Introduction to spectroscopy, energy and bonding Autumn semester

Question 1

What are the equations relating wave length, frequency and speed of light

Answer 1

c = λv

c = speed of light (m/s)
λ = wavelength (nm)
v = frequency (1/cm)

E = hv

E= energy (joules)
h = plancks constant
v = frequency

E= hc/λ

Question 2

What is rotational spectroscopy?

Answer 2

Gaseous molecules can rotate and it is quantised, the absorbed wavelengths correspond to microwaves and infrared regions of electromagnetic spectrum.

Question 3

What is the gross and specific selection rule for rotational spectroscopy?

Answer 3

gross > must posses a permanent dipole
Specific selection > ΔJ = + or - 1

Question 4

How to calculate allowed rotational energy levels?

Answer 4

Ej = hBJ(J+1)

E = energy of rotational quantum number (joules)
h = planks contant (joules/seconds)
B = rotational consent (1/seconds)
J = rotational quantum number

Question 5

How do you calculate the rotational constant (B)?

Answer 5

B = h/8π ^2I
I = moment of inertia (Kg m^2)

Question 6

How do you calculate moment of inertia (I)?

Answer 6

I = ur(o)^2
u = reduced mass (Kg)
R(o) = bond length (m) need to convert any measurement to m normal need to or form nm to m x10-9

Question 7

How do you calculate reduced mass of atoms?

Answer 7

1. work out the mass of the atoms in Kg by Mr divided by 1000 (converts it to Kg mol-1) / avocadros number
2. then substitute into
   U = m1 x m2 / m1+ m2

Question 8

What is the centrifugal distortion coefficient?

Answer 8

Although in calculations bond length is treated as a constant there slight changes which accounted for by the centrifugal distortion coefficient

Ej = hBJ(J+1) - DJ^2(J+1)^2

D = centrifugal distortion coefficient

Question 9

What is vibrational spectroscopy?

Answer 9

This is the molecular vibrations of the individual chemical bond, it is quantised and correspond to the Ir on the electromagnetic spectrum.

Question 10

What is the gross and specific selection rule for vibrational spectroscopy?

Answer 10

gross > must be a change in the dipole moment, thus must have a permanent dipole
specific selection > ΔV + or - 1

Question 11

How do you calculate vibrational energy levels?

Answer 11

Ev = (V +1/2) hv

V= vibrational energy level
v = frequency

Question 12

How do you calculate the force constant for vibrational energy?

Answer 12

v = 1/2π (k/u)^1/2

v = frequency of vibration
k = force constant (stiffness) (kg/s^2)

Question 13

How do you calculate potential energy in vibration energy?

Answer 13

Epe = 1/2kx^2

k = spring constant
x = amount of extension,displacment

Question 14

How do you calculate anharmonic vibrations?

Answer 14

EV = (V+1/2)hv - Xe (V +1/2)^2hv

Xe = anharmonic constant

anharmonic oscillation means that ΔV + or - 1 is exactly true

Question 15

How do you calculate the vibrational modes of freedom for a linear and non-linear molecule?

Answer 15

Linear = (3N -5) Modes
non linear = (3N-6) Modes

N = number of atoms

Question 16

What is electronic spectroscopy?

Answer 16

This is the process of a beam of light being passed through an atom exciting an electron to a higher energy state resulting in colour. It corresponds to the uv/vis spectrum.

Question 17

What is the beer-lamberts law?

Answer 17

This allows us to calculate the molar absorption coefficient and wavelength for different absorbance points and whether an electronic state is forbidden.

A = εcl

ε = molar absorption coeffeinct (dm^3 / mol cm)
c = concentration (mol dm^-3)
l = path length (cm)

Question 18

how does the molar absorption coefficient say whether a electronic state is forbidden of not?

Answer 18

ε is negative its strongly forbidden
ε is 10-100 weakly allowed
ε is 500+ its strongly allowed

Question 19

What is a chromophore?

Answer 19

The part of the molecule that is responsible for the absorption of light, it is normally a conjugated system. With the longer the conjugated system the longer the wave length and the smaller the energy of absorption.

Question 20

What is NMR spectroscopy?

Answer 20

Nuclear magnetic resonance Spectroscopy uses radio-frequencies to cause transitions between the Zeeman energy levels of nuclei in a magnetic field.m

Question 21

how do you calculate the energy change between the ground state to exited state of an nucleus?

Answer 21

ΔE = hyB(o) / 2π = hv

h = placks
y = magnetic ratio
B(o) = strength of the magnetic field

Question 22

What impacts the the chemical shift?

