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	edta^4-, 4O: and 2N:
more than one	Multidentate	many, mostly organic, O: & N:

· The combined molecule made of a central metal ion surrounded by ligands is called a complex. You might have to state the co-ordination number and name one in the exam so here’s how:

1. First, how many ligands are attached? The number attached is called the co-ordination number.

2. Secondly, in alphabetical order, identify the molecules acting as ligands by their special ligand names. Anions (negative ions) swap –ide for –o at the end of their names and neutral ligands keep the same name, except water and ammonia because they’re very common.

How many ligands	Prefix	Ligand	Special name
1	Mono	Cl^-	chloro
2	Bi	CN^-	cyano
3	Tri	F^-	fluoro
4	Tetra	OH^-	hydroxo
5	Penta	H2O	aqua
6	Hexa	NH3	ammine

3. Thirdly, name the central metal ion. Use the English name if the complex has an overall positive or neutral charge and use the Latin name if the complex is overall negatively charged.

4. Show the oxidation number of the central metal ion in brackets in roman numerals.

5. Combine stages 1-4 to make a whole name, such as hexaaquacopper(II) = [Cu(H₂O)₆]²⁺.

Further examples follow:

· [Fe(CN)₆]^4- = hexacyanoferrate(II)

· [CuCl₄]^2- = tetrachlorocopper(II)

· [Ni edta]^2- = mono-edta-nickelate(II)

· [FeCl₃(NH₃)₃] = triamminetrichloroiron(II)

Basic Oxygen Steelmaking process

Industrial production of steel is a batch process. Impurities in the blast furnace are removed with oxygen gas and basic oxides, hence BOS.

· Sulphur is removed first with magnesium

Mg (s) + S (s) → MgS (s)

· Excess carbon, silicon, manganese and phosphorus impurities are removed by reaction with oxygen gas, unavoidably along with some iron

Si (s) + O₂ (g) → SiO₂ (s)

Mn (s) + O₂ (g) → MnO (s)

4P (s) + 5O₂ (g) → P₄O₁₀ (s)

C (s) + O₂ (g) → CO₂ (g)

2Fe (s) + O₂ (g) → 2FeO (s)

· Acidic oxides SiO₂ and P₄O₁₀ are removed by basic oxides CaO and MgO.

SiO₂ + CaO → CaSiO₃ calcium silicate (component of slag)

SiO₂ + MgO → MgSiO₃

P₄O₁₀ + 2MgO → 2Mg₃(PO₄)₂

P₄O₁₀ + 2CaO → 2Ca₃(PO₄)₂