite spins.

Question 13

State Hunds rule

Answer 13

Every orbital in a sublevel is singly occupied before any orbital is doubly occupied.

Question 14

State the general requirements for significant overlap between atomic orbitals and state what happens to energy of bonding MO and antibonding MO as overlap of orbitals increases

Answer 14

Significant overlap only occurs between AOs with:
- similar size
- similar energies
- correct symmetry

As overlap of orbitals increases
- energy of bonding MO is lowered
- energy of anti-bonding MO is raised.

Question 15

What are the features of bonding molecular orbitals ?

Answer 15

• Overlap between orbitals is in phase or constructive.
• Concentrates electron density between nuclei
• Energy of MO is lower than that of AOs from which it is formed.

Question 16

What are the key features of anti-bonding molecular orbitals ?

Answer 16

• Overlap between orbitals is out of phase or destructive.
• Electron density pushed away from favourable internuclear region →
  nodal plane.
• Energy of MO is higher than that of AOs from which it is formed.

Question 17

Sketch a bonding and antibonding MO for H2.

Answer 17

Check sketches

Question 18

Sketch the MO diagrams for O2, O2- and O2+

Label the HOMO AND LUMO
Identify the bond order of O2+ and O2-

Answer 18

Check diagram

Question 19

Define the ionisation energy for an element. Explain why the ionisation energy of sodium is smaller than that of magnesium.

Answer 19

Energy change when an electron is removed from an atom of the element in the gas phase.Sodium has a smaller effective nuclear charge than magnesium. Valence electrons sit further away from nucleus. Easier to remove.

Question 20

State the general trend for electron gain energy across period 2 and discuss any anomalies that occur.

Answer 20

Electron gain energy generally decreases from Li to Ne

Be, N and Ne have higher than usual electron gain energies. Be has a higher electron gain energy because the electron gained must occupy a 2p orbital, which is higher in energy than the 2s orbital. N has a higher electron gain energy because the electron must pair with a second. Ne is a full octet and does not accept electrons readily.

Question 21

Which of the following pairs of elements has the higher first ionisation energy?

Be or B

Answer 21

Be has a higher first ionisation energy than B. General trend of higher effective nuclear charge normally means electrons are closer and have a stronger interaction with the nucleus making them more difficult to remove. However, since the extra electron in B is in a higher energy orbital (2p) that is further away from the nucleus compared to the 2s electrons, this one becomes easier to remove

Question 22

Electrolysis is the process of using an electric field to split a chemical compound in water, and is used to extract sodium from rocksalt, but not potassium from potash (KOH). Discuss why electrolysis is unsuitable for potassium extraction.

Answer 22

Potassium is more reactive than sodium. Electrolysis would allow potassium ions to migrate to the cathode but formation of potassium metal on the surface would cause an exothermic reaction with water.

Question 23

Write a balanced chemical equation for the reaction of a group 2 metal with water.

Answer 23

M + 2H2O → M(OH)2 + H2

Question 24

Account for the fact that group 2 peroxides are generally more stable down the group

Answer 24

Like sized ions stabilise one another, generally speaking. Peroxide is a large anion that is stabilised more easily by larger cations.

Question 25

Draw a Lewis symbol for H , Ne , F and Pb

Answer 25

Check Lewis structures

Question 26

Sketch the Lewis structure of chlorine

Answer 26

Check Lewis structures

Question 27

What are the general trends when assigning oxidation states ?

Answer 27

Oxidation is usually -2 except in peroxides and F2O

Hydrogen is usually +1 except in metal hydrides where it is -1 (eg NaH)

Fluorine is always -1

Chlorine is usually -1 except in compounds with O or F

Group 1 and 2 metals are respective.

Question 28

Explain why boron halides are excellent Lewis acids and rationalise why BI3 is a better Lewis acid than BF3.

Answer 28

Boron Halides are good Lewis acids because the Boron atom does not have a full octet, because boron halides are more electron deficient compounds. Meaning they are excellent Lewis acids because they accept electron pairs.