A-level Chemistry/Printable version

The Pattern of Periodic Table - Periodicity

In the periodic table, all elements are arrived in order of increasing relative atomic mass and elements with similar properties recurred at regular intervals. The vertical column of similar elements are called groups and the horizontal row is called period. If there is one outermost electron, that element belongs to Group I. If there are 7, it belongs to Group VII.

In Group 0, the electron structure is -s(square) or -s(square)p(to the power of sixth). Because of their stable electron arrangement, noble gases have large first ionisation energies and exist as monatomic molecules. They have very low melting points and very low boiling points. The melting point increases down the column. The melting point of Helium is the lowest (3K), the 'highest' melting point of noble gases is (211K).

Group I and Group II - Alkali and Alkaline-Earth Metals The chemical properties in Group I and II metals are very similar. There are one / two s-electrons, which are held weakly by the positive nucleus. The atoms readily lose the outermost electrons and form positively charged ions.

e.g. Na = Na+ + e-

e.g. Mg = Mg2+ + 2e-

The first ionisation energy is lower than the second, as the number of shells increases, the distance between the nucleus and the electrons increases, thus the force that held the atoms and nucleus together decreases. They can form ionic bond with Group VI / VII elements and metallic structure. In metallic structure, a 'sea' of electrons between each atom held them together. This arrangement allow the atoms to 'slide' above one another without breaking the bonds. It explains the electrical and thermal conductivity of Group I and II metals.

Order and rate equations

The reaction is investigated by running several series of experiments. In each series just one of the reactants is varied so that we may observe the effect on the rate of reaction. Here there are two reactants A and B. The units of rate are change of concentration divided by time (e.g. moles per litre by seconds). For any one reactant the change in the rate of the reaction is categorised as:

• Zero order
  • Any increase in the concentration of the reactant has no effect on the rate of the reaction.
• First order
  • The rate of reaction increases in direct proportion to the increased concentration of the reactant. This can be observed by comparing runs 1 and 2. The concentration of A remains constant whilst the concentration of B is doubled (x2). Consequently the rate of the reaction is doubled from 0.8 to 0.16. We would say that this reaction is first order with respect to B.
• Second order
  • The rate of the reaction increases in proportion to the square of the concentration change. This can be observed by comparing runs 1 and 3. The concentration of B remains constant whilst the concentration of A is doubled. Consequently the rate of the reaction is quadrupled (x4) from 0.8 to 0.32. This reaction is second order with respect to A.

Run	Initial concentration of A	Initial concentration of B	Initial rate
First	0.01	0.02	0.08
Second	0.01	0.04	0.16
Third	0.02	0.02	0.32

It should be made clear that rates can only be determined by experiment.

Constructing Rate Equations

Rate equations allow us to predict the rate of a reaction under different conditions. They take the form

Rate = k[A]^a[B]^b ...etc.

Where

• [A] is the concentration of reactant A (in moldm^-3) and a is the order of reaction with respect to A
• [B] is the concentration of reactant B (in moldm^-3) and b is the order of reaction with respect to B
• k is the Rate Constant. This depends on the reaction itself - how inherently fast it is - temperature, and any catalysts.

The rate equation can be rearranged in the normal manner to find any one of the required variables. At A level chemistry this is often finding k given data like above. The overall order of the reaction is the sum of the indexes a+b ..etc.

Rate determining step

Mechanisms can have one step or a series of steps in which each step can have a different rate. The overall rate is decided be the step with the slowest rate known as the rate determining step. Aromatic chemistry is the study of reactions involving the benzene (phenyl-) group C₆H₆. An aromatic compound containing this group - such as benzene, methylbenzene or TNT (trinitrotoluene) - is called an arene, and a functional group containing a benzene ring is called an aryl group.

Structure

From around 1865, the accepted model of benzene was the Kekulé structure - effectively cyclohexa-1,3,5-triene. It was thought that there were three double bonds in each benzene ring, with each of the 6 carbon atoms being attached to a single hydrogen nucleus. This model was later revised to suggest that the double bonds oscillated rapidly between the two possible positions (i.e. 1,3,5 and 2,4,6) in the molecule before the 1930s...

...when it was discovered through experiment that bonding in benzene was actually hybrid - neither single nor double - for all C-C links. It was found that, rather than localised π bonds between carbon atoms, a delocalised ring is formed - with each carbon atom donating one p-orbital electron. From this, the skeletal symbol for benzene (a hexagon with internal circle) is derived - and although rarely used, benzene can also be drawn out in full with each carbon atom shown and an internal ring representing the bond.

As there are three areas of electron density around each carbon atom, repulsion theory dictates that the molecule is planar with a 120° bond angle.

This structure is evidenced by:

• Bond length data - experiment has found that C-C bonds are 0.154 nm long, whereas C=C bonds are 0.134 nm. Observation of benzene showed that only one bond length of 0.139 nm was present - uniform to all carbon-carbon links in the molecule.
• thermochemical stability - hydrogenation of cyclohexene (i.e. cyclohexane with one double bond) produces an enthalpy change of -120kJ per mole. Hydrogenation of benzene would therefore be expected to produce an enthalpy change of -360kJ per mole if the structure contained three double bonds - but in reality only -208kJ pre mole is observed. The ring structure makes benzene more stable.

Halogenation

When a benzene ring is exposed to HCl in the presence of AlCl₃ or FeCl₃, halobenzenes are formed.

Acylation

Polymers are very long chains formed by the combination of small molecules (monomers) - and have many varying uses in modern daily life. There are three main types of polymer that you need to be aware of - polyalkenes, polyesters and polyamides - and two main types of polymerisation reaction as described below:

Addition Polymerisation

This is the simplest mechanism - whereby unsaturated monomers (i.e. alkenes) combine to form a much larger saturated polymer. The most famous examples of this process would probably be the production of poly(styrene) from phenylethene and poly(ethene) from ethene. Note that in each case it is the double bond that splits and bonds to the next molecule in the chain - so poly(stryene) has a central alkyl chain with phenyl side groups attached to it.

Addition polymers are non-biodegradable, and can produce toxic products in disposal - especially by burning, such as in the case of poly(chloroethene) [which is also referred to as "poly(vinylchloride)" or pvc]. However, they generally have very useful properties and so are ubiquitous despite their environmental impact.

Condensation Polymerisation

This mechanism is so called because a small molecule such as water or hydrochloric acid is released with each addition to the chain.

For polyesters, the usual esterification mechanism is used - between acid and alcohol groups - but with each monomer molecule having a pair of the groups. Most often, a combination of dicarboxylic acids and diols is used, but sometimes monomers can be hydroxy dicarboxylic acids (i.e. with one of each group). Each end of the molecule bonds to one other - and so a chain is formed.

For polyamides, much the same mechanism occurs, but with a peptide link between monomers (i.e. CONH) with a nitrogen atom in place of the oxygen present in polyesters. The reactant groups are amines and carboxylic acids - and synthetic polyamides are mostly created from dicarboxylic acids and diamines a similar way to polyesters. However, you also need to be aware of natural polyamides - in biological systems, amino acids combine via peptide links to produce proteins. The only difference is that in nature amino acids are used whereas artificially dicarboxylic acids are combined with diamines.

Because of their ester or peptide bonding, condensation polymers are polar - therefore they often have very high melting points and can be extremely strong, depending on the regularity of their structure. Kevlar®, for example, is a polymer made from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine (1,4-diaminobenzene).

Condensation polymers can be broken down by hydrolysis to recover the original monomers - which means that they are biodegradable and so easier than addition polymers to dispose of, but also are inappropriate for some uses involving long term regular exposure to water.