Unit 11- testing and analysis

Question 1

Aldehydes

Answer 1

• can be found using Fehling’s or Benedict’s solutions, which both reduce from blue to brick-red. Adding 5 drops of it into a warm test tube containing substance
• Tollens reagent also works, this time it costs the tube in a silver mirrored precipitate

Question 2

Ketones

Answer 2

-can be found using the same test as they do exactly the opposite of an aldehyde

Question 3

Carboxylic acids

Answer 3

• add CaCO3 to them and run the product through the limewater test for Co2
• if positive it’s an acid

Question 4

Alkenes

Answer 4

-can be distinguished by the bromine test as Br2 undergoes electrophilic addition and changes from orange to colourless

Question 5

Mass spectrometry

Answer 5

• electrospray ionisation/electron guns can ionise a species to give them all a +1 charge
• Ions accelerated by an electric field as detector plate has negative charge
• all ions given same energy, so heavier ions accelerate slower
• ions take one electron from the detector and produce a current equivalent to their mass

Question 6

Mass spectrometry graphs

Answer 6

• taller peaks = more abundant, Mr given by M/z on axis
• often M+1 peaks due to isotopes
• bonds can be fragmented in spectrometry forming other ions
• almost every possible fragmented ions will be produced and detected, as there is a roughly equal chance of each one bring produced
• Peaks must then be analysed to find the right peak

Question 7

IR spectroscopy

Answer 7

• covalent bonds absorb IR energy and stretch or bend, causing a detectable wave
• every bond has a different wavenumber (1/wavelength)
• sample is pressed between salt plates and has IR fired at it like an X-ray scanner
• what isn’t absorbed is detected, graph shows upside down peaks where more energy is absorbed
• this detects functional groups using known wavelengths
• 1000-1500 cm^-1 is fingerprint reigin where every molecule is unique