There are two terms that are used:

1. Bond enthalpy – this is the average of a bond of a certain ‘type’ For example the C-O bond. This occurs in many diferent molecules and has a slighlty different value on each, as the actul environment of a bond affects the energy needed to break it. Thus the bond enthalpy is the average taken over a whole range of different C-O bonds.

The values that are quoted in data tables are for Bond enthalpy per mole of bonds in kJ mol^-1

2. The bond dissociation enthalpy: This is the energy required to break one mole of specific bonds. For example, when CH₄ is broken to give CH₃ + H. This requires a different amount of energy than say when the ‘fragment’ CH₃ is broken into CH₂ and H. Notice that the products are simply the fragments that are expected on breaking the bond.КартиниИдея за подаръкикониикониПравославни иконииконописikoniсвети георгихудожник на икониИкони на светциХудожникОткъде да купя иконаидея за подаръксондажилак за паркет

atomic absorption spectrum

To understand atomic absorption it is necessary to take a look at some of the concepts involving the nature of light itself.

Light is just a form of electromagnetic radiation. The term electromagnetic radiation refers to the propagation of energy through a medium (or a vacuum) in the form of oscillations of an electrical nature with a corresponding perpendicular magnetic field (motion of electrical fields causes a magnetic field and vice versa)

So electromagnetic radiation is a form of energy that passes from one place to another at the speed of light in the form of waves.

Waves have various characteristics, such as velocity (the speed of light),  wavelength (the distance from a point on one wave to the equivalent point on the next wave), frequency and amplitude.

The amplitude is proportional to the intensity of light and not particularly important at the moment. The frequency is the number of wavelengths that pass a specific point per second. There is a very simple relationship between the velocity of a wave, the wavelength and the frequency.

c (the velocity) = lambda (the wavelength) x f (the frequency)

c is a constant (the speed of light)

The energy of a wave is difectly proportional to its frequency and hence, inversely proportional to its wavelength. Both are connected to the energy be the Planck constant ‘h’.

Energy = h x f

Energy = hc/wavelength

In atomic absorption events, the energy of the incoming wave must have the exact energy that is needed for an electron to be promoted to a higher level. It absorbs this energy and gets promoted. Detection systems see this specific wavelength (or frequency) removed from the spectrum.

An absorption spectrum thus appears as a series of dark lines superimposed on a continuous spectral background. This process need not necessarily take place in the visible region of the spectrum (which is tiny compared to all the available electromagnetic spectrum, it occupies the wavelengths from about 400 -700 nm) in which case the detection cannot be visible, it must rely on apparatus designed to detect absorption in specific regions of the electromagnetic spectrum.

The system is as follows:

The oxidising agent reacts with the iodide ions (usually introduced in the form of potassium iodide).

H₂O₂ + 2I^- + 2H+ 2H₂O + I₂

The iodine produced is then absorbed by reaction with a fixed amount of thiosulphate ions:

2S₂O₃^2- + I₂ S₄O₆^2- + 2I^-

As soon as the thiosulphate ions are used up the free iodine then reacts with some starch indicator that is added right at the beginning. The reaction produces an almost instantaneous blue/black colour. Thus it is possible to time fairly accurately the time taken for a fixed amount of iodine to be produced (from the thiosulphate moles present initially)

If the oxidising agent concentration is kept constant and all other factors are constant except the iodine concentration then the rate equation simplifies to:

Rate = k[iodide]x

where x is the order of the reaction with respect to [iodide], and k is a kind of super constant combining all of the other constant factors, the rate constant and the other components of the reaction.

So, as rate is porportional to 1/time taken then

1/t = k[iodide]x

if you take logs throughout

log(1/t) = xlog[iodide] + logk

This now has the form y=mx+c (a straight line graph)

So a plot of log(1/t) on the y-axis against log[iodide] on the x-axis gives a straight line of gradient x and intercept k (although k is pretty useless in this case)

The theoretical yield can be simply calculated froma knowledge of the stoichiometry of the reaction(s) involved in the preparation.  The final percentage yield is calculated from actual yield/theoretical yield x 100%

C₆H₆ + NO₂⁺ C₆H₅NO₂ + H⁺

It may easily be seen that 1 mole of benzene s expected to produce 1 mole of nitrobenzene. However, this is rarely the case.

