Organic Analysis

Question 1

how can Tollen’s reagent be made?

Answer 1

1. get some silver nitrate solution (colourless)
2. add a few drops of NaOH - pale brown precipitate forms
3. add a few drops of dilute ammonia until the precipate dissolves

Question 2

outline the process of testing for alkenes. how does this work?

Answer 2

1. add bromine water to an alkene
2. shake
3. if an alkene is present, colourless solution is formed

• bromine water reacts with the double bond and a dibromoalkane (colourless) is formed

Question 3

describe how to test for carboxylic acids. why may further tests be needed?

Answer 3

1. add carbonate (e.g. solid sodium carbonate) to the carboxylic acid
2. carbonate should fizz - CO₂ is produced
3. bubble gas produced through limewater - if cloudy, CO₂ preseny

• other acids also react in the same way, so it does not prove that carboxylic acid is present

Question 4

when is high resolution mass spectrometry useful?

Answer 4

when identifying different molecules with the same molecular mass rounded to the nearest whole number

Question 5

what is the purpose of high resolution mass spectrometry?

Answer 5

used to measure the relative mass to several decimal places

Question 6

how does infra-red spectroscopy work?

Answer 6

uses infra-red radiation to increase the vibrational energy of a covalent bonds in a sample

Question 7

what does the frequency of infra-red radiation absorbed by a covalent bond depend on?

Answer 7

• atoms that are either side of the bond
• the position of the bond in the molecule

Question 8

describe how to use an IR spectrum to identify a molecule

Answer 8

• identifty peaks (troughs) and use data sheet to find corresponding bonds
• use the bonds present to identify the molecule present

Question 9

what is the “fingerprint region” on an IR spectrum? why is it useful?

Answer 9

• lies between 500cm^-1 and 1500cm^-1
• size and position of the peaks are unique to a particular molecule
• can compare the fingerprint region generated against a known library of spectra to identify the molecule

Question 10

what are extra peaks in the fingerprint region of a sample indicative of?

Answer 10

impurities

Question 11

what effect does a stronger bond have on the frequency of vibration?

Answer 11

vibrate with a higher frequency (faster)

Question 12

what effect do heavier atoms have on the frequency of vibration?

Answer 12

vibrate with a lower frequency (slower)

Question 13

how is infrared radiation re-emitted?

Answer 13

• electromagnetic radiation from the sun reaches the Earth and is absorbed by the land and sea
• some of this radiation is re-emitted as infrared

Question 14

list 3 examples of greenhouse gases

Answer 14

• carbon dioxide (CO₂)
• water vapour (H₂O)
• methane (CH₄)

Question 15

what causes the greenhouse effect?

Answer 15

the covalent bonds of greenhouse gases absorb the infrared radiation and re-emit this back towards the Earth