Answer 22

Electrons is the surrounding bonds shield the nucleus from the full effect of the magnetic field given a range of chemical shifts. The more shielding present the lower the chemical shift.
- electron withdrawing groups remove electron density and increase chemical shifts meaning higher energy of radio waves absorbed
- electron donating groups increase shielding.

Question 23

What is spin-spin coupling?

Answer 23

pin coupling between NMR active nuclei that are not chemically identical. Different spin states interact through chemical bonds in a molecule to give rise to this coupling.
n +1 rules gives us the number of surrindign hydrogens.

Question 24

What impacts the intensity if peaks?

Answer 24

Mainly useful for proton NMR, the size of the peaks in the NMR spectra can give information concerning the number of nuclei that gave rise to that peak. This is done by measuring the peak’s area using integration

Question 25

How do you calculate bond order?

Answer 25

Bond order = 1/2(number of bonding electrons - number of anti-bonding electrons)
if the bond order is 0 the molecule won’t bond/exist.

Question 26

Whats boyles gas law?

Answer 26

At constant temperature and a fixed amount of gas the volume of the gas will reduce proportional to an increase in pressure.

Question 27

What is Charles gas law?

Answer 27

At a constant pressure, the volume of a fixed amount of gas will increase with increasing temperature.

Question 28

What is Avogadro’s gas law?

Answer 28

Equal volumes of gas at a consent temperature and pressure contains equal numbers of molecules.

Question 29

What is the ideal gas equation?

Answer 29

pV = nRT

p = pressure (pa)
V = volume (m^3)
R = ideal gas constant
T = temperature (K)

Question 30

What is Daltons law of partial pressures?

Answer 30

The pressure exerted by a mixture of gases is the sum of the partial pressure of the gases.

Question 31

How is partial pressure calculated?

Answer 31

P(A) = mole fraction of A x total pressure

mole fraction (A) = number of gaseous moles of A/ total number of gaseous moles

Question 32

What is kinetic molecule theory with gases?

Answer 32

This describes the gases behaviour in terms of molecules that are in constant random motion in every direction. Evidence is
- flow (fills a volume)
- thermal expansion
- brownian motion (collision with Microparticles create random motion)
- diffusion

Question 33

How can you calculate the kinetic energy of. a gas molecule?

Answer 33

for a singel molecule
E(kE) = 1/2 ms^2

m = mass (kg)
s = molecular speeds (m/s)

For per molecule
E(kE) = 1/2 mc^2

Question 34

How do you calculate the average molecular speed of a gas?

Answer 34

for mean kinetic energy and pV use molecular speeds
C = (s1^2 +s2^2…/N)^1/2

for effusion, colliosn frequency and mean free path use mean speed
C = s1 + s2…/N

Question 35

How to calculate the average molecular speed?

Answer 35

C = (3RT/M)^1/2

C = root mean square speed of molecules (m/s)
R = gas constant
T = temperature (K)
M = molar mass (kg/mol)

Question 36

How can you plot the difference in modular speeds?

Answer 36

At any instant molecule will move at different speeds it distributed by plotting speed (s) on the x axis with number of molecules on the y axis.

Question 37

What affects molecular speeds?

Answer 37

• temperature (increase temp increase kinetic energy)
• mass (as mass increases molecule moves slower)

Question 38

What is Grahams law of effusion?

Answer 38

At a given temperature and gas pressure the rate of effusion is inversely proportional to the square root of the molar mass.

Rate od effusion = 1/M^1/2

Rate of effusion A/rate of effusion B = (M(B) /M(A))^1/2

Question 39

How does the hard sphere model show bonding?

Answer 39

When two molecules collide they can only get as close as their van Der Waals radii will permit for the orientation of that collision

width of molecule = 2 x van Der Waals radius (2x1.5A)
Length of molecule = 2 x van Der Waals radius + bond length

Question 40

How do you calculate the collision cross-section?

Answer 40

σ = π (2r)^2 = π d^2

Question 41

What is collision frequency?

Answer 41

The mean number of collisions that a molecule experiences per seconds (Z)

Z = Square root of 2 x N (A) x (Cσ)(p/RT)

N = number of molecules

Question 42

What is mean free path?

Answer 42

The average difference a molecule travels between collision at a given temperature and pressure.

λ = average speed (C) / average time between collisions (Z)

Question 43

What is the Van Der Waals equations for gases?

Answer 43

(p + a(n/v)^2)(V-nb0 = nRT