Chemcial Analysis

Question 1

How does infrared sprectroscopy work?

Answer 1

• Shines a range of IR frequencies, one at a time, through a sample of organic compound
• At some frequencies, the energy will be absorbed
• A detector on the other side of the sample will show that some are being absorbed, whilst others aren’t
• Different energies correspond to different functional groups

Question 2

What happens when the energy of IR is absorbed by a bond?

Answer 2

The bond vibrates more

Question 3

What IR radiation can molecules absorb if they have covalent bonding?

Answer 3

It can absorb IR radiation at the same frequency as the natural frequency of the bond

Question 4

What are the axis on an absorption spectrum?

Answer 4

• Horizontal axis shows the wavenumber of the radiation absorbed in cm^-1
• Vertical axis shows transmittance, which is the percentage of radiation that passes through the sample

Question 5

What is the shape of the peak on an IR spectrum of an alcohol?

Answer 5

• Absorbance peak is curved and broad
• Within range of 3200-3600com-1 (data sheet)
• Peak represents O-H group!

Question 6

What is the shape of the peak of an IR spectrum of an aldehyde or ketone?

Answer 6

• Absorbance peak is sharp and thin
• Close to 1700cm-1 (data sheet)
• Peak represents C=C!

Question 7

What is the shape of the peak of an IR spectrum of a carboxylic acid?

Answer 7

• One absorbance peak broad and pointy between 1630-1820cm-1
• Represents O-H group!
• Second absorbance peak sharp and thin near 1700cm-1
• Represents C=O!

Question 8

What are the advantages of IR spectroscopy?

Answer 8

• Small sample size

- Sample is easy to prepare

Question 9

How to answer an exam Q based on IR spectroscopy?

Answer 9

• Mention whether peak is sharp or broad
• Mention range of peak
• Refer to functional group represented
• Identify organic compound

Question 10

What are the real life applications of IR spectroscopy?

Answer 10

• Breathalysers

- Monitor gases and air pollution

Question 11

What are the main three greenhouse gases?

Answer 11

• CO2
• CH4
• H20 (water vapour)

Question 12

What is the normal function of greenhouse gases in the greenhouse effect?

Answer 12

• Absorb IR radiations emmited from Earth’s surface at the same frequency as the natural frequency of their bonds
• Then re-emit IR

Question 13

What are the bonds responsible for absorbing IR in greenhouse gases?

Answer 13

CO2-> C=O
CH4 -> C-H
H20 -> O-H

Question 14

How does mass spectrometry work?

Answer 14

• Separates atoms according to their mass
• Shows relative numbers of different atoms/fragments present
• Before atoms can be detected, they are converted into positively charged ions then separated by deflection

Question 15

What is the m/z value?

Answer 15

• Mass to charge ratio

- It is numerically equal to the mass of the ion

Question 16

What does the height of a peak on a mass spectrum show?

Answer 16

-Height of each peak measures the relative abundance of the ions that gives rise to that peak

Question 17

Which peak represents the molecular mass for the whole molecule?

Answer 17

Clearest peak FURTHEST TO THE RIGHT

Question 18

Which species aren’t well detected by the mass spectrometer and why?

Answer 18

Radicals because they re not accelerated or deflected by the mass spectrometer

Question 19

How does the analysis of fragments work in mass spectrometry?

Answer 19

• When an organic compound passes through the spectrometer its molecules are broken into positively charged fragments
• Each +ve fragment gives a corresponding line in the mass spectrum
• So, we can work out its mass from its position
• By piecing together fragments, we can deduce the structure of the parent molecule

Question 20

What happens when a molecular ion fragments?

Answer 20

A positive ion and a radical form

Question 21

What are the real-life uses of mass spectrometry?

Answer 21

• Monitoring air pollution

- MOT emission testing

Question 22

How to write an equation to show the formation of a molecular ion

Answer 22

Compound —–> Molecular ion + Electron (e^-)