Polymers, Proteins and Steel

sketchy Exam Revision

 

Recrystallisation

Questions about recrystallisation are worth about 3-4 marks each, but often have an extra mark for the quality of your written communication, so they’re well worth getting right.

 

1. Dissolve your impure crystals in the minimum possible amount of solvent.  The solvent is usually water.  Gently heat the solution until all the crystals have dissolved.

 

1. Leave the solution to cool very slowly, perhaps overnight.

 

2. If any debris remains, decant most of the clear solution from the debris into a second flask.  Then heat the solution again to redissolve the crystals.

 

3. Cover and leave to recrystallise.

 

4. Collect the crystals by vacuum filtration and leave them to dry on a watch-glass.  You can speed the process up by placing it in an oven or on a hotplate.

 

The gist of it is…

v      dissolve impure solid in minimum of solvent

v      cover and leave to cool slowly

v      vacuum filter the crystals and leave to dry

 

Hydrolysis

Hydrolysis is the splitting of amide and peptide linkages into their original constituents.  It’s the reverse process of condensation reactions.

 

·        Dipeptides can be hydrolysed into their two constituent amino acids

·        Condensation polymers can be hydrolysed into the constituents monomers,  especially nylon(6,6) → 1,6-diaminohexane + hexanedioic acid (DP2.1)

·        Hydrolysis takes place best under reflux, warmed, for several hours, with moderately concentrated (4M-6M) hydrochloric or sulphuric acid or sodium hydroxide pretty much like in your stomach

 

 

Esterification

Esters (RCOOR’) are formed when alcohols react with carboxylic acids.

 

 

Rusting

 

 


·        Iron(0) is oxidised to iron(II) ion

 

·        Oxygen and water are reduced to hydroxide

 

·        Iron(II) hydroxide forms as a precipitate

 

·        Iron(II) hydroxide is oxidised to iron(III) oxide

 

 

 

 

Overall iron undergoes two-stage oxidation from Fe to Fe2+ to Fe3+.  Both water and dissolved oxygen are required.  The dissolved oxygen concentration is greatest near the outside of the droplet, so electrons flow here from the iron being oxidised to iron(II) ions in the centre of the droplet where the [O2] is lowest.  At the outside, OH- is formed.  Rust forms from Fe(OH)2 by oxygen-catalysed processes leading to Fe2O3 crystals containing any number of water molecules of hydration (xH2O).

 

 

Colorimetry

 

There is often a question about colorimetry in the PP&S exam, and it usually carries 4 or 5 marks, around 5% of the total.  You are given a sample solution of [some chemical] of unknown concentration, and some pure, solid [chemical]. You have to find the concentration of the unknown sample by colorimetry.  The more intensely the unknown sample absorbs a colour of light, the more concentrated it is.

 

·        Weigh the solid to find its mass

·        Dissolve the solid in distilled water up to a precise volume

·        Make up a range of solutions of different known concentrations by serial dilution

·        Choose an appropriate filter of a complementary colour to the solution

·        Use a blank reference cell filled with water to zero the colorimeter

·        Measure and record the absorption of the solution at your range of concentrations

·        Plot a calibration curve of concentration vs. absorption

·        Measure and record the absorption of the solution of unknown concentration

·        Read off the sample’s concentration, from its absorption, on the calibration curve

 

 

 

 

Complexes

 

·        Ligands are molecules or ions which surround a positively-charged central metal ion and form (dative/coordinate) covalent bonds with it. 

 

·        A ligand, by definition, has at least one lone pair of electrons, which it needs to donate to the metal ion form the dative covalent bond.

 

·        Ligands are given generic names based on how many points of attachment they contain.  A general rule of thumb is that any atom in the ligand molecule with a lone pair of electrons can be a point of attachment, although this is not true every time.  In the following table, a colon ( : ) means a lone pair of electrons.

 

Points of attachment

Name of class of ligand

Example

1

Monodentate

water, H2O:, ammonia :NH3

2

Bidentate

ethanedioate, 1,2-diaminoethane

6

Hexadentate

edta4-, 4O: and 2N:

more than one

Multidentate

many, mostly organic, O: & N: