A2 Chemistry Term 3

Question 1

Describe the different types of chromatography

Answer 1

Thin Layer chromatography:
- Solid stationery phase usually alumina or silica that is dried to make a coating on plastic or glass
- mobile phase is a solvent which moves up the surface
- quicker than paper chromatography and can be used on smaller samples.
- Rf value can be measured
Gas-Liquid chromatography:
- Stationery phase is non-volatile liquid
- Sample is moved through by an inert carrier gas
- Retention time is measured and is the time taken for the substance to move through the coloumn.

Question 2

State the formula for finding the number of carbons using the M+1 peak

Answer 2

n = 100/1.1 x abundance of M+1/abundance of M+

Question 3

Describe the peaks formed due to Cl and Br atoms

Answer 3

• if M+2 peak is about 1/3 of M peak, suggests presence of one chlorine atom in the molecule
• if M+2 peak is about the same as M peak, suggests presence of one bromine atom in the molecule.
• if M: M+2 : M+4 is about 9:6:1 suggests 2 chlorine atoms.
• if ratio is about 1:2:1 then there are 2 bromine atoms.

Question 4

Describe the use of TMS

Answer 4

It is an inert, volatile liquid that is used as a reference compound and so has a chemical shift value of 0.

Question 5

Describe how deuterium is used in proton NMR

Answer 5

• due to rapid reversible proton exchange in OH and NH groups they only show up as single peaks.
• their ranges of chemical shift also overlap with chemical shifts of other types of proton.
• By adding D2O the OH and NH groups exchange with the deuterium.
• The deuterium does not absorb in the same region of the spectrum as protons.
• The new spectrum can be compared to the original to see if OH or NH groups were present.

Question 6

Define a transition element

Answer 6

A d-block element that forms one or more stable ions with an incomplete d subshell.

Question 7

Describe and explain the properties of the transition elements

Answer 7

• melting point is high
• density is high
• atomic and ionic radius is small
• first ionisation energy is high
• harder and rigid
• good conductors of electricity and heat

Question 8

Define a ligand

Answer 8

A species that contains a lone pair of electrons that forms a dative covalent bond to a central metal ion. Species that form one co-ordinate bond are called monodentate ligand. Two bonds are called bidentate.

Question 9

Define a complex

Answer 9

a molecule or ion formed by a central metal atom/ion surrounded by one or more ligands.

Question 10

Define the co-ordination number

Answer 10

The number of co-ordinate bonds that are formed around the central metal ion.

Question 11

Describe the substitution of ligands and the colour changes that occur

Answer 11

• Blue solution to pale blue ppt when hydroxide is added to [Cu(H2O)6]2+
• Adding ammonia to this pale blue ppt forms a deep blue solution
• Yellow solution forms when adding Cl- to [Cu(H2O)6]2+
• Blue cobalt chloride becomes pink when water is added to it.

Question 12

Describe why transition element complexes are coloured

Answer 12

• In an isolated transition metal ion the d orbitals are degenerate meaning they are at the same energy level.
• The coordinate bonding from the ligands causes the d orbitals to split into two sets of non-degenerate orbitals.
• the difference in energy between the non-degenerate orbitals is a part of the visible spectrum of light.
• an electron absorbs this energy to jump to the higher energy level meaning light of a specific wavelength is absorbed/.
• The rest of the visible spectrum causes a colour

Question 13

describe the medical significance of cis-platin

Answer 13

anti-cancer drug that binds to sections of DNA in cancer cells, preventing cell division