Any discrepancy bertween theoretical yield and practical results is due to the following causes:

The reaction may not produce a stoichiometric amount of product. This could be due to equilibrium, slow rate, decomposition of reactant, or product, or evaporation. The reactants may also not be 100% pure. A common problem in synthesis is the issue of side-reactions producing an undesired product.

During a work-up which may involve solvent extraction or crystallisation some of the product is invariably discarded.

During the purification stage some of the product may be lost in distillation, or by decomposition, or even evaporation. Some may remain inside the reaction, or purification vessels, filtration systems, chromatographic columns etc etc.

All of the above will lead to a reduced yield.

Higher than expected yield can only be due to the presence of impurities caused by inadequate purification. (assuming that no mathematical errors have been committed).

It just remains to ascertain how such impurities have arrived there.

It is usually because the product still contains solvent, but clearly it could still be mixed with other products or even reactants. It could absorb components from the air such as water vapour or carbon dioxide. The vessel used to collect the product could even be contaminated.

All other atoms are unstable on their own under normal conditions. Metal atoms exist in giant structures, giant covalent elements exist as just that, giant and covalent, simple covalent molecules are arrangements of atoms in molecules etc. etc. Nothing exists on its own.

So, if we want to represent carbon we don’t write down C_{(very big number)} we just understand that it is a single macromolecule and express it as C, pure and simple.
The same goes for all giant structures, be they metals, or macromolecules. In the case of phosphorus, sulphur and certain other simple molecular elements we are faced with a choice. Do we refer to them by the molecular formula, or do we understand that they exist as finite simple molecules, but continue to express them as if they were free entities? Most texts and chemists choose the latter.

It is understood (within the chemists fraternity) that sulphur is usually found as S₈ molecules and that phosphorus is P₄ pyramids, but that there is no real advantage to be had expressing them as such. Indeed sulphur changes its form several times while being heated through to the vapour state.

S₈ crowns give way to S₈ strings that entangle into S₈ cross linked structures that turn to S₈ vapour.

When sulphur is reacting with (for example) oxygen which is the ‘correct’ way to represent it? What form does it actually have at the moment of reaction? We don’t know (and probably don’t care).
The sensible answer to the first question is that there is no ‘right’ way to express the chemical strucure. For this reason the majority of texts (and chemists) simply refer to sulphur as ‘S’, using the same logic as used for giant and similar stuctures.

Nitric acid is an oxidising agent and the reaction is not the usual acid + metal reaction. The products are oxides of nitrogen instead of hydrogen. The actual nitrogen oxide formed depends on the concentration and temperature of the acid.

There are actually two equations for the reaction of copper with nitric acid. It depends on whether the nitric acid is concentrated or not. If it is concentrated and in excess then the ratio is 1:4 copper to nitric acid. If it is dilute then the ratio is 3:8.

Cu + 4HNO₃ –> Cu(NO₃)₂ + 2NO₂ + 2H₂O

3Cu + 8HNO₃ –> 3Cu(NO₃)₂ + 2NO + 4H₂O

Nitric acid when concentrated is a strong oxidising agent so it makes sense that a higher oxidation state of nitrogen (IV) oxide is formed when the nitric acid is concentrated.

The ethanoic acid dimer has an effective RMM = 120. This means that considerable Van der Waals force would be expected between particles. Compare with a totally non-polar molecule with a similar relative mass such as decane. The boiling point of decane is 174ºC.

Were ethanoic acid simple monomers (RMM = 60) then comparison with pentane (bp=36ºC) suggests a much higher degree of intermolecular force.

It seems that the boiling point of ethanoic acid (118ºC) is somewhere between that expected from a simple monomer (with hydrogen bonds) and a dimer.

Clearly the situation is not simple and there will be contributions from hydrogen bonding, Van der Waals forces, monomers and dimers. It is difficult to state categorically which is the most important.

When someting acts as an oxidising agent is gains electrons (removing them from the oxidised species). This can be shown by the relevant half-equations:

H₂O₂ + 2e –> 2OH^-

or in the presence of acid:

H₂O₂ + 2H⁺ + 2e –> 2H₂O

Now damn well stay out!

CC

Where has spectroscopy and instrumental analysis gone from the main syllabus? It’s been exiled to the options. This seems strange to me as it’s probably the most used (and useful) area of chemistry in research and industry.

What about radioactivity? It’s been banished altogether. There’s no interest in nuclear chemistry nowadays in the world is there?

Perhaps it’s all physics… I don’t think so!

CCикони