Doc Brown's Advanced Chemistry

PART 15.5 Ultraviolet & visible spectroscopy

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

Sub-index for this page

(a) More examples of organic molecule absorption spectra

(b) More examples of inorganic uv-visible absorption spectra (mainly transition metal ion complexes)

(c) Examples of applications of uv-visible absorption spectra in chemical analysis and identification

(d) Examples of applications of uv-visible absorption spectra in chemical kinetics

(e) The analytical uses of flame emission and absorption spectroscopy

(d) Examples of applications in chemical kinetics

A change in reactant or product concentration can make the basis of studying the rate of a chemical reaction.

Ideally, a single coloured reactant changes into a colourless product, or,

colourless reactants produce one coloured product.

The concentration of the coloured reactant or product can be followed by sampling the reaction mixture and measuring the absorbance at appropriate time intervals.

The two graphs above might represent results for a 1st order reaction.

The graphs assume

(i) Only one reactant is coloured and forms a colourless product, so absorbance decreases with time.

(ii) The absorbance is proportional to the concentration of the coloured reactant.

Graph A represents the raw data i.e. measuring the absorbance of samples removed from the reaction mixture - this must be done quickly and efficiently.

From the graph A data and a concentration versus absorbance calibration curve you can then convert absorbance to a concentration to give graph A.

The graph data (and other sets)  can then be analysed just like any other sets of kinetics data to determine the order of reaction and rate constant.

See Advanced A level chemistry notes - sections 5. to 